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Tachometer affected by brakes/blinkers  Rate Topic 
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 Posted: 08-19-2005 01:47 pm
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StabnSteer
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I figure there must be a ground issue here somewhere...when I hit the brakes, the tach stops working (drops to zero). When I use the blinkers, it intermittently stops working with each blink, so it bounces madly...

BUT

This only happens when the car is cold. After running awhile, these symptoms slowly bleed away so that after about 20 minutes, the blinkers have no effect and the brakes have a tiny effect on the tach reading!!

I have completely redone all the wiring for the rear lights - soldered grounds and the like - so I suspect there is a connection issue with the tach...and as the car came to me completely disassembled in that area, it is certainly possible. Any ideas jump to mind out there?

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 Posted: 08-19-2005 03:26 pm
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Mark Rosenbaum
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Very peculiar.  Does the problem occur when only when the turn signals are used, or also when the hazard lights are flashing?  Is the big ground cable from engine to chassis still in place and making good contact?

Since the problem appears to be temperature related, it's logical to assume that something that changes as it warms up would be involved.  This could be the tach itself, the voltage stabilizer, a miswire connecting the tach-to-coil wiring with a light bulb, etc.

I would approach the problem assuming that there might well be several things wrong at the same time.  Probably the first thing I'd try is to undo, examine, and repair if necessary, all of the wiring plugs and sockets under the dash, playing very close attention to the insides, where there might be loose wire shards or other conductive debris.  Electrical contact cleaner, or brake cleaner, works well there.

If that didn't resolve the issue, I'd disable entire systems, e.g. the turn signals, and see if that told me anything.  This might lead to ONLY the tach/ignition circuit and fuel pump wiring remaining connected -- in which case one then reconnects systems, one at a time, until the problem reoccurs.  (Though bear in mind that occasionally when this process is used, the problem goes away and never reappears.  This isn't magic, just insufficient attention and observation on the mechanic's part.  Or so it says in the fine print.  :^}  )

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 Posted: 08-24-2005 11:58 pm
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Arvin Appelman
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Do the head lights and/or the blower fan effect the tach.?  Is your battery in good condition? 

Don't forget to check both ends the battery cables. 

Arvin Appelman

19492

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 Posted: 08-26-2005 01:20 pm
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StabnSteer
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Interestingly, no other electrical bits seem to effect the tach...so it seems to point toward the taillight assembly. The hazards also cause the fun display of tach dancing. Note that this started a few months after I rewired the taillights. So things were great for awhile.

The battery is fine. Main grounds appear fine and have been cleaned in the past...perhaps I'll give them another go. The switch for the blower was destroyed when I got the car, so I don't know if it would have an effect (plus the PO superglued it into place...grrrrr).

I'm going to put a new temp gauge into the car soon, so that will give me a chance to get behind the dash and see if there's anything obvious there.

Mark - you noted a voltage stabilizer...can you elaborate? I know there was the stabilizer for the fuel and temp gauge...where would one be for the tach (or is it built into the tach?).

I do have a spare tach as well...perhaps I'll try swapping them out...

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 Posted: 08-26-2005 03:59 pm
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Mark Rosenbaum
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Re the voltage stabilizer.  Sorry for the confusion, I didn't explain that at all well.  My thought was that the stabilizer used for the temp and fuel gauges was also providing power to the tach, or to the ignition current line that runs to the coil.  This could be due to a miswire by a PO, to some sneak current path involving a bad ground, or to some strange wiring fault where two unrelated circuits are somehow connected.

The blower switch is pretty independent of everything else.  It provides a ground for the blower motor either directly, or through the big printed-circuit resistor on the side of the heater box.

Your comment that things were working fine for a while suggests that a miswire is not the cause of the problem.  That being the case, I'd look first at the connectors for broken wires or pins, stray wire strands, etc., and if necessary check the wiring harness for signs of damage that might suggest damaged wires inside.

One other thought.  I failed earlier to reflect upon your comment that once things warm up, applying the brakes has only a small effect on the tach.  This could be related to the normal slight change in engine speed that occurs whenever it must pump air from (i.e. "apply vacuum to") the brake booster.  This can be proved or disproved by disconnecting and plugging the vacuum line to the booster, then applying the brakes.

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