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Discussion Starter #1
Quick question,
If a PCM relay went bad, other than the obvious (engine will crank but not start) is there a way to take an educated guess that is the problem without testing it?

For example, I am thinking that the idiot lights would NOT come on when one turns the ignition key to ON/RUN, but I'm not sure.
 

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normally fuel pump would not come on. possible, no power at injectors

idiot lights i would think should come on...normally they're not powered by the ECM

check engine light probably would work because it's only grounded by the computer

naturally it depends on the wiring of the specific vehicle...the above is in general
 

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Discussion Starter #3
RickMN had written...
Cranks but won't fire
It cranks but won't fire? Look at the check engine lights while cranking. If it's ON, then the computer is not getting a good signal from the crankshaft position sensor. If it's OFF, then the sensor is good. Next step would be to check fuel pressure.

I'm thinking that if the CEL does not light, that it might also be due to the fact that the PCM may not be getting power, but I don't know.
 

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Discussion Starter #4 (Edited)
Just for the heck of it, I pulled my EEC relay in my 99 Crown Vic (to simulate a bad relay), and the CEL did NOT come on when I turned the ignition key to ON/Run.

My guages worked and I think all the other lights came on... Air bag, ABS, Battery etc.
 
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