Working on my fathers 1995 F-350 7.3L. It intermittently has no power, goes into limp mode, and turns the CEL on. I cannot pull the codes out of the ECM with my bluetooth scanner and torque. I've even tried an expensive snap-on machine and cannot pull codes out of it. The truck sat for a few years, and recently we got it back running with a new FICM and some more replacement parts that I can't think of off the top of my head. I know for sure the FICM was replaced, CPS was replaced. I cannot get it to cough up the codes as to why the check engine light is on. I can't get it to maintain a stable link between the scan-tool and the ECM.

When the truck runs, it runs fine. However it will drop itself into limp mode and runs like garbage. I'm beginning to suspect that due to old age that the computer may be on it's way out. Is there anything I can check to confirm this before I go hunting a new one? Any ideas are welcome, and I'm at a loss here as to what to try next.

 

If your CEL light is on you have codes, the reason you cannot communicate with the PCM is because your PCm is pre OBD2 and has to get flashed for it to work properly with some scanners, the dealer and some Snap On scaner can read our trucks.check your UVCH and see of they aint got burned or loose connectors.

 

Thing is, I've had both the bluetooth obd tool and the snap-on pull codes at random. Right now I can get nothing out of it at all. Both pieces of equipment fail to link with the PCM at the moment. Should've included this in the first post but was typing it it out before I headed to work. I also suspected that the intermittent random running conditions might be the UVCH related. I've already replaced one of the pigtails for the UVCH before, due to a melted glow-plug pin. I do need to yank the VC's off of it and look at those harnesses too.

The intermittent no-link from the OBD/PCM is what got me thinking about that. But you do raise a good point with the UVCH possibly being a culprit.

 

Might want to check the #16 fuse in the cabin fuse panel as it protects the data link and the cigar lighter. If the fuse is good, make sure the contacts in the fuse panel are grabbing the spades of the fuse.

 

I've pulled and swapped the #16 fuse already. Cleaned the contacts in the fuse box as they looked a bit corroded, and etc. Still getting an intermittent data link from this thing. Still can't get codes out of it with either scan tool. I'm at a loss for what else it might be save for wiring somewhere?

 

swap PCM relay with one beside it

also check this wiring out with ohm meter

 

Sorry about the long delay on responding. Life kinda got in the way for a little bit, so I didn't have much time to play around with this thing. However, I did get around to swapping the underhood relays around and I still cannot get a consistent OBD link from this thing. Sometimes it will work, Sometimes it does not. While I was messing with it I went ahead and ordered new valve cover gaskets, new UVCH's and replaced all the main harness pigtails. One of the valve cover gasket connectors was melted, drivers side front. It runs, but still like garbage. I'm thinking the IDM is suspect, but I'd just be guessing without a way to test it. And since I still can't get codes out of the computer even with a check engine light on I'm lost as to what the symptom is, that it is throwing a fit about.

Any more suggestions? I really don't know what to do.

 

Do you know someone that has a 7.3 Stroke? If you do put your IDM in that truck and see if the crappy running follows to the other truck. If it follows, the IDM is the suspect.

 

Turns out the IDM on the truck was recently replaced by my father when the truck was DOA in the yard. He bought it from autozone as a reman and I went and warranty swapped it today b/c we do not have access to another truck with one. Haven't installed it yet, but i'm hoping that it fixes this mess. When the truck stopped running initially it was doing the same thing, rough running at first, then not running. Replaced the IPR,CPS, IPC, fuel pump and it never fired. Replaced the IDM after a while of fooling with it and it fired right up and ran like new.

However, we never knew at that time the VC connectors were melted up front. I'm thinking that when the IDM was replaced, and since we didn't know about the damage to the VC connectors it fried the thing again. I'm at a loss as to what else it could be.

 

Did you ever get back to this?

Here's some information about which pins do what in the OBD connector. Maybe you lost connection to the DLC. I was looking at some more diagrams like these but don't know where. Troubleshooting the DLC is a topic in the manuals.

https://forscan.org/forum/viewtopic.php?f=4&t=6142