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Inaccurate tach & speedo


greenmachine
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Just got my son's 90 TGP running. The tach & speedo both read higher than actual. HUD speedo is accurate. Oil, temp & boost seem accurate. Fuel may be off - too early to tell. Check guages is the only light on. Is this a common problem? Any ideas?

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Just got my son's 90 TGP running. The tach & speedo both read higher than actual. HUD speedo is accurate. Oil, temp & boost seem accurate. Fuel may be off - too early to tell. Check guages is the only light on. Is this a common problem? Any ideas?

pretty common.
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Yes, unfortunately, it is very common. I have about 5 TGP clusters lying around, and the most accurate is between 3-5 MPH over the actual speedo. The one currently in my TGP reads 90 when I'm doing 50 or so. :lol:

 

As many times as it has been asked, I have yet to run into a solution for the speedo/tach issue. BUT....most people have cured the "check gauges" or "low fuel" lights syndrom with this procedure...

 

http://www.w-body.com/service/checkgauges.html

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It's not the vss is the HUD is correct. I had a fix that someone told me about, it involved soldering a resistor across 2 pins of the speedo. I don't remember what ohms, I'll try and find the fix.

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It's not the vss is the HUD is correct. I had a fix that someone told me about, it involved soldering a resistor across 2 pins of the speedo. I don't remember what ohms, I'll try and find the fix.

 

This is what you're looking for.

 

I jumpered a 1 or 10megohm resistor across the calibration resistor on my cluster and then it was pretty close to dead-on accurate.

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On the quote from gnatgosplat - was it a 1 or 10 megohm resistor. Did it fix both the speedo and tach? Where is the calibration resistor? Thanks for the info!

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On the quote from gnatgosplat - was it a 1 or 10 megohm resistor. Did it fix both the speedo and tach? Where is the calibration resistor? Thanks for the info!

I heard theres a place you send the cluster to and they fix everything and charge you $300. I just use the HUD though because to fix it right you would need new speedo and tach cables and if you need those, you probably need a new TV cable first. what I am saying is I think it may be a heat soak issue with the cables stretching out and throwing it off. Its hard to say though. my new car is almost dead on and the cables werent ever twisted up behind the motor. but part of it may be who assembled your car back in 1989-1990 in detroit. some of the cars might have had the cables routed funny or twisted up sometime during tune-ups. my sunroofed TGP is about 15 mph off at all times and the tach is like 1000 rpms over or more. and the cables are way more twisted up behind the motor. they also are brittle from heat soak so I dont dare un-twist them.
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On the quote from gnatgosplat - was it a 1 or 10 megohm resistor. Did it fix both the speedo and tach? Where is the calibration resistor? Thanks for the info!

 

Shawn(Gnat) said that he could not remember which one of those he spliced in, that's why he stated it that way. I'm pretty sure using resistors on that calibration resistor will only fix the speedometer issue. I'll PM him again and ask where that calibration resistor is located, he told me a few months back, but I've forgotten where it is now. :redface:

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On the quote from gnatgosplat - was it a 1 or 10 megohm resistor. Did it fix both the speedo and tach? Where is the calibration resistor? Thanks for the info!

to fix it right you would need new speedo and tach cables and if you need those, you probably need a new TV cable first. what I am saying is I think it may be a heat soak issue with the cables stretching out and throwing it off. Its hard to say though.

 

With all due respect, what are you talking about? You make it sound like there are literally cables for the speedo & tach. The signal that drives the speedo comes from the vss. The signal that drives the tach comes from the Ign.

module I think. There is a two wire plug on the cluster that goes to the HUD; it just feeds the same signal the dash

gets (vss) to the HUD. There is no way for the HUD to be off because it just counts the number of pulses per second.

The problem is the dash & that's the same for almost everyone in varying degrees. The speed & tach use servo motors and there a special circuit on the pc board that drives them. They are two separate, but nearly identical circuits and both developed the same problem in time. I emailed Jeff M some time ago with pictures of a jig I made to calibrate the two dashes I own & both are dead on now but I never got a response. It took allot of work to figure it all out & had to built a circuit to generate the pulses to mimic the pulses in the car so I can tell you to try to calibrate one in the car would just be hit or miss. Electronics repair is what I do for a living so I had the resources to do it all.

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On the quote from gnatgosplat - was it a 1 or 10 megohm resistor. Did it fix both the speedo and tach? Where is the calibration resistor? Thanks for the info!

to fix it right you would need new speedo and tach cables and if you need those, you probably need a new TV cable first. what I am saying is I think it may be a heat soak issue with the cables stretching out and throwing it off. Its hard to say though.

 

With all due respect, what are you talking about? You make it sound like there are literally cables for the speedo & tach. The signal that drives the speedo comes from the vss. The signal that drives the tach comes from the Ign.

module I think. There is a two wire plug on the cluster that goes to the HUD; it just feeds the same signal the dash

gets (vss) to the HUD. There is no way for the HUD to be off because it just counts the number of pulses per second.

The problem is the dash & that's the same for almost everyone in varying degrees. The speed & tach use servo motors and there a special circuit on the pc board that drives them. They are two separate, but nearly identical circuits and both developed the same problem in time. I emailed Jeff M some time ago with pictures of a jig I made to calibrate the two dashes I own & both are dead on now but I never got a response. It took allot of work to figure it all out & had to built a circuit to generate the pulses to mimic the pulses in the car so I can tell you to try to calibrate one in the car would just be hit or miss. Electronics repair is what I do for a living so I had the resources to do it all.

 

So what would it cost to get the tach and speedo repaired????

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On the quote from gnatgosplat - was it a 1 or 10 megohm resistor. Did it fix both the speedo and tach? Where is the calibration resistor? Thanks for the info!

to fix it right you would need new speedo and tach cables and if you need those, you probably need a new TV cable first. what I am saying is I think it may be a heat soak issue with the cables stretching out and throwing it off. Its hard to say though.

 

With all due respect, what are you talking about? You make it sound like there are literally cables for the speedo & tach. The signal that drives the speedo comes from the vss. The signal that drives the tach comes from the Ign.

module I think.

 

I dont know :confused: I just solved my problem another way so I dont really care

 

:leaving:

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With all due respect, what are you talking about? You make it sound like there are literally cables for the speedo & tach. The signal that drives the speedo comes from the vss. The signal that drives the tach comes from the Ign.

module I think. There is a two wire plug on the cluster that goes to the HUD; it just feeds the same signal the dash

gets (vss) to the HUD. There is no way for the HUD to be off because it just counts the number of pulses per second.

The problem is the dash & that's the same for almost everyone in varying degrees. The speed & tach use servo motors and there a special circuit on the pc board that drives them. They are two separate, but nearly identical circuits and both developed the same problem in time. I emailed Jeff M some time ago with pictures of a jig I made to calibrate the two dashes I own & both are dead on now but I never got a response. It took allot of work to figure it all out & had to built a circuit to generate the pulses to mimic the pulses in the car so I can tell you to try to calibrate one in the car would just be hit or miss. Electronics repair is what I do for a living so I had the resources to do it all.

 

:werd:

This guy knows what he's talking about.

I built a similar pulse generating circuit using a 555 timer IC to generate the pulses as well.

 

Yeah, I don't remember if it was a 1 or 10-megohm resistor. Try both, they are cheap. It's likely you might need a different value altogether, each cluster would be calibrated differently. It just so happened the resistor I tried on my cluster made it dead-on accurate. Beginner's luck?

 

Basically, putting that resistor in parallel with the existing laser-etched resistor will reduce the resistance across those 2 connections. The laser etched resistors you're looking for are white dummy IC's (chips) with 14 or 16 pins and black paint on top (the paint is resistive and is the calibration resistor).

 

Yes, the tach is an identical circuit and you can calibrate it in a similar way.

 

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  • 5 years later...

Hi...i see that this is an old thread but i take my chances.

I drive this 93 Grand Prix and the speedo is 10 km/h off..it shows more than actual speed.

I have a spare cluster i can work on, i found the resistors..but...on which pins should i solder the megaohm resistor(s)?

 

thanks,

Cosmin

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on the resistor "blocks", there should be two areas that have the paint on them, for both the speedo and tach, they use the largest of the two areas. you should see the only two pins that the large areas connect to, solder a resistor or potentiometer across those two terminals. 1 Mohm is probably too much for just being 10KM/H off, but if done correctly, you'll see the effects and you'll just have to keep swapping resistors until you get it correct or use a potentiometer.

 

i bypass the resistor block completely and use a 500K 25 turn potentiometer on my bench i have setup.

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  • 3 months later...
  • 2 months later...

I know this is an old thread, but first I have seen of it.

 

just to give you an idea of how common this problem is...

 

I give you.. The dueling needles!

 

th_TGP1.jpg

 

 

th_TGP2.jpg

 

Also notice that it says that I am turning about 10000 RPMS! WOOT!

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yeah, the 90GP was bad enough to where we cruised at 55MPH at ~5000 RPM... it was pretty bad. speedo was tripled as well.

 

i first repaired that cluster ~3 years ago now? still kicking along just fine.

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for the love of god let this thread die! I would be surprised if the green machine hasn't been crushed. wasn't that car the green tgp with the racing stripes?

 

mods, go ahead and lock er up! :thumbsup:

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