Are these symptoms of a bad ECM???

VIPERBLUELX · Jan 13, 2004

I just bought a 93 5.0 5-speed coupe:

My car idles great with either the TPS unplugged or the IAC motor unplugged. If both are plugged in together the idle jumps to 3,000 rpms, but will drop back down if you push the clutch to the floor.

I checked TPS voltage and it's at .97, but it only goes to 2.61 at WOT and swapping to a new TPS doesn't fix it either.

Also, I can't scan the codes with my Snap-On diagnostic scanner. The scanner works fine on my other Mustangs.

I'm thinking the ECM is fried or some computer wiring is bad. The car runs fine with the TPS completely unplugged and it even drives good with it unhooked. The car does feel weak compared to others i've had though.

What do you think?

ashford · Jan 13, 2004

with the iac unplugged the engine should die or barely idle, either the base throttle position is set high or the iac is sticky. do you have anything to measure duty cycle?with an idling warm engine it should be somewhere in the neighborhood of 10-25%. as for the clutch thing it almost sounds like an auto computer was put in it. an auto when the neutrac safety switch isn't hooked up will rev to 3000 rpm when started then fall back down after a few seconds, well the one i retrofitted into my car did anyway took me awhil to figure that one out

VIPERBLUELX · Jan 13, 2004

The car will continuously rev at 3000 with both plugged in.

All the Mustangs i've owned would idle normal with the IAC unplugged, I thought it only functioned when driving or with the A/C on? It shows on the Corral to set the idle where you want it with the IAC unplugged, then plug it back in after you get it set.

ashford · Jan 13, 2004

whats your signal voltage it should be at 5 volts, if its not check it at some other sensor like the plug on the egr if its the same check it at the ecm(dont remember what wire it is), if the voltage is low there you do have a bad ecm.
as for the setting of the idle witout the iac plugged in, thats mainly for modded engine that have surging problems.

jrichker · Jan 14, 2004

ashford said:
whats your signal voltage it should be at 5 volts, if its not check it at some other sensor like the plug on the egr if its the same check it at the ecm(dont remember what wire it is), if the voltage is low there you do have a bad ecm.
as for the setting of the idle witout the iac plugged in, thats mainly for modded engine that have surging problems.

Vref (5 volts) for the TPS is pin 26 on the computer.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

Are these symptoms of a bad ECM???

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Are these symptoms of a bad ECM???

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