Author Topic: Factory Tach Circuit Board Component Specs  (Read 502 times)

Offline bshrubb

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Factory Tach Circuit Board Component Specs
« on: March 20, 2017, 06:09:57 PM »
I have a 1980 Chevy shortbox. I installed a full gauge cluster from a dump truck that had the factory tach. I do not know the year of the dump truck. All the gauges work fine except the tach. When the engine is idling the needle sits pretty much where it should but it doesn't take much throttle and the needle buries itself as if I were bouncing the engine off the rev limiter. Just wondering if anyone out there knows what values I should be seeing on the multi meter if I were to test all the components on the circuit board? There is one resistor in particular that concerns me because its obviously gotten really hot and is burnt. Its hard to tell what colors the bands are supposed to be because they are discolored from getting burnt. I'd like to get this tach functioning properly again. Any help is greatly appreciated.
« Last Edit: March 20, 2017, 07:02:54 PM by bshrubb »

Offline Irish_Alley

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Re: Factory Tach Circuit Board Component Specs
« Reply #1 on: March 23, 2017, 09:37:46 AM »
subscribed. just cause i want to know if its fixable. doesnt look good
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Offline hatzie

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Re: Factory Tach Circuit Board Component Specs
« Reply #2 on: March 23, 2017, 12:07:12 PM »
That top left diode got pretty hot too.  Looks like there's still a number on that one.  Z9??...  Maybe a Zener

It's a single sided board so it may be better than it looks even tho it's crispy under the two components. 
Are the traces on the other side still attached to the circuit board?

The needle moves so it sounds like the Obsolete LM181 chip is still functional.  That's good because the NTE replacement for that chip is obsolete too.

I'd try clipping the resistor and diode leads off close to the board and getting a look at the backside of the resistor and part #s off the diode it may be in better shape or it may be worse.   You don't know til you look.
Wiping on some light oil may bring out the color too.
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Offline bshrubb

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Re: Factory Tach Circuit Board Component Specs
« Reply #3 on: March 29, 2017, 09:23:07 AM »
I've done a continuity test on all the pathways and they are all in good shape still. The soldier for the two leads for that top left resistor was burned right off so that's where the heat was created I'm assuming. All that heat could have wiped out that diode beside it as well being as its so close. Ok I will pull those two components off the board to see if there are any part #s on the backsides of them and I'll try the light oil trick as well. Thanks.