So after frying another ecu I figured that it is time to tear down my harness to see if there is a glitch in it. A couple things I am wondering though. Is there some special tape I should use to wrap the harness? Does anybody have a picture of the pin readouts for the 90 mustang harness so I can start to trace everything back to were it should go? Also I am in need of another harness firewall grommet if anybody has one laying around. Any help would be greatly appreciated.

Thank you

Bobby

Here's a good diagram. Don't get hung up on wire colors, they may not match.

What is getting fried? What ECM's have you used?
88-91eecPinout.jpg

So where do you plan on starting since the fuel pump circuit in the computer is getting toasted?

The one thing I know of that fries ECU's from those model Mustangs is that manual transmission and automatic transmission harnesses are set up differently for O2 sensors.
The fix is a jumper wire.
Sorry I don't know more than that, I decided to go a different way for EFI.
But it may give you enough to start searching.

I have used 3 different ecms. I had an a9l but it might have been bad from the get go so I used it as a core from autozone. That one says its a a3m. They seem to work for a few months but then boom. I forget what model Kirks is that he let me borrow for testing and to get home the other day.

I don't think its a the fuel pump wire. When Gary looked at it last he said it was a ground wire issue for the ecm and it just lost complete power to the whole thing. So I don't think its a fuel pump wire.

I was wondering about the ground system for EFI, I started tracing my way through the diagram Viper posted, since TFB wants EFI I might as well start learning these things.

When I repaired it the solder joints between the PCM connector and the board had burnt on pins 40 and 60. Those are your 2 main grounds. Not sure what would cause it except for maybe high resistance like a loose ground wire. I would also check the case ground (pin 20), perhaps it's bad creating additional load on the other 2? Just grasping at straws since I'm not sure what caused it. But, if it's the same problem I can likely repair it again.

Well I'll be In touch with you again Gary to get you my computer. I'm going to pull the harness and check those grounds from the pcm and see if there is some thing else going on. What's the perfect way/location to ground it. I have it ground to the firewall and also a ground strap In the same spot going to the frame. Figured that would be enough.

The other thing I think is weird is how I can drive it for 1500 miles and then it just suddenly does this. I would think it would happen sooner.

I prefer a 3 way ground post that is easily accessible, a nice bolt used as a stud, bring large gauge (2-4) wire from frame, engine and body to it and be done. I can tell you this solved the ground problems we were experiencing on Bubba Smurf and I run a monster 00 gauge on my diesel starting system.

I'll hazard a guess at to why it takes a few miles before it pops. If it is a high resistance ground problem I bet it's "eroding" the connection over time until it can't carry the load anymore, when too thin it breaks.

Where did you put this post in bubba. Is it on the frame or more on the body and everything connects to that.

Kirk helped me with my grounding post and we did mine on the body (passenger wheel well). You can see it if you zoom in on the picture.

I think we put it in a similar spot, inner fender well. Passenger side makes sense with most of the high draw items converging over there. It's not a new idea, I've seen it before, likely a Bronco.

Hmmm, that's a good idea. Need to put in a new starter anyway...

Could having a bad ground on my O2 sensor possibly be causing this? I just pulled my harness and was testing grounds and I am not getting anything from it

On side I do get a circuit the other I don't.

I have a break out box if you want to borrow it. Makes pinning out easier.
Rob C
Shop # 303-679-1610

Garry did some rearranging and cleaning. Installed it with my new pcm and it fired right up. Looks like a couple wires got crossed on the 02 sensors and possible be the cause. We will see how this goes.