>"Subject: RE: Re: Re: Tachometer reading 500-2000 RPM high!
>
>Does that mean a bad transistor in the tach?"
>
Probably but could also be poor connections in the under-dash wiring. A
connection heating up and going resistive might also show the same symptoms.
So after cleaning all the connections in that circuit, using the Haynes or
Bentley schematics for reference, I would then retest for the same failure.
If it still occurs, then you have clean connections!! And then you must work
on the tach.
The way I would troubleshoot this heat related problem is to heat the
circuit board and parts with a hair dryer...since it is very difficult to
get too much heat from this tool. And then take a can of cool spray (Radio
Shack) or can also substitute the can of compressed air for cleaning out
computers, and spray each part _one at a time_ slowly while watching the
tach operate. When there is a change from normal to abnormal operation, you
have uncovered the defective part. Continue this process to check out ALL
the parts so that you can verify that you have found all heat sensitive
parts.
If you don't get satisfaction using the hair dryer, you may have to resort
to using a real heat gun. Remember the transistors are made to operate up to
150 degrees C and the circuit board and other components will take quite a
bit of heat for a short time. (Just don't stop the heat gun in one location
for while!!)
Kind regards,
Tim Economu
Senior Design Engineer
1969 MGB/GT mona
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