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Discussion Starter #1
Tach went "dead" a couple of weeks ago (floats at about 750 with the ignition off, at 0 when engine is running).
Now ECM fuse blows as soon as engine is started, and disables ECM. What could be shorting to blow the ECM fuse?
87Dave
 

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Could be the ECM itself. Most of the time if it is one of the input sensors, the fuse doesn't blow. Try disconnecting the battery, unpluging the large ECM connector, replaceing the fuse and battery connection, and seeing if the fuse pops. If it does not pop, a guess is that the module has a problem. I keep a spare one for my Olds. Found it at pick-a-part for 25.00. ECMs have a part number on the label. I found one that had the same part number on it.
 

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Did you fix you fuse/tach problem? You can also try this. The electronic spark timing (EST) distributor starts in the bypass mode (the spark timing is controlled directly by the pickup coil amplifier output) after starting the ECM sends a signal to the HEI module to switch control to the ECM. This signal may be grounding in the dist. You can disconnect the pigtail connector at the dist. The engine should start in the bypass mode, if the fuse doesn’t blow the problem is in the dist. Also disconnect the tach lead at the dist to see if the tach stays at 0 with the ignition off.
 

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Discussion Starter #4
1st, thank you both for your fast responses. It makes membership in this organization more than worthwhile.
John-I unplugged the larger connector from the ECM, put a new fuse in, turned the ignition switch on and the fuse blew. I then installed the ECM from my other 87 El Camino, turned on the ignition switch and the fuse blew (I hope that the other ECM was good, as I am in the middle of an engine transplant, and didn't "screw" up the other ECM).
Elree-I haven't been able to fix the tach problem (removed it from the instrument panel to see if it was blowing the fuse, it did'nt change anything, and reinstalled it). I can't find the tach lead at the distributor to disconnect. I removed the EST pigail from the distributor, pulled the cap off to see if anything was evident, replaced the cap, put in a new fuse, turned on the ignition switch and the fuse blew.
I can't find anything in the factory service manual addressing the tach and its circuitry.
Any more help will be greatly appreciated.
87Dave
 

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I'm not at the shop today because of New years so I can't look at a wiring diagram so this is only a quick guess if you wanna run out and check.The ecm powers the fuel pump relay so maybe something is up with that circuit.I'm 'sorta' positive the ecm on GM cars only supplies ground to power up most solenoids to protect it from short circuits and to keep "induced voltages" from triggering things at the wrong times but maybe not with the fuel pump circuit.I have worked on tons of fuel injected cars and have NEVER seen the ecm fuse blow ever.So that is why this is only an 'off the wall guess'.I can't imagine anything else blowing the ecm fuses??? The ecm sends out 5 volt reference voltages to it's sensors so if even one of those 5 volt wires got grounded before the sensor the computer is still protected because it sends that 5 volts through a resistor inside of it.I'm curious to see what you find.Please keep us informed.
 

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Discussion Starter #6
Thanks SSCHEVYFAN,
My El Camino (factory Super Sport Decor) is a carburated 305 V8. The V6 was fuel injected.
87Dave
 

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"John-I unplugged the larger connector from the ECM, put a new fuse in, turned the ignition switch on and the fuse blew."

Stupid question.
Are you saying that wihout the ECM plugged in the fuse pops? Something more simple as a shorted input (12v) line to the ECM?
 

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Discussion Starter #8
John, #1=yes, #2 tell me more, I have a feeling that the tach may be a key to the problem.
87Dave
 

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I don't know if I'm seeing things right but if your fuse pops without the ECM plugged in, it's not the ECM. That is, it might be the power line to the ECM. If that's what's occuring, unplug the ECM, disconnect the battery, remove the popped fuse, and measure the fuse clip to ground. If it is a short, I can't guess where it could be. I don't have a schematic on that model and can't tell if the fuse powers only the ECM or something else.
 

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Discussion Starter #10
Thanks again everyone. Sorry if I confused anyone! There are two (2) 10 amp ECM fuses. One - EMC Power Supply (which hasn't blown) and One - EMC Power Supply-Ignition. The EMC Power Supply-Ignition is the one that keeps blowing. With the ingition off there are about 30 ohms resistance, and 12 volts with the ignition on.
I also checked the tach wiring in the engine compartment. There is continutity between the distributor and the coil, the splice connector between the splice and the coil, but no continutity through the tach filter.
 
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