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Discussion Starter #1
Thanks again for all the responses. The "ECM Power Supply/Ignition" fuse problem was taken care of (Tip- Poor factory routing-"buried/hidden" wires to the air diverter valve melted on the exhaust manifold, shorted). Still having problems with the tach though. ~750 rpm with the ignition off, 0 with the ignition on (engine running or not). No blown fuses-the problem was intermitent at first. The circuit board looks good (only 63,000 miles). Any tips?
Dave
 

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You might want to post this problem over at www.montecarloss.com and see if someone there can help.

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Leo Paugh
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progress has little to do with speed, but lots to do with direction.
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Discussion Starter #3
If there is no "continuity" between the radio filter and the tach, is the filter shorted, resulting in no reading at the tack?
Dave
 

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Still just guessing around. The filter or filter network may not show continuity through it. It'd probably a little circuit with coils and capacitors inside. Measuring like you are doesn't really tell you much. It might be good, it might be bad. Sorry best I can say without knowing exactly what's inside of it.
 

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Discussion Starter #5
Thanks John - I "bypassed" the filter. My parts source told me that Chevy quit stocking them in '96. According to the service manual, the filter "reduces radio interference" and "rounds off the pulses and removes spikes". Will I ruin the tach by not using the filter?
87Dave
 

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You probably won't ruin the tach. However, the "noise" that gets through will keep the tach from running acurate and might be jumpy. Almost a waste to hook it up.
Like I said I'm not sure what's inside of them. If GM used strictly passive filtering, they should last for a long time. I'd hit the junkyards and pull another one out. Shouldn't cost that much.
 

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I would not try to connect the tach without the filter.

According to my references, that filter is not purely capacitive.

It consists of 2 resistors in series. At the junctions of the two resistors there are 2 capacitors. The capacitors are clearly to shunt extraneous noise. The resistors are what have me concerned. You should not be reading an "open", Try your meter on X100 or X1000 and see if you have a current flow. The tach "sees" the ignition system directly thru the series resistors. This is how it was in 1980, I don't have 1987 info. Just to be safe, I'd hit the bone yards or try a GM parts counter for "tach filter".

[This message has been edited by Coppertop (edited 01-20-2002).]
 
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