A while back I posted about a problem I was having with my 91's tach... it's reading too high of an RPM. I discovered here that it's very common, and also discovered that nobody knows how to fix it other than to pay somebody to repair it.

So, I did more digging, and found a thread on thirdgen.org pertaining to the same problem with the tachs on later thirdgen f-bodies. In virtually all situations, their high-tach problem was isolated to a specific resistor that had gone bad. Somebody there chimed in that the later C4 tachs used the same resistor chip, so I'm guessing that this resistor chip will be the source of my tach's problems.

With camera in hand, I'll be tackling this issue this weekend, and I hope that by Monday I'll have a complete report of whether I was sucessful or not, and if I was, I'll document it in pictures for others who would prefer to fix it themself for $.99 instead of paying hundreds to somebody else.

Stay tuned...

 

Hey Jim, your problem is indeed quiute common. PopEvans just posted this thread with exactly what your talking about.

http://forums.corvetteforum.com/c4-g...ch-repair.html

A really good write up with pics. See if its the same for yours.

 

Other ways of taking care of this is to replace the tack from you're guages? If so I've got one from a 91 if you need one just pay for shipping.

 

Is it already adjusted? If so, I'll take it if he doesn't want it. TIA

 

no problem.

 

Thanks for posting that link. I am at Carlisle right now and have had no luck finding a 92-96 instrument cluster to replace/repair the crazy tach in my 91. I was going to search for that post and fix it myself. So far the only used one I have found is from someone here who wanted $200 fo one.
This just saved me some cash!

 

Yup, looks like PopEvans beat me to it, though I do have something to add that I will also post in his thread.

It may not be obvious from his pictures, but those two terminals that he suggested soldering to are electrical equivalents of the two pins on the resistor chip itself, which is actually located below those pins on the board. I went a step further and removed the entire printed board from the cluster in order to gain access to that chip on the other side of the board. The reason for this is that the resistance across those particular pins has changed over time, which is what caused the tach to become inaccurate in the first place. Simply soldering a new resistance in parallel will likely only be a temporary fix because that resistance will continue to drift. You need to cut two pins off of the chip soldered to the board, THEN solder the new resistor into place in order for this to remain reliable.

Once I cut the correct pins off that chip, I determined that 300k is the proper resistance to re-insert into the circuit. Since there's no parallel resistance thanks to cutting the pins, this should be a constant value that will work on everybody's car and won't change over time.

Pics:
This first pic shows the resistor chip on the printed board. It's the white & black chip below the bulb:


The two pins that I have my DMM probes on are the pins across the faulty resistor within:


Once I cut those pins off of the chip, I soldered the resistors right into the same holes:


Ultimately, because I was using the wrong tip on my soldering iron, I managed to burn the copper trace around one of the holes, and had to relocate one end of the resistor to one of the spots that PopEvans used. To be honest, his spots are better because there's a bigger solder area and there are no nearby pins to deal with.

 

What you guys don't realize is the tach is driven by a stepper motor which wears out over time and all you're doing is a temporary fix. By not replacing the motor you wind up with an inaccurate tach over it's full range-not a good tool if you race. When we repair a cluster we use a GM test machine to calibrate and test all functions. Cost is $225 or $250 with a new lens.

 

Do you have a fix for the blank display?

 

Who is "we"? I can't find your website.

 

From what people on thirdgen.org found, one specific resistor within that chip fails. It's documented and common, and replacing that resistor with a known value seems to provide accurate and consistent results. If it was the stepper motor, people would not get consistent results by all using the same resistor value... everybody's would be different. Dozens of guys measured that particular resistor and found numbers all over the place. When they replaced it with a 200k resistor, their tachs all became very accurate again. If this was an issue with the stepper motor, they wouldn't all have found the same value of resistor to solve their problems. Likewise, those with good functioning tachs have measured this resistor value to be at 200k, whereas the measured values on malfunctioning tachs were always at a different value.

I'm not saying that the stepper motors don't wear, I'm sure they do, but that doesn't seem to be the cause of the majority of the problems... it seems that these resistors are.

 

Yes, GMAUTOCOMPUTERS.com

 

GMAUTOCOMPUTERS.com

 

The F body is a different cluster. Just about every 90-91 Corvette cluster,especially the ZR1's, needs repairs. Delphi, who built the clusters, has a definite set of service repairs to be done to each cluster to bring it up to standards. If you're wanting a quick cheap fix, then your method is a good one. Can't blame owners wanting to save money-but-if you got a keeper, then we're offering an alternative and a factory, warranteed, fix.

 

What does it cost to go thru the entire set of gauges and make sure everything is at spec? Also, what is the turnaround time?

 

 

 

Yes, the F body is a different cluster, but both clusters, as well as a ton of other GM clusters, all use a similar circuit configuration and utilize the same type of DIP resistor chip, which is prone to failure. I'm not trying to say that there aren't plenty of other failure-prone components on this circuit... I know Delphi electronics well enough to know that they have a LOT of problems, however, if the tach issue can be traced to a failed resistor chip, then my fix is perfectly adequate for those who want to try the DIY approach. It's not just for those of us who want a "quick cheap fix".

Unless I can measure the resistance of this chip on a known-good cluster (or have somebody else who is interested in helping the DIY community do it for me with their known-good cluster ), then I can never know whether my particular issue was due to a bad resistor, or I simply did a "band-aid" fix by changing that resistor value.

However, on the f-body cars, which use a very similar configuration, it's a documented, known issue that the particular resistor in question fails, and when people replace it with the original resistor value, their tachs function correctly again. That is a clear indication, that in the vast majority of those cases, that the resistor, and not other components, is what is at fault.

For those who want the entire cluster refurbished, or who simply don't have the desire to diagnose this themselves, a service such as yours is invaluable. But, plenty of us like to do things ourselves, and if so, there's no harm in tossing a DMM across a couple spots on a circuit board to see if the resistor may be the failed component.

 

$225, 5 days or less

 

Is $225.00 to repair it or just diagnostic??