85 monte ss tach reading high intermittently

[DmcL]

The stock tach in the car reads normal when driving normally but sometimes (maybe all the time) when you give it full throttle it suddenly reads high. Like say 7k when it should be 4 or 5. I don't have the car yet but it's an issue I saw on the test drive and that was pointed out to me.

Is this a common issue? What's likely to be the cause?

 

[pontiacgp]

I had a tach that had a similar problem and gave it to Bonnewagon as I was putting a VDO tach in my GP. Bonnewagon found the problem was the jumper was lose or maybe it was the connectors. You might want to ask him about it.

 

[DmcL]

Ok probably something on the dash cluster then by the sound of it. Since were kind of on the subject can I just swap in a higher reading speedometer and get rid of the stock 85mph one?

 

[fleming442]

Verify voltage at the rpm where it jumps. I had this problem on a 79 t/a, and it turned out to be a bad alternator. The needle on mine was also erratic at times. ..

 

[Pit]

Check this out:

http://www.montecarloss.com/community/u ... ber=646206

Easy fix, work fine...

 

[DmcL]

Ok after getting the car on Monday and driving it a bit the tach is high all the time so it looks like the resistor trick will probably fix it.

I have some other gremlins to fix but I will make a separate thread.