Tach is off??
Thread Starter
Registered User
Joined: Mar 2004
Posts: 567
From: Mission, TX
Tach is off??
My tach indicates that at idle im at 1500 rpms....and it doesnt sound like its more than 1000.....plus, when I floor it, the rpms dont stop at redline...it keeps on and it shifts at **8000 rpms***! The tach only goes to 7K, but it keeps on to where 8K would be if it was there....it sounds like its really at around 5000 when it shifts.
Its got to be wrong...any suggestions?
Its got to be wrong...any suggestions?
Registered User
Joined: May 2004
Posts: 139
From: Greenville, SC
Re: Tach is off??
The best thing to do is to first verify that it is truly the tach that is off. You can use a scan tool, laptop with WinALDL, aftermarket tach, etc. But, I bet that the tach really is off because it is a very common problem on 3rd gens. No one really know the exact answer to the problem though. My advice is to try and get another gauge cluster or factory tach and see if that fixes it.
Thread Starter
Registered User
Joined: Mar 2004
Posts: 567
From: Mission, TX
Re: Tach is off??
Originally Posted by Ben91RS
The best thing to do is to first verify that it is truly the tach that is off. You can use a scan tool, laptop with WinALDL, aftermarket tach, etc. But, I bet that the tach really is off because it is a very common problem on 3rd gens. No one really know the exact answer to the problem though. My advice is to try and get another gauge cluster or factory tach and see if that fixes it.
Where can I get the software for free?
How much for the cord that connects the laptop to the car?
Is there anything else I need besides the cord and the software? (I have a laptop..)
Thanks
-Chris
Registered User
Joined: Dec 1999
Posts: 2,022
From: MD
Re: Tach is off??
Which vehicle? The 91 or the 84? I guy I went to college with a guy that had a similar problem with his 87Z28, and asked me for help after seeing my car. Turns out a previous owner swapped in a new instrument cluster, from a V6 car. The V6 and V8 tachs are calibrated differently. The V6 tach wants to see 3 pulses per engine revolution, while the V8 tach wants to see 4 per revolution. The tach will see one extra pulse per engine revolution, and thus read 1/3 higher than it should.
Last edited by Marc 85Z28; Sep 13, 2004 at 05:44 PM.
Thread Starter
Registered User
Joined: Mar 2004
Posts: 567
From: Mission, TX
Re: Tach is off??
Originally Posted by Marc 85Z28
Which vehicle? The 91 or the 84? I guy I went to college with a guy that had a similar problem with his 87Z28, and asked me for help after seeing my car. Turns out a previous owner swapped in a new instrument cluster, from a V6 car. The V6 and V8 tachs are calibrated differently. The V6 tach wants to see 3 pulses per engine revolution, while the V8 tach wants to see 4 per revolution. The tach will see one extra pulse per engine revolution, and thus read 1/3 higher than it should.
Well its supposed to shift at 5300-5500 so take 5500 and multiply that by 1.333 and you get 7333 rpms....so thats about right...it my be the problem.
The guy I bought it from said its all stock, but Im finding little knicknack stuff thats I suspect isnt stock....
What can I use to check the pulses?
Registered User
Joined: May 2004
Posts: 139
From: Greenville, SC
Re: Tach is off??
I'm not sure if you can check the pulses to the tach, but I'm not sure. Also, most of the tachs that people seem to have problems with read about 1500 to 2000 RPM too high (like mine), which is about how much yours seems to be off by. People know what causes the tach to be off, it's a capacitor on the back of them, but the problem is that no one knows the correct rating of capacitor to replace the bad stock one with. People have gone into lenghty discussions on Thirdgen.org and there is still no definite answer. I think I am going to experiment with this soon.
About getting the car scanned or hooking it up to a laptop, most reputable shops have scan tools and can check the RPMs that the computer sees and compare it with the tach, but they'll charge about $60. You can get cords that connect your laptop to the car for about $30 on Ebay (should be called WinALDL or just ALDL cables) and the software is free but I can't remember the website offhand. Thirdgen.org has the info in the stickies on the DIY-PROM board.
About getting the car scanned or hooking it up to a laptop, most reputable shops have scan tools and can check the RPMs that the computer sees and compare it with the tach, but they'll charge about $60. You can get cords that connect your laptop to the car for about $30 on Ebay (should be called WinALDL or just ALDL cables) and the software is free but I can't remember the website offhand. Thirdgen.org has the info in the stickies on the DIY-PROM board.
Thread Starter
Registered User
Joined: Mar 2004
Posts: 567
From: Mission, TX
Re: Tach is off??
Originally Posted by Ben91RS
I'm not sure if you can check the pulses to the tach, but I'm not sure. Also, most of the tachs that people seem to have problems with read about 1500 to 2000 RPM too high (like mine), which is about how much yours seems to be off by. People know what causes the tach to be off, it's a capacitor on the back of them, but the problem is that no one knows the correct rating of capacitor to replace the bad stock one with. People have gone into lenghty discussions on Thirdgen.org and there is still no definite answer. I think I am going to experiment with this soon.
About getting the car scanned or hooking it up to a laptop, most reputable shops have scan tools and can check the RPMs that the computer sees and compare it with the tach, but they'll charge about $60. You can get cords that connect your laptop to the car for about $30 on Ebay (should be called WinALDL or just ALDL cables) and the software is free but I can't remember the website offhand. Thirdgen.org has the info in the stickies on the DIY-PROM board.
About getting the car scanned or hooking it up to a laptop, most reputable shops have scan tools and can check the RPMs that the computer sees and compare it with the tach, but they'll charge about $60. You can get cords that connect your laptop to the car for about $30 on Ebay (should be called WinALDL or just ALDL cables) and the software is free but I can't remember the website offhand. Thirdgen.org has the info in the stickies on the DIY-PROM board.
Is there anything that will allow me to tune it myself?
Registered User
Joined: May 2004
Posts: 139
From: Greenville, SC
Re: Tach is off??
It won't allow you to tune it, but it's a step towards it. You will also need an actual PROM burner and spare PROMs, if I remember correctly. The DIY_PROM board on Thirdgen.org has all the info you need to get those and make your own PROMs. I'm no expert on PROM burning, though, I need a laptop first. 
Registered User
Joined: Mar 2003
Posts: 691
From: Miami, FL
Re: Tach is off??
the tach problem pretty much happens to everyone eventually. my tach was wrong too. idled at 1500... shifted at 8000+... so instead of having to go throught he hassle of taking out my inst. pannel and messing with everything, i bought a new tach, mounted it, and wired the tach wire to the msg box. works perfect 
when the tach reaches 5,200, the new one reads only about 3. it's off by quite a lot, lol
when the tach reaches 5,200, the new one reads only about 3. it's off by quite a lot, lol
Banned
Joined: Mar 2001
Posts: 3,249
Re: Tach is off??
Until it was destroyed this last July, my tach has never been off by more than a few as verified by a snap-on timing light which has a tach in it and by several other Auto-Meter tachs over the years. Try the timing light approach first. Next borrow a scantool to see what the ECM sees.
Registered User
Joined: Nov 2000
Posts: 3,978
From: MI
Re: Tach is off??
speaking of the tach...anyone have problems with the mph cluster? mine has stopped working lately. just lays flat at zero until i drive for about 10 miles then it kicks on and shows how fast i'm going. thinking about just redoing the whole cluster with something different.