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 Posted: 03-12-2020 09:27 pm
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cjwilson
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Hello and thanks in advance for input.

I have a 73 JH I am trying to bring back. My tach is giving troubles. Before I dive in, I thought I'd glean some wisdom here. The tach appears to work while starting the vehicle, but then immediately drops to zero while driving.

Any thoughts or advice where to start looking?

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 Posted: 03-13-2020 01:38 pm
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DonBurns
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I'm sure other members will be able to help you work it out with the original tach, but I'll mention one option- I was having issues with both the tach and speedo. I tried to rebuild the tach myself and failed miserably. Ended up buying both new from SpeedHut. GPS speedo. They are custom built and you can specify faces to pretty closely match the original appearance.Cheaper than sending out to be professionally rebuilt if that turns out to be necessary. I am holding on to originals in case I ever want to restore.

Last edited on 03-13-2020 01:39 pm by DonBurns

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 Posted: 03-13-2020 01:50 pm
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cjwilson
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Thanks Don. I feel like it might be related to the key being in the start position. That seems to be the only time the tach is energized.

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 Posted: 03-16-2020 04:19 pm
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discogodfather
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What are you running in terms of ignition coil? Still have the ballast resistor?

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 Posted: 03-16-2020 04:22 pm
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cjwilson
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Yes. Original points and resistor. I had pulled them out once and put in a Pertronix. But then fried it months later. So the points and resistor went back in. It's been converted back for years.

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 Posted: 03-16-2020 10:55 pm
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redracer
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It is possible the green wire entering your tach from the #2(middle fuse--check it) may not be getting any power. The car will still run with the white wire to the tach sending current out to the coil either through a ballast resistor(W/S: white with slate line) or nickel chromium resistive wire(white with pink stripe)--not sure which wire system you have).
I would advise pulling out the tach out through the dash; if you're "not lithe & limber" maybe a grand kid to go upside down under the dash to loosen the two knurled finger nuts should work.
keep us posted, bruce

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 Posted: 03-16-2020 11:00 pm
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cjwilson
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OK. I'll check power from the middle fuse first while standing next to the car. If that's functional, I'll find someone lithe to assist.

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 Posted: 03-17-2020 12:45 am
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cjwilson
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I went and checked again. First, I have 12v to both sides of every fuse. And the tach is not really registering the engine rpm's at all. It is bouncing a bit, but mostly staying at zero. Independent of during starting or idling or stabbing at the gas pedal while it's running. I did get it to bounce to 4000 rpm once, but it mostly stays at zero.

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 Posted: 03-17-2020 01:44 am
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cjwilson
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I dusted off my schematic. 4 wires to tach. Ground. Fused power off Fuse #1. Hot in from Ignition Switch. And finally hot out to the Coil. It looks like if the car is running ok, then the last 2 are working, signal into and out of the coil. So I'm thinking either hot from Fuse #1 or Ground is my culprit.

And of course to check these, I'll have to find someone lithe.

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 Posted: 03-17-2020 01:51 am
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redracer
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yes, the tach must be grounded(this is actually one of the main problems with the fuel & temp gauge: the 10v voltage stabilizer has a bad ground contact between the housing/case and the sheet metal screw attaching to the metal backing plate).So pull the tach(and maybe the gauge panel--much more difficult) and do some preventative maintenance

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 Posted: 03-17-2020 02:00 am
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cjwilson
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Gotcha. So, I'm probably looking first for the voltage stabilizer ground as the culprit? Is it nearby? What does it look like?

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 Posted: 03-17-2020 02:01 am
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cjwilson
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Or is it internal within the tach?

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 Posted: 03-17-2020 02:04 pm
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redracer
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The "voltage stabilizer: controls ONLY the temp & fuel gauges(NOT the tach). If the ground for that is not working, the needle for those two gauges will read 22 1/2 degrees higher(I.e. CCW rotation-your temp gauge will look "HOT").
ALL the gauges MUST be grounded for not only the night lights to work but any internal parts that need grounding(such as the TACH)

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 Posted: 03-17-2020 03:24 pm
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cjwilson
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Ok. So I just need to trace the ground from the tach. On my schematic, that is a black wire labeled B-126-C. It heads off to a wire nut then on to terminal 4 of a 5 way plug.

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 Posted: 03-17-2020 04:42 pm
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cjwilson
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Redracer, I have both good ground and good power to the tach. And fuel and temp gauges are both working and registering at key on.

So I'm wondering if I need to rebuild my tach.

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 Posted: 03-17-2020 09:16 pm
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Tom Bradley
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cjwilson wrote:
So I'm wondering if I need to rebuild my tach.

Possible, but unlikely. When I did a Spiyda conversion on my tach, the tach internals looked very robust. What you described earlier sounds to me like a poor or intermittent electrical connection. The main culprit I have found over the years is the slide-on connectors that are used all over the place on these cars. Before going to messing with the tach internals I would check to make sure that the connections to the back of the tach are clean and tight. The connectors should be difficult to pull off. If not, try tightening them with a pair of pliers so it takes a bit of muscle to push them back on. If the connectors on the wire ends are tight enough, they should push through any oxidation on the spade connectors on the tach. To be safe, though, it is better to clean off the spade connectors with fine sand paper or steel wool. If possible, also check the wires going into the connectors. Normally these connectors do not move much, so the wires should be OK, but if they have been moved much for some reason, the wires could be loose or frayed in which case the connector needs to be replaced.

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 Posted: 03-17-2020 09:19 pm
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redracer
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Assume you have pulled the green wire from the back of the tach and push it in & out a few times to slightly clean the connection? If all that has been done(including the ground connection for the tach) then you'll need to pull it.
Either get it reworked/whatever or get a good used one(I have about a dozen N.O.S. ones in case you want a "new" one; somewhat pricey but they're "new").

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 Posted: 03-17-2020 10:19 pm
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cjwilson
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Thanks Tom. I pulled the dash and the tach and have reinstalled it a couple of times. All of the connections are tight. The only issue I see is the wires in the male bullet connector are starting to fray. I'm guessing that 50% of the wires are still connected. It's at least a good enough connection for the car to run. I'm guessing that's not my issue.

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 Posted: 03-17-2020 10:21 pm
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cjwilson
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And the tach internals looked good. Nothing frayed. Solder joints still solid. Nothing to my untrained eye looked amiss.

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 Posted: 03-17-2020 11:11 pm
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cjwilson
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I do have 12V DC into the tach. Someone told me that I need to check the signal wire into the tach for AC voltage. It could have up to 90V AC on it. That will confirm if I have signal to drive the tach. And if I do have AC voltage present, the tach is the problem.

Can any of the much more experienced people here confirm this?

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