ElectronicsNeed help wiring something up? Thinking of adding an electrical component to your car? Need help troubleshooting that wiring glitch?
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THANK YOU SO MUCH to all those that put their time and effort not only into researching the issue, but in posting it for the rest to see. Your efforts are greatly appreciated!
Okay tried it and failed. tach was reading super high and pegging out(originally). I clipped the #4 pin and solder'd 2 100k 1/2w ohm in parallel. started car, tach just pegged out. took it apart and solder'd them in series. Tach still pegs out. What the hell did I do wrong?
Okay tried it and failed. tach was reading super high and pegging out(originally). I clipped the #4 pin and solder'd 2 100k 1/2w ohm in parallel. started car, tach just pegged out. took it apart and solder'd them in series. Tach still pegs out. What the hell did I do wrong?
The resistors should be in a series (1 after the other) Also, IIRC you need to clip pin 10 also, everything worked fine when i did it. just read closely.
Quote:
For anyone still viewing this helpful thread, i have successfully repaired my tach (89 Iroc) by clipping pins 4 and 10 off, and soldering two 100k Ohm resistors in a series to bring me to 197.1k Ohms. Soldered them onto the board and tested it against my autometer tach. 99% accurate, revved in 200 rpm increments to 4000 rpm and it's dead on.
Resistors used were Radioshack
100 Piece
Carbon-Film Resistor Assortment
1/2 watt - 5% tolerance
The resistors should be in a series (1 after the other) Also, IIRC you need to clip pin 10 also, everything worked fine when i did it. just read closely.
I tore it apart again and saw the #4 pin didn't solder properly. I was just too freaking excited to get it going.
My 1992 RS has this problem but it seems a little more harsh than others. My idle is at 3,000 RPMs... which I know is waaay off. Do you think this mod will help still or do you think
that I would need a lower/higher resistor value?
I just had the dash out on Saturday too. (Dang Engine Warning Light, well it's out now).. should have searched harder for this thread.
-AJ
My 1992 RS has this problem but it seems worse than any of your problems. I'm 350 carb swapped and my idle on the inside gauge reads 2,250- 3,000 RPM (depending on cold start or when engine temp is up). I know that this is off. Do you think I would need a differant resistor value?
The resistor value is a constant. What happens is that the resistor pack can fail by either going to a complete open state (infinite resistance) where your tach will just peg or it can partially fail and you just end up with a higher resistance. Mine was around 3k at idle as well and I found that these pins on the resistor pack were reading around 500k.
Regardless of how far off the resistor pack is, the fix remains the same. Just remember to check the resistance of the resistors with an Ohm meter as they do vary by +/- %5 or so (usually -). With 2 100k resistors, you should be pretty close to 193k.
I performed the fix on mine yesterday and had the tach working well with several shutdowns and re-starts over the course of a few hrs. I went back out to the car after it sat for awhile yesterday evening and it had quit altogether. I just pulled the cluster back out and don't see anything wrong or burnt. I don't get it. What now?
Short story, Make sure everything is plugged in well, remeasure the resistance on your fix (check for short), check for voltage and ground on the plug to the cluster.
Given the scenario, my best bet would be a short / bad connection in the cluster or a bad solder connection / resistor. Solder off the resistors, ensure the pins are not contacting anything they shouldn't (i.e. other pins, solder areas) and solder on a new one.
You may try soldering a wire with a connector in each pin location so that you can plug in a POT, set the resistance to match the gauge to the actual measured RPM (using a timing light with readout, etc), unplug the POT, measure its resistance and plug in the appropriate resistor.
Just did this on my 90 rs v6 and so far it seems to work. I used two 100k ohm resistors in series sodered to the back of the board and the pins clipped on the front. My reading before the fix was over 1M ohm resistance.
My tach showed the car idling at 3000rpm and when I was driving at 70mph, the tach was pinned.
I pulled the instrument panel out of my '90 vette and at first couldn't find the small circuit board that had the chips on it. A little closer look and I determined it was built directly into the main circuit board of the cluster! Strange. Anyway, I measured across 4 and 10 and didn't get a reading so I clipped pin 10 and soldered 2 100k resistors in series to pins 4 and 10. When I tested it out, she was sitting idling at 500rpm. I hooked up the computer and ran the diagnostic software and it showed 925 rpm. So, I pulled the cluster again, took off the resistors and measured them. 194k total. so I took another 100k resistor (all I bought) and threw it in the mix to get 298k and threw the cluster back in again. This time it sat idling at about 700 rpm and the computer showed 750 rpm. I brought it up to 3000 rpm and the computer showed 2800 rpm. Well, close enough to my liking!
Strange how the chips were part of the main circuit board, compared to pictures of others I found here. Also, that it took 298k instead of the 196k everyone else reports. Anyhow, I'm happy it's fixed!
WoooHooo!!! add one more fixed tach to list. My tach has been getting steadily worse since i bought it 7 years ago. Started about 500 off then it was about 4000 off. Idled over 4500 rpm, pegged if you rev it. Just never knew how to fix it and didnt want to change the cluster cause its got low miles. Thank you all for figuring out this fix. I'm so happy now.
im gonna go ahead and do this resistor replacement, even though my cars an 88 not a 90-92, but i measured the resistor on the tach board and got nothing back, i figured i should get something so it lead me to believe the resistor chip is bad, instead of getting resistors i was gonna go with the potentiometer, i pulled one from an old busted up guitar, took out my multimeter and turned the knob until i got about 192k ohms, is it ok to use this?
also just incase im not reading the multimeter right(im a multimeter newb, finally went and bought one not to long ago), i have a picture
*edit* now when i test it with the multimeter, it says .192m ohm, is that the same as 192k ohm?
i know the reading of the multimeter is probably obvious, but still.. gimme a break, lol
Last edited by wwwyzzerdd; 11-15-2009 at 11:39 PM.
well i went ahead and did it, and it worked!... sort of, said i was idling at 500, i think im gonna have to use another 100k ohm resistor along with my pot, or i might just get 3 100k ohm resistors and solder them in
anyway this worked, my tach did have a faulty resistor, i avoided this thread for so long because my car isnt a 90-92, shouldve just tried it out a long time ago
