I have had an intermittent Check Engine Light for a couple of days --- the van started and ran fine, but the CEL would come on after about 15 minutes of driving. It would sometimes go off before I reached my destination, but it was always off the next time I started the van. No sweat.

I went to read the codes this afternoon and now have a problem. I put one end of a jumper wire into the Self Test Input (STI) and then accidentally let the other end of the wire touch the 12V connector of the starter relay (why is that thing unprotected anyway?). I saw a little arc so I know there was some current flowing.

After this, I connected the STI to SIG RTN for reading the Key On Engine Off codes and got no pulses at all. I rechecked the jumper wire but got the same result. I then removed the jumper and started the van --- it ran quite rough and would only stay running by giving it plenty of gas. The CEL was on the whole time. I did this a couple of times just to be sure.

I guess there's nothing more annoying than turning a little problem into a big one... crap.

I couldn't find an electrical circuit diagram showing the STI in my Haynes manual, so I'm not sure what is likely to have been damaged. I assume the engine control module (ECU) has been damaged, but that is just a guess.

Some specific questions:
- where is the ECU?
- Should I spring for a new one (Rock auto shows $80 to $175, depending on the ID number), or is one from a junk yard just as good?

Has anyone out there been in this position? Any advice would be most welcome.

Thanks in advance....

 

my Aero CD manual has failed on my but one of the other guys will come along that has one running.

may be as simple as blown ECU power fuse.

check the Autozone site. you have to sign up to access their veh ele. diagrams.

ecu location
Fuel Injection Technical Library » EEC Vehicle Locations

 

Engine side of firewall. Single bolt releases the PCM (aka ECU) box connector, no need to touch the two nuts on either side.


Drivers side of firewall, under dash, left of steering column. The PCM is the silver box. On this example it was slightly rusted to the bracket that is held in place by those two nuts just talked about above.


The rear edge of PCM is confined by a large clip. You might be able to get the box in and out without removing the clip, but I doubt it.


STI should be a straight shot into pin 48 on the box and it should be a high impedance input, so I'm surprised about your experience with the arc'ing. I suspect other issues.

*** encore ***

 

Aha!
Thanks, Red, for that pic of the plastic ECM clip.

I've found those things laying in various Aeros I've owned for years, and never could figure out where they went. I had one laying under the driver's seat of my '89 from the time I bought it until I sold it a decade later!

 

Thanks for the ideas, and especially for the photos!
I couldn't find a blown fuse, and the Owner's manual indicates the ECU is powered by a 20A fusable link. Is there an easy way to confirm the ECU is getting power?

A google search turned up a diagram of the ECU connections here
1990 Ford Aerostar won't start - Engine Discussions at Automotive.com

This diagram shows Fuse Link J on ECU pin 1, and the PCM power relay controlling input from Fuse Link E to ECU pin 57 (if I'm reading the diagram correctly).

Where is the PCM power relay?

Thanks again...

 

VOM on the engine harness side of the connection. Pin 1 keeps the PCM memory alive, pins 37&57 get power with key-on.

 

Fantastic photo and annotations!
Thanks Rojo.

If I don't have to work late (again), I'll check it out tonight.

 

Any advice on how to get the ECU out?

I have it loose, but there are so many wires in the way I can't remove it (and would have little hope of re-installing it). I disconnected the fuse panel, but there is still a large wire bundle in the way.

 

It can't be that bad. Show those wires who's boss. PM if you need a house call.
I gather though that the harness checked out and you are on to a swap strategy.

 

Got it --- woot!
Thanks for the encouragement.
Yes, the harness showed 12V on 37 and 57 with key on, and 12V on pin 1 at all times. I decided to get a replacement ECU --- fingers crossed.

 

Just finished successful installation and test drive --- it is now running well, just like before I started. Thanks for the help!
BTW, I decided to get the replacement ECU from a local parts house (Winchester Auto in San Jose), so that I could compare the old one and new one while at the counter.

This forum is a great resource!

Cregg