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edward_davis
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My speedo had the ol' weeble-wobbles, so I went to the trouble of pulling out the rusted-in dash pod to check out the cable connection at the back.  After painting the dash pod spike and lubing it with some bicycle gear grease (good all-purpose stuff), I can now take out the dash pod easily.  I opened the speedo and dusted some graphite inside and put everything back together, wiping off the grime with a damp cloth. 

Now my tachometer doesn't work.  At first it was reading too high, by about 500 rpm.  Then when the weather cleared up and I took her out for a spin, the tach gave up completely as I wound out second gear.  There's power coming in on the hot lead, and the electricity is making it through the tach and to the ignition, since I have no trouble with the engine.  The ground connection seems to be good, too.

All of the other gauges work, and I've reduced the weeble-wobble in the speedo (still there, but smaller), but I'd like to know if there's any chance I can fix this myself.  If not, who is a good fixer of these things?  How much can I expect it to cost?

Thanks, Edward Davis

Mark Rosenbaum
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The British car club sites have quite a bit of information posted on Smiths tachs, and much of it is pertinent to the JH.  Here's part of a post made back in August 1994 by Mark W. Olson, who apparently was one of the first LBC owners to 'reverse engineer' the Smiths tach:





"[...]  For those of you interested in the tach itself, the Smiths tachs of that  vintage have an interesting design. The ignition wire loop acts as the  primary winding of a transformer. On the inside of the tach is a coil that picks up the current pulses from the ignition wire. Each ignition pulse triggers a one-shot made with two germanium transistors. The output of the one-shot drives an ammeter. The more frequent the pulses, the higher the duty cycle of the one-shot output, the higher the reading on the ammeter. The tach is calibrated by varying the one-shot's pulse width via a pot. They tend to calibrate well at low and high RPMs, but they tend to read a couple of hundred RPM high when the needle is nearer to vertical.

"The design was obviously originally done for a positive ground car, as the circuit is referenced to the 12V and it floats off the ground through a resistor. [...]

"The most common failure mode I've found so far is failure of the main timing capacitor, 
which can lead to erratic behavior or total failure. Mechanical failure of the ammeter is the second most common failure. [....]"




If the above makes sense to you, then you can probably fix the tach.  Other options are to find someone who can fix it, or who can replace the electronics with something more modern; here, the internet is your friend.

Finally, try eBay: most Smiths RVI-type tachs will work without changes, and Smiths RVC-type tachs will work with only minor wiring alterations.  Often you can even exchange tach faceplates so the replacement will look original.  In any event you should re-calibrate the new tach by comparing its reading to a known accurate tach or tach-dwell meter.

Good luck.

Mark Rosenbaum
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For what it's worth, here's a schematic of the tachometer circuitry, from classictiger.com:

 

Attachment: Tach Schematic.jpg (Downloaded 161 times)

edward_davis
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I've disassembled my tach, and it turns out to have much simpler circuitry than the old Sunbeam Tiger tachometer that the diagram comes from.  There's only one transistor and one capacitor.  All of the individual elements seem to work in my tach, but the capacitor is no longer connected to anything.  There's a place for a connection in the circuit board, but nothing there.  I didn't find any loose parts, either, but I might have lost the bit the first time I took the gauge out of the dash.  I didn't realize how easily the bezel comes off, so it slipped off while I was looking at it; could have lost the circuit element then.

I've bought a natty old '76 MGB tach on e-bay to "reverse engineer".  If it really is the same sort of design, hopefully I can make one working tach from the two units.  Once I get it worked out (if, I should say), I'll post the circuit diagram.

There is a site online that has a circuit diagram for a DIY conversion to a voltage trigger, so that the tach can be used with an MSD system.  If I can't get the old-school system to work, I might try that instead.  It apparently works fine for old Jags and MGB's.  The big problem with MGB's is that the tach has the ignition warning light in the lower part of face, interfering with the placement of the new circuit board.  Good ol' Jensen folks, putting our ignition light on the console...

Does anyone out there have a photo of the insides of their original Jensen tachometers?  If I saw a working unit (or a unit broken in a different way), it would help with troubleshooting mine.

Mark, thanks for the pointers; they were good leads that will hopefully lead to a happy DIY resolution to my problem.

edward_davis
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Here's another quick question:

Can an old MGB speedometer work in a JH?  I've seen some on ebay that look just like my unit except for the silver bezel.  Would they have to be recalibrated?  Is that sometihing I can do myself?

John Finch
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Do you happen to remember the web address for the MSD/Tach site? I will be installing an MSD in 18309 this spring and will likely need the diagram. Thanks

Mark Rosenbaum
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Attached is a rather poor photo of the interior of a JH tach.  Either I'm an even worse photographer than usual, or my camera isn't working all that well tonight -- this was the best of eight tries.  You're right, only one transistor, so obviously it has to be a different circuit than the one I provided.  I've spent more than forty years in electronics, yet I missed that small but vital detail in several previous examinations... aargh!

Several owners have reported what sounded like MG tachs in their JHs, that worked just fine.  Since both cars have 4-cylinder engines no recalibration should be necessary.  But then you don't know what a PO might have done to the MG tach, so it wouldn't hurt to check its calibration, if you can.

Attachment: tach interior.JPG (Downloaded 148 times)

edward_davis
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John,

Here is the MSD tach site I found.  It's sometimes hard to navigate, but it has a lot of information and some good circuid diagrams:

http://members.shaw.ca/tsmit/tachmod/tach_other.html#MG

You can buy the prebuilt circuit board from the folks or you can make it yourself.  I have no association with these people; I just found them on a somewhat-random Google search.

edward_davis
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Mark, 

Could you tell what component attached to the negative end of the capacitor in your tachometer?  I don't seem to have any components attaching there; there's an empty hole in the board and the other side of the board doesn't have any soldered connections there.  In your photo, it looks like the hole is supposed to be empty, so it could be I just traced the circuit wrong: this is only a hobby and I haven't done anything like this in 10 years.

The capacitor seems to be OK; it charged up when I tested it with my ohmmeter, and it slowly discharged when I applied the voltmeter to it.  The break in continuity in my tachometer seems to occur in the meter itself: if I test the resistence across the black and red wires that attach to the "meter" part of the device, I get an open circuit. 

I'll know more when I can get my hands on another tachometer; I don't want to pay too much since I'm just going to open it up and take it apart.

Jensen Healey
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I may have some tachometer guts for sale. Let me check the back shelves. Send me an email klhoush@comcast.net

 

Kurt

San Anselmo, CA

Mark Rosenbaum
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Edward,
I've just taken a thorough look at my spare tach.  To respond to your most recent post:
(1)  The negative end of the capacitor connects to +12 volts, the positive end to a pad on the board that does not go anywhere.  This seems odd, but perhaps the cap was hooked up for some special application.
(2)  The meter movement should measure about 8 ohms, if yours reads open, that's probably the problem.

I've traced out and drawn up a schematic for my tach, hopefully correctly, though I've left out the capacitor referenced above.  This drawing is attached below.  Component values reflect item markings, or presume compliance with international color code standards.  The unknown device could be a capacitor, thermistor, or something else entirely -- I can't tell with the means at my immediate disposal.

The other thing I'm not sure about, is the transistor.  I presume that MT59/A is the part number, and that MT stands for Mullard Transistor, but I can't find any data whatsoever for the device.  Possibly MT59/A is a Smiths house number for some standard part.

The circuit makes sense if I assume that the transistor is a PNP germanium device, but this has not been confirmed.  (It's possible the device is an SCR or UJT, but I consider this unlikely.)  The pinout is also uncertain.  Pin 3 must be the collector, but it's unclear whether pins 1 and 2 are emitter and base, or base and emitter, respectively.  Arguments can be made for both arrangements.

Meter zero is a mechanical adjustment.  Because of the uncertainties about the transistor, it is not apparent whether the potentiometer is a calibration adjustment or a sensitivity adjustment.  If the latter, then tach calibration is purely mechanical as well, probably by adjusting pointer spring tension at time of manufacture.

For a large number of subtle reasons, this tach is a sophisticated and brilliant combination of mid-20th Century technologies.

Attachment: JH Tach schematic.jpg (Downloaded 122 times)

edward_davis
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Another link explaining Smith's Tachometers for those who care.  From the Golden Gate Lotus Club:

 

http://gglotus.org/ggtech/smith-tach/smithtach.htm

edward_davis
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I have replaced the insides of my original Smith's tach (RVI 1439/O3A) with those from an MGB tach I bought on e-bay (RVI 1439/O1A).  The ad said it was in working condition, and there seems to be no problem with the meter itself, like my old one was having.  We'll see when I get her all hooked up this weekend.  Interestingly, there were only two differences in the gauges, except for the design of the faceplate.  The MGB gauge was oriented upside-down relative to the JH gauge (the faceplate attaches 180 degrees).  And there was no mystery capacitor in the MGB gauge.  I did not check to see if the resistors were the same, though, and the potentiometer was of a slightly different design.  Hopefully it'll work.

edward_davis
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Well, it all seems to work.  Once again, the club has saved me money while providing me with good guidance for tinkering.  Thanks, everyone!

jls
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I have just fixed the tacho for my 1970 MG Midget which is the same as yours.  The fault was with the component you have omitted from your circuit which was a 2.5uF 16V electrolytic connected between pins 1 and 2 of the transistor (+ve end to pin1 which is the + rail of the circuit).  The original component was an axial type which went from the pin2 (base?) connection to the +ve rail, with the +ve connection looped under the edge of the board.

The fault on mine was that the capacitor had gone short circuit (old age).  I replaced it with a 2.2uF 63V aluminium electrolytic in a radial package which fitted the holes available in the PCB nicely.

I tested and calibrated it with a pulse generator.  The 50 ohm pot is for calibration.  The repaired instrument seemed to be quite accurate and surprisingly insensitive to pulse width and to ignition circuit current.  I haven't fathomed how it works or exactly what the feedback winding of meter current to the transformer winding does.

d.dewdney
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Hello gents, this is my first venture into the message board. I live in the North....way North than most of you, King City Ontario Canada. I'm new to the Jensen family but not new to british autos (1st car was an MGB and I never looked back. Now to my problem.

I bought a 1973 Jensen Healy in June (Just as I blew one of my own valves) I have yet to put her on the road but have managed to tinker here and there. One of my problems is the Tach, On start up it looks good and as she warms up the tach does not return but stays at 2000 rpm, If I rev it to 3000 she stays there. When the engine is shut down it again stays at 3000, if I leave it for period it eventualy goes back to zero.

 

Any thoughts you may have would be greatly appreciated.

Thanks Dave

In King City

Last edited on 01-02-2008 01:48 am by d.dewdney

dwalls1
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K den, I can assume that my dead tach needs repair and that the signal from the ignition is most likely present and is not the problem. Have used Mr. Rosenbaum's excellent schematics in the past, but on this one I am lost as to how the tach signal gets from the coil to the tach and the factory schematic is tough to follow. So, tell me where to start. Should I begin by removing the tach from the dash? The speedo R&R took three visits to the chiropractor to recover from. Maybe I'll wait 'til I've got time to remove the seat first.

My apologies for confusing Mr. Kimbrough and the late Mr. Rosenbaum. I am a dolt.

Last edited on 08-10-2008 09:41 pm by dwalls1

edward_davis
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The factory tach gets its signal through the input wire for the coil.  The wire runs from the ignition to the tach and then onward to the coil.  The tachometer gets its reading by measuring the changes in current in the ignition wire: inside the unit is a little transformer, with the ignition wire wrapped around one side and another wire with many more windings on the other.  This is not at all how modern tachs work, so going to an aftermarket tach requires a bit of rewiring. 


When you pulled the speedo, did you get the whole gauge cluster out?  I pulled mine out with great difficulty the first time, but after sanding down and painting the "spike" that holds everything together, and adding a bit of grease when I reassembled it, I can now pull it out with two fingers and get easy access to the back of the gauges.  See my above post from 7-05.

Cheers!
Edward

dwalls1
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Thank You for responding with an alternative to working upside down with my back bent in the wrong direction. No, I did not remove the entire cluster when I repaired the odometer. I will definately look into doing that to get at the tach. BTW and FWIW I found with the speedo that the sealing ring from the oil filter canister on my TR7 makes an excellent replacement for the o ring under the bezel when mounting the gauge back into the dash cluster.

dwalls1
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Got the tach out without doing the Dashboard Limbo. Didn't see any fried components so that about exhausts my diagnostic capability. Since my tach just suddenly quit altogether is there a component that most likely is the culprit, or would it be just as well to replace the capacitor, the resistors and the transisters. (what, no vacuum tubes?) Should I be able to source these components at Radio Shack? In reading the posts on this thread it seems that erratic readings are more common than total failure. If it comes to it can I send it somewhere for repair without replacing the whole circuit board for $165.00 plus? Then again now might be the time to upgrade to a unit that will work with the inevitable pointless ignition though my ignition is OK now. I'm glad I checked here first as I was able to figure out that I needed to plug the white and white/grey wires together to run the car without the tach from info on this thread. Guess I'll bring my multimeter home this weekend and make like I know how to use it and check some resistance readings. Would appreciate any thoughts you anyone might have on my ramblings and Thanks for allowing me to ramble. Dale

Could anyone help me identify the main timing capacitor? I think I may just replace it and not try to test it.

Last edited on 08-15-2008 04:26 am by dwalls1

JodyFKerr
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No readings = crappy readings.

Bsed on my understanding of the tachometer (I took one apart a while back, so colour my ingorance for me) the issue is most likely in the wiring reaching the unit, not the tachometer itself. It is, in general terms, too simple to fail. First off, clean the terminals on the back of the tach to see if you get any improvement. If not, pass a volt meter across the connectors to determine if the to tach is reading any varations. (good variations = bad tach). If not, then it's diagnostics floating back through the wiring. I can't find my diagrams right now, but it's not a clean trace IIRC.

 

Jody

dwalls1
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Hello Jody,

   From what I have gleaned on this thread, if the tach isn't getting a signal then the coil isn't either and the engine won't run. I will be playing with it tonight and see if I can narrow it down to a single component failure, perhaps the capacitor mentioned in earlier posts on this thread. Your dead on about this being a simple unit. The technology was about 10 years obsolete 35 years ago, which makes it about 45 years ahead of my understanding of electrical gadgets. I'll let you know how I make out. Dale

 

dwalls1
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FWIW I've sent my tach to MOMA in Alburquerque. I've asked them to repair it as a stock unit without the capability of using it on a pointless ignition. I'll probably regret it, but hope to run the points for as long as I own the car. I can have it reworked later to work with the pointless system, but it would be more money. Oh well, what isn't.

dwalls1
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Got the tach back in this evening and it works fine. MO-MA got it on the 21st and returned it to me on the 22nd. Wow! talk about your turn around time.

maddog55
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Did you ever find out what kind of transistor that was?

I'm working on repairing the smith tach I've got. Everything is fine now but still need to know what the part is PNP probably but not sure.

atgparker
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Hey, do a search on RVC tachometer and you will find a few listings for Smiths Tachometer RVI to RVC conversion boards.
Spiyda Design does pay pal and lists on eBay. There is dynoplex.org which has the bits and pieces for a DIY solution as well. I'm going to swap out the board on my Smiths RVI 1439/O3A so that the Pertronix ignition will trigger the tachometer correctly as it is RVI at the moment it doesn't work much above 5,000 RPM.

answerman
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I've been looking at the Spiyda Design board, just haven't gotten around to pulling the trigger on it yet. Though I'm still running the stock ignition, I'd rather hook up the tach in the RVC fashion anyway. Let us know how the conversion goes.

atgparker
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The Spyida Board replacement was started this week.  I'll have snipped the white wire that goes through the torrodial doughnut inside the tachometer and replaced the main PCBA with the new one.  The screws go through the heat-sinks on two transistors on the new board.  Pretty cool mounting scheme.  The male and female wires from the ignition switch and the one that runs to the +Ve coil terminal are plugged together and no longer run through the tachometer.  I have run a new wire from the -Ve coil terminal and this will plug into the new PCBA with a resistor to knock down the voltage a bit from the Pertronix Flame thrower coil and ignitor unit.  Spyida provides three resitors to try and an audio cable to run from your PC while running a wave file at maximum volume to calibrate the Tachometer at either 3K or 6K for a 4 banger.  The audio cable was missing from the kit and the engine is not back in the car yet so I haven't gotten any further with this.  But the trim ring and glasss was cleaned up and I cut a pair of O-rings I had and placed them into the groove so as to provide some squish when I remount the ring to the housing and seal up the front of the tachometer.  The white paint inside has some flakes that have dislodged so I'm deciding about what to do with the finish inside the casing.  The kit has great instructions and email with Spyida have been imediate and complete to aid in my getting this far.  There is alternative seetings concerning a jumper for use with a ignition module that has a tach-out signal so it would seem this RVI to RVC conversion will cover most all ignition systems.

Jensen Healey
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VDO Tach: Connect to coil or MSD. Looks good but not as cool as the stock tach. $90


http://www.summitracing.com/parts/vdo-333051/overview/

atgparker
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Ran into a few issues with the Spyida RVI to RVC repalcement PCBA kit. The VDO is looking a bit more appealing at this point! I discovered the heatsinks which are screwed to the frame violate the swept area in which the moving coil swings about that round anular disk magnet. In operation it would swing up to about 2,500 RPM and stop altogether because of the collision. Then on the other side of the needle swing it would hit the second heatsink at around 5 or 6,000 RPM. So, I had to take it apart and snip off the corners of the tansistors heat sinks to make enough clearence. The calibration cable arrived but with the wave file playing and a 12 volt feed it isn't working. So, I'll try to calibrate the Smiths Tacometer in the car to 3,000 RPM using the potentiometer (POT) with a Suntune diagnostic meter I have assumming that its decent in as far being accurate as a Tachometer. That is if I can rig it up so I can read it in the car while the POT gets twidled on the PCBA.

The 40 miles I put on the car over the weekend with it uncalibrated. This left me with an overall impression that the electronics are a bit overly damped to stop needle bounce and therefore it lags a bit in displaying the true engine speed when trying to observe unloaded reving of the engine when I bump the RPM during down shifts and the like.

Dakota123
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Well that's a bit disconcerting.  I have a Spiyda conversion kit I was planning to use once I do away with points-based ignition.  (Mine did come with the calibration cable at least.)

Maybe damping is adjustable?


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The Spyida conversion kits needle damping can be changed but Chris who I have been in dialog with has indicated that to do this requires changing a surface mounted capacitor.  So as the PCBA is doped with a coating of anti fouling goop it would be slightly more involved.  Chris indicated that the chosen value was used to stop needle bounce at idle and that the tachometer can read quite accurately at the calibrated value and indicate the RPM effectively while the drive train is under load.  Its the quick accelerator jabs with the clutch pedal depressed that may not get recognized as the engines response will beat the capacitors charge/discharge rate i.e. thus the needles movement is subdued to what ever that rate of change is that is dictated by this capacitor! 

Last edited on 06-15-2017 01:31 am by atgparker

Dakota123
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Interesting.  Thanks for being a trailblazer and figuring this stuff out.

Mike

answerman
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Well, my Spiyda kit arrived today. Very fast shipping considering it came from England. I'll probably try to get it swapped out tonight, we'll see how it goes. Fortunately I have a spare tach so I can use that one to get it built/calibrated etc before I actually pull the existing one out.

answerman
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The board swap went just fine, had the tach disassembled and rebuilt in about a half hour. I didn't run into the issues with the transistor heat sinks being in the way (in fact they aren't even close) but I haven't been able to test it yet. Went back to the Spiyda site to download the wav file for calibration and the site was down, so I just abandoned the project for the night.

The site was back up this morning, so I was able to download the file. Taking another crack at it tonight.

answerman
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OK, for the first time since I have owned Ms. J (almost a year to the day) I FINALLY have a working tachometer.

My experience seems similar to atgparker's. I mentioned above that I had trouble downloading the wav file from the Spiyda site, but I was able to get it yesterday. I temporarily hooked up the tach directly to the battery with a wire to the coil minus. However, I could not get the calibration to work with the cable. Maybe my laptop isn't loud enough.

Anyway, today I went out to Harbor Fright (yes that's how I spell it) and picked up an automotive multimeter that, among other things, has a clamp-on attachment to read RPMs (I needed to get it for my boat anyway). Hooked that up, and was able to use it as a baseline to calibrate my tach with the Spiyda board. It seems quite accurate in comparison to the clamp-on, though as atgparker noted it is a little slow to respond.

Nevertheless, I now have a stock looking, Smiths RVI1439/03A tach in my dash with RVC internals, and connected in the RVC way, which was my goal. I just plugged the two wires that connected to the internal loop together (they were thinking, making one a male bullet and one a female) to bypass the tach that way, and ran a new wire from the tach directly to the coil. Works nicely. Even though I have the stock ignition with points, I think this is a more effective, reliable way to connect the tach (plus now I'm covered if I do go to Pertronix at some point), and I would recommend the Spiyda board to anyone with tach issues.

dbeliveau74
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Greetings, I have an electromotive XDi ignition systen installed by the previous owner. Nice system no more distributor under the delorto carbs. But the tach now only goes to 3000 rpm and won't read any higher. On the tach body is written Modified for HPV-1 or Tec.
I was wondering if anyone had any ideas?
Thanks,
Dan

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I wonder if the modification to the tachometer has interferences like I discovered with the SPIDA kit? You would have to disassemble the tack and look inside to see what's going on.

Last edited on 06-15-2017 01:32 am by atgparker

Tom Bradley
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Hi Dan,
Do you know if the tach is reading correctly at the lower RPMs? The XDi ignition system uses two coils for a 4-cyl engine instead of one. So if the signal to the tack is only coming from one coil, the RPM reading will be 1/2 of the actual engine speed unless the tach has been modified to account for this or if the XDi system is sending a separate (composite) signal to the Tach. In the latter case, the tach may be maxing out because the signal is too low and it is not picking up all the pulses at the higher rates. Possibly the tach has a sensitivity adjustment or the XDi has a signal level adjustment to correct this.

dbeliveau74
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I was informed of the tach malfunction by the owner when i picked it up. He made it sound li!e this was a new issue.
He did the restoration/upgrade 20 + years ago.

Tom Bradley
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I assume then, that everything was put together correctly and working originally. If the rev reading goes up to 3000 and then has a hard stop it might be something has come loose inside the tach and is blocking movement. Easy to happen since it probably has the SPIDA conversion inside or something like it. If it is more of a soft limit my guess would be electrical in nature like the low signal level mentioned above. This might simply be due to a bad electrical contact somewhere along the line. If the 12V or GND lines going to the tach are loose, rusty or corroded, that could be the source of the problem. The 12V line I would especially check since it was not originally designed in and may not have been connected up very well.



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