Tach reading way high

Spaceboy1980 · May 6, 2011

My car idles at around 5500 rpm going by the tach. I found a page about repairing a later style tach but mine has a blue square box at the back and not a circuit board like in the tutorial. Anyone have any insight on this. Tach is the factory one residing in my 79 Elky.

Thanks,

TURNA · May 6, 2011

Do u have a V-8 in the car and the car was originaly a V-6?

Spaceboy1980 · May 7, 2011

Good question. When I bought the car it already had a racing head service 320 horse motor so a swap was done but I believe it to be a factory V8 car due to the way the fuel lines were ran and other clues. The fifth vin digit is an H, that's factory V8 I think?

t5montecarlo · May 7, 2011

Motor VIN H = 305 LG4.

If the tach was from a V6, it would read wrong, not that much. For every rotation of the V8, it would register 8/6 (1.333) times higher.

Doober · May 8, 2011

It's likely the resistor 'pack' (looks like a white IC chip on the tach circuit board) or one of the capacitors. One of the traces usually goes bad on the pack, and eventually opens or shorts, causing the tach to stay at 0 (short) or peg at its max (open). The capacitor also has something to do with the linearity of the tach, but I don't remember that much about it... give this a read:
http://www.thirdgen.org/techboard/electronics/357893-90-92-tachometer-fix.html

The board doesn't look identical, but the same resistance applies. I tried this with a pair of resistors (temporary fix, didn't keep the tach in the car) and it got the tach dang close to what it should be.

Spaceboy1980 · May 8, 2011

Well I found a tach in a junkyard today and pulled it. The face was trashed but I hooked it up and it showed a 800 rpm idle and revved properly so I swapped the circuit board over to mine and I now have a working tach!

I spent a lot of time fixing the circuits and lights on my dash cluster, I have back lighting now I didn't know it had. Even my turn signal indicators work now.

I also found a dome light delay module in the same car in the yard but it too is bad. I wonder if there is anyway to trouble shoot that, but that's for another topic.

Doober · May 9, 2011

There has been a little talk of the delay modules. I think for the trouble of finding an OE one you can build your own, or buy an aftermarket (non-OE) one.

One of the guys painted the back side of the 'shadow' portion of his cluster and got a lot more light... originally I believe it's a light blue, but a flat white would help it light up a bit more. I did the same thing with the circuit board, found one from a slightly newer car that was in great shape at the junkyard because the one I had was bad.

Spaceboy1980 · May 9, 2011

Funny you should mention painting the back of the shadow box, I spritzed mine with some of that faux chrome paint. I'll start a new topic about the delay module because that is something I would like to repair.

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Tach reading way high

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