Tachometer Does Not Read Correctly (Page 2/2)
vertigo1 SEP 30, 06:47 PM

quote
Originally posted by reinhart:

Most likely a bad tach gauge. GM 80's gauges are notorious for this. I had a similar issue where the stock tach was fine and then slowly got off until it was about 40 percent too high. I replaced it with a junkyard tach and now it's off by just 10 percent or so which is close enough for me.

One interesting thing I learned is that there are 2 different 86-88 V6 tach designs. My 88 tach would go to 0 immediately when I turned the engine off. The 86 replacement slowly drops to 0 when I shut the engine off (takes about 2 seconds). Everything else is identical.



Now when you say "tach gauge", are you talking about the physical gauge itself or the board that Mr. phonedawgz sells because I've already replaced two of those boards to no avail.
The Ignition coil looks brand new (i just checked) so it looks like it was replaced right before I got it last year. However looks can be deceiving and it IS an aftermarket "BlueStreak" model so who knows.

If it ends up NOT being the ignition coil, where do you suggest I look after that? I'll take a photo to show but the ignition system has definitely been messed with. I believe the ignition chip thing (ICM - the unit that fries all the time) was moved from below the distributor to a separate location away from the engine heat. Maybe he miswired something?

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"The last 29 days of the month are the hardest" - Nikola Tesla

[This message has been edited by vertigo1 (edited 09-30-2021).]

creaky78 OCT 02, 06:11 PM
When I got my 87 GT up and running, the tach nnedle would fluctuate and bounce around. It was simply a poor connection to the tach filter.
Patrick OCT 02, 06:21 PM

quote
Originally posted by vertigo1:

I believe the ignition chip thing (ICM - the unit that fries all the time) was moved from below the distributor to a separate location away from the engine heat. Maybe he miswired something?



The Ignition Control Module itself gets a whole lot hotter than "the engine heat". It would be interesting to see how and where your ICM has been re-located.
vertigo1 OCT 05, 06:20 PM

quote
Originally posted by Patrick:

The Ignition Control Module itself gets a whole lot hotter than "the engine heat". It would be interesting to see how and where your ICM has been re-located.



Oh it's a feat of engineering he did here, lol.
Indeed, I believe that's why he added an aluminum heatsink from a Mac Pro (he told me). Very creative.
Patrick OCT 05, 06:44 PM

quote
Originally posted by vertigo1:

Oh it's a feat of engineering he did here, lol.



Thanks very much for posting those images!

Years ago I had a thread Here where ICM cooling was discussed, and larryfiero posted that he had relocated his ICM. (I thought maybe you might've acquired that particular Fiero.) Larry's thread is Here, and it contains many images with great info regarding ICM relocation. (I might add that if Larry's Photobucket hosted images ever "disappear", that I've saved them all and can supply them to anybody who might wish to see them.)

[This message has been edited by Patrick (edited 10-05-2021).]

vertigo1 OCT 06, 04:20 PM
So given that photo of mine, it is possible the issue with my Tach is related to wonky wiring of the ICM?
reinhart OCT 07, 06:21 PM
I doubt this particular relocation spot makes any difference in heat of the ICM. If it's relocated by the airbox or definitely within the trunk I can see it making a difference. Also adding a heatsink under the distributor as I've seen a few people do has likely no effect since the air around the distributor is about the same temperature as the distributor.
Patrick OCT 07, 07:07 PM

quote
Originally posted by vertigo1:

So given that photo of mine, it is possible the issue with my Tach is related to wonky wiring of the ICM?



I can't answer that question directly, but I was hoping that maybe something larryfiero might've posted in his thread could help you with a wiring issue, be that to do with wire shielding and/or a proper ICM ground.

The ICM gets much hotter than the engine bay itself. Anything that's done to help draw heat away from the ICM is a bonus. Whether the ICM is relocated (and attached to a sufficient heat sink), or is left in the stock location with added cooling, it's definitely a worthwhile venture. My ICM is at least a dozen years old (as it was a used one I had kicking around when I installed it in my distributor back in 2008) and it's still working just fine.