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1G Mpi relay buzzing


10+ Year Contributor
Jul 28, 2012
maple shade, New_Jersey
hello I hope someone can help. I'm installing a 90 harness in to a 91 and i changed the plug for the ignition to fit one of the 91. Long story short I have everything is place and after I start it and shutting it off. My ecu started smoking and the mpi relay was not stop buzzing. I narrowed it down to c-60 ecu harness. But from there I'm lost. I keep reading the write up 19eclipse90 did but can find my answer. Any help would be awesome thank you. When it was running it can fine.


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10+ Year Contributor
Dec 9, 2010
wampum, Pennsylvania
Are you sure that its your mpi relay that was buzzing? that is around the area on the board where some of those ic's are used to drive the iac stepper motor, a bad winding in an iac motor will burn up those ic's.


DSM Wiseman
Feb 3, 2002
Boulder, Colorado
From here is looks like IC105 is blown because the B coils in the ISC were shorted or the wiring for them was.

Depending on how badly that driver is blown the ECU may not be able to power up correctly now.


10+ Year Contributor
Jul 28, 2012
maple shade, New_Jersey
Yes it was the relay. And yes the driver for the isc was burnt . And there is where I found the issue two fold. One a 90 harness in a 91 can you need to switch the white and black wire on the isc. If not just guess what you do.....? Burn out that driver cause it tries to adjust it the wrong way. Two be lazy after all the work and not test the isc in the first place. And guess what it was bad also. So kids dont be like me LOL. But it is up and running again. Going to see if it holds up for now. Thanks for trying to help.
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