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Tachometer Does Not Read Correctly by eti engineer
Started on: 11-28-2019 02:37 PM
Replies: 17 (345 views)
Last post by: Patrick on 10-07-2021 07:07 PM
eti engineer
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Report this Post11-28-2019 02:37 PM Click Here to See the Profile for eti engineerClick Here to Email eti engineerSend a Private Message to eti engineerEdit/Delete MessageReply w/QuoteDirect Link to This Post
I haven't been in here for a while, but I have a question that I am sure I can be helped with by you all.

I have a 1988 Fiero GT with T-tops. The car is in excellent condition, so there is not really anything that needs my attention. I have one small problem that bothers me, however, and this is that the tach reads about 3 times higher than it should for any given engine rpm. I have replaced the tach filter and it did not cure the problem. If anyone else has had this issue and solved it, I sure would like to know what it takes to make it work correctly. Thanks...
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Report this Post12-02-2019 10:28 AM Click Here to See the Profile for Cliff PennockClick Here to visit Cliff Pennock's HomePageClick Here to Email Cliff PennockSend a Private Message to Cliff PennockEdit/Delete MessageReply w/QuoteDirect Link to This Post
Is it really three times higher? It could be that you actually have a dash for a 4-cylinder installed. But then it should read 1.5x too high.
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vertigo1
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Report this Post09-29-2021 04:35 PM Click Here to See the Profile for vertigo1Click Here to Email vertigo1Send a Private Message to vertigo1Edit/Delete MessageReply w/QuoteDirect Link to This Post
Revising this one for hopefully some input;
The above links do not help much as I've already replaced the Tach board itself twice (supposedly both pre-calibrated for the V-6 Fiero).
At idle, it has some semblance of accuracy but when rev'd or under load, it can jump sporadically from 2-5k when I know I'm almost certainly somewhere in between.
How does the tach get the signal from the engine and can that sender be replaced (as I'm sure this is where the issue is)?
Thanks all,

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Report this Post09-29-2021 07:37 PM Click Here to See the Profile for buddycraiggClick Here to Email buddycraiggSend a Private Message to buddycraiggEdit/Delete MessageReply w/QuoteDirect Link to This Post
I'm pretty sure the tach gets the signal directly from the pickup coil.
Can someone post a schematic please?
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Report this Post09-29-2021 10:39 PM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by vertigo1:

Revising this one for hopefully some input;
The above links do not help much as I've already replaced the Tach board itself twice (supposedly both pre-calibrated for the V-6 Fiero).
At idle, it has some semblance of accuracy but when rev'd or under load, it can jump sporadically from 2-5k when I know I'm almost certainly somewhere in between.
How does the tach get the signal from the engine and can that sender be replaced (as I'm sure this is where the issue is)?
Thanks all,



You have a bad ignition coil. The spark is jumping from the secondary to the primary. It does this when there is more air in the cylinder, ie under higher load. It will eventually take out your ICM.

The only thing the tach signal runs through is the tach filter.


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phonedawgz
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Report this Post09-29-2021 10:43 PM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post

phonedawgz

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quote
Originally posted by eti engineer:

I haven't been in here for a while, but I have a question that I am sure I can be helped with by you all.

I have a 1988 Fiero GT with T-tops. The car is in excellent condition, so there is not really anything that needs my attention. I have one small problem that bothers me, however, and this is that the tach reads about 3 times higher than it should for any given engine rpm. I have replaced the tach filter and it did not cure the problem. If anyone else has had this issue and solved it, I sure would like to know what it takes to make it work correctly. Thanks...


I make replacement tach boards. $49 for an adjustable board. $79 includes a precision oscillator to allow you to properly adjust the tach so it's right on. $89 and I include the circuitry to also give you a low fuel light. Works with all three versions of stock Fiero tachs and with 4, 6, or 8 cylinder engines. Prices include US shipping.

https://reddevilriver.com/Tachometers.html

You could also add a pot to your existing tach board. Instructions are here. $9.00 and I can ship you the pot needed.

http://www.fieros.de/en/articles/tach.html

[This message has been edited by phonedawgz (edited 09-29-2021).]

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reinhart
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Report this Post09-30-2021 07:06 AM Click Here to See the Profile for reinhartClick Here to Email reinhartSend a Private Message to reinhartEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.
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Report this Post09-30-2021 05:09 PM Click Here to See the Profile for vertigo1Click Here to Email vertigo1Send a Private Message to vertigo1Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by phonedawgz:


You have a bad ignition coil. The spark is jumping from the secondary to the primary. It does this when there is more air in the cylinder, ie under higher load. It will eventually take out your ICM.

The only thing the tach signal runs through is the tach filter.



Interesting if so because the car drives great otherwise.
So you're saying $80 could fix my problem? Sign me up! Lol
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Report this Post09-30-2021 06:47 PM Click Here to See the Profile for vertigo1Click Here to Email vertigo1Send a Private Message to vertigo1Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
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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[This message has been edited by vertigo1 (edited 09-30-2021).]

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Report this Post10-02-2021 06:11 PM Click Here to See the Profile for creaky78Click Here to Email creaky78Send a Private Message to creaky78Edit/Delete MessageReply w/QuoteDirect Link to This Post
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.
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Patrick
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Report this Post10-02-2021 06:21 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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.
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Report this Post10-05-2021 06:20 PM Click Here to See the Profile for vertigo1Click Here to Email vertigo1Send a Private Message to vertigo1Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
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.

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Patrick
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Report this Post10-05-2021 06:44 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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).]

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vertigo1
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Report this Post10-06-2021 04:20 PM Click Here to See the Profile for vertigo1Click Here to Email vertigo1Send a Private Message to vertigo1Edit/Delete MessageReply w/QuoteDirect Link to This Post
So given that photo of mine, it is possible the issue with my Tach is related to wonky wiring of the ICM?
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Report this Post10-07-2021 06:21 PM Click Here to See the Profile for reinhartClick Here to Email reinhartSend a Private Message to reinhartEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.
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Patrick
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Report this Post10-07-2021 07:07 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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.
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