1993 GMC Sierra Unable to read ECM

1993 GMC SIERRA
216,000 MILES • V8 • 4WD • MANUAL
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SIRDANOMAN
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My mechanic is unable to communicate with the ECM on my truck. He says the ECM could be failing but it is very rare. Would it be worthwhile to change the ECM and PROM?

Overall, the truck runs very well despite having 216, 000 miles. Thank you
Jul 30, 2010 at 12:24 PM
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DOCFIXIT
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Hi System is OBDI was he using an ODBI scanner? if not an OBDII will not comunicate. Jumper a wire between term. A and B on ALDL see diagram


https://www.2carpros.com/forum/automotive_pictures/198357_Graphic_536.jpg

key on does check engine light flash? Let me know Thanks for donate
Jul 30, 2010 at 12:42 PM
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SIRDANOMAN
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Thanks for the quick reply. I have now used an OBDI coder reader (glorified jumper). I simply get code 12 (the handshake code), but nothing else. My idle continues to flucuate primarily when I've been driving the vehicle for a number of miles and have shut it off. When I restart, the idle "hunts" from 200 to 1000 RPM and will settle but the engine still sounds like it is surging mildly. I've had several vacuum leaks repaired, leaky injector cover reparied, and replaced nearly every sensor but MAP sensor. I'm stuck! Thank you!!
Sep 8, 2010 at 9:02 PM
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DOCFIXIT
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Was IAC replaced or cleaned? EGR checked and cleaned? ECT checked or replaced?Fuel pressure good?
Sep 9, 2010 at 10:21 AM
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SIRDANOMAN
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IAC replaced and TBI removed and cleaned with new gasket. EGR cleaned and no carbon buildup anywhere. Diaphragm seems to be intact. ECT (both) replaced, but haven't checked with multimeter. Fuel pressure check was good. I'm stumped. Will check the timing next week. Could a failing pickup coil and/or distributor problem be an issue? Thanks.
Sep 9, 2010 at 8:14 PM
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DOCFIXIT
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Two things to check Ignition ,module can be tested at Auto Zone the other pick up coil check wires to coil for cracks in insulation if present replace
Sep 10, 2010 at 12:21 PM