Tach Reads High

[farley463]

Got an odd problem I'm trying to sort out. When idling warm the tach will read about 1100rpms, which had me chasing my tail trying to figure out this "high idle" problem I thought I was having. I just got a scanner from eBay today, and when I was looking at live data my actual idle speed was at about 850ish. When I turn the key on with the engine off, it'll usually sit right at 0 rpm like it should, but some times the needle will point to around 200, but if I turn the key back off and let the needle fall below 0, then turn the key back on, it will read normal again. Has anyone had this problem before and is there anything I can do to fix this?

[caffeine]

Older tachs sometimes start reading high from extra resistance somewhere in the gauge IIRC. I'm not sure if that's what's happening in your case, but when that happens the tach will read high by a certain % all through the RPM range. I have this issue in my Fiero where the gauge reads about 30% too high, so by the time I reach redline the tach is completely pegged. The fix for this is to either take apart the tach and fix it (I think there's a writeup somewhere for 3rd gen F-bodies), or replace it with a working one.

[robertisaar]

the 1.5 W-body clusters weren't the most accurately built clusters GM produced...

since it's relevant, the 89-93 grand prix UB3 clusters all tend to lose calibration for both speedo and tach(my wife's grand prix read something like 3X actual speed on both), i've been repairing them for a couple of years now, the laser cut calibration resistor seems to be the culprit every time. replacing it with a 500K 15 turn pot has worked for me every time.

[pocket-rocket]

the 1.5 W-body clusters weren't the most accurately built clusters GM produced...

I don't really trust any gauge 100%, I look at them as a "close enough" kind of deal and keep them all out of the red.

[robertisaar]

you would think at least the speedometer would be close to accurate though.... after all, these aren't cable-driven.

[pocket-rocket]

Mine read around 110 (I think, Ben might remember) when I got clocked at 117 years ago....

[robertisaar]

6.4% error.... that's not too bad. at 55MPH, speedo would actually read 58.5.

[pocket-rocket]

Back then I figured the error would go the other way. The look of shock completely overtook the look of fear in my face as the county sheriff informed me of it, I'm sure.

[robertisaar]

actually, you're right. when it shows 58.5, you're actually at 55.

[pocket-rocket]

Actually I believe it was 105, not 110. That's why I was more shocked when he said 117. Not that it really makes a difference when you are doing over twice the speed limit (at the time).