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| Tachometer Does Not Read Correctly (Page 1/2) |
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eti engineer
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NOV 28, 02:37 PM
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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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Gall757
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NOV 28, 03:50 PM
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Send a PM to forum member J Gunsett
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pmbrunelle
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NOV 28, 06:57 PM
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Cliff Pennock
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DEC 02, 10:28 AM
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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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SEP 29, 04:35 PM
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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, ------------------
"The last 29 days of the month are the hardest" - Nikola Tesla
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buddycraigg
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SEP 29, 07:37 PM
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I'm pretty sure the tach gets the signal directly from the pickup coil.
Can someone post a schematic please?
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phonedawgz
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SEP 29, 10:39 PM
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| 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,
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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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SEP 29, 10:43 PM
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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... |
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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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SEP 30, 07:06 AM
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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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vertigo1
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SEP 30, 05:09 PM
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| 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.
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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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