Tach Reading Double
Thread Starter
Instructor
Joined: Feb 2005
Posts: 106
Likes: 0
From: Longview Texas
Tach Reading Double
I have 1991 Corvette with L-98 and the Tach is reading double of what it should be reading. Has anyone had this problem before and what does it take to fix it? 
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
Pro
Joined: Mar 2007
Posts: 526
Likes: 1
From: San Angelo Texas
There is someone out there that recals them. I'm sure I posted it somewhere in here. I think it was about $185 for him to do it too.
http://www.corvetteactioncenter.com/...d.php?t=101543
http://www.instrumentclusters.com/
Shoot, just google "corvette tachometer repair" and look at what comes up.
Last edited by mechguy79; Feb 21, 2008 at 09:12 AM. Reason: added stuff
6th Gear
Joined: Jan 2006
Posts: 6
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Tach Problem
Intermediate
Joined: Nov 2006
Posts: 49
Likes: 0
From: UCKFIELD EAST SUSSEX
here is John's picture that he kindly set me and his fix underneath. Thanks John
Hi, this is a 90 circuit board, see Capacitors number C3 and C4, between them is the IC CS9252 black part, and the white part with the black etching on it, this is the problem part. GM used a laser beam to calibrate it. If you look close you'll see a very thin 1/4 long line. this is the calibration. What happening is that black carbon is deteriorating and the more it does the more it reads higher .To fix the problem, you need to get some 1/4 watt resistors, 100,000 Ohms, or 100 K, 250,000,or 250 K and 500,000 or 500 K ohms.,. ( Radio Shack ) What you need to do is piggy back the resistor or maybe a combination of resistors onto Capacitor C-4, the gold color one . I would start with a 250,000 ohm or 500,000 ohm resistor,.this will bring it down about 1 , to 1 1/2 inches .This is trial and error method I have a tested it easy. So let say you went too low now, now you add 100,000 more, this will bring it back up a little. Get the picture the less you add the more it goes down lower, the more the higher up it goes. If you play with it you'll see. You really need some equipment to get it perfect, but you should get pretty close this way. Let me know how you make out. John Gardner
Hi, this is a 90 circuit board, see Capacitors number C3 and C4, between them is the IC CS9252 black part, and the white part with the black etching on it, this is the problem part. GM used a laser beam to calibrate it. If you look close you'll see a very thin 1/4 long line. this is the calibration. What happening is that black carbon is deteriorating and the more it does the more it reads higher .To fix the problem, you need to get some 1/4 watt resistors, 100,000 Ohms, or 100 K, 250,000,or 250 K and 500,000 or 500 K ohms.,. ( Radio Shack ) What you need to do is piggy back the resistor or maybe a combination of resistors onto Capacitor C-4, the gold color one . I would start with a 250,000 ohm or 500,000 ohm resistor,.this will bring it down about 1 , to 1 1/2 inches .This is trial and error method I have a tested it easy. So let say you went too low now, now you add 100,000 more, this will bring it back up a little. Get the picture the less you add the more it goes down lower, the more the higher up it goes. If you play with it you'll see. You really need some equipment to get it perfect, but you should get pretty close this way. Let me know how you make out. John Gardner
Advanced
Joined: Mar 2009
Posts: 54
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From: New Orleans LA
I've seen postings like this on some camaro board with lots of others trying it and it succeeding. I don't see a lot of vette owners trying and reporting on this. So, if I understand this fix, there is no clipping of the old gold resistor? Simply straddle a 100-500K 1/4 watt resistor contacting the same points that the old gold resistor contacts. No need to remove the gold resistor?
thanks
thanks
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Intermediate
Joined: Nov 2006
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From: UCKFIELD EAST SUSSEX
I did try this fix myself as my tach was reading low. After reading up all the information and buying the resistors I spent over five hours trying to fix the fault without sucess. The main problem is that you have to do this as a trial and error. One resistor may not work and you have to remove and dismantle the dash several times only to find it still doesn't work. In my opinion either get a secondhand cluster, the 1991 is a bit rare, or just get a new exchange cluster that way you only have to do the removal once. I am sure some people have managed to do it themselves but I think it is best to let someone else fix it
Melting Slicks
Joined: May 2010
Posts: 2,082
Likes: 3
From: Conowingo Maryland
I found this when a lady droped off her truck. She said at times it does not feel like it has enough power. Have to remember this was a V6 engine and max RPS were around 5000 before the rev limitor woud start shutting it down. Test drove and noticed that I had to let off the gas for it to shift. Besides that the truck ran well. I took it back to the shop hooked up scan tool I wanted to see where the shift points were and see if anything else stood out. what I noticed was the more I accellerated at times the RPMs seem way off. I was getting like 6000 under med throdle and it was kicking on the Rev limitor there for low power or wrong shift points at times. I went right to where the ECM picks up RPM single and that is the cam sensor replace problem fixed. I forgot to say this truck did not have a tach in it. Trying to shorten up a long story but I would hook up a scan tool a see what RPMs the ECM is seeing. If it is reading the same as your tach then you need to find out where your ECM is receving it reading from and that most likely will be the cam sensor
