Are you removing/disconnecting the WHOLE dash, or the _cluster_? (Just so we know what to narrow it down to.)

 

Are you using the OBS tank and pickup or the one original to the truck? Originally the truck didn't have an electric fuel pump in it did it? I'm grasping at straws here.

Is the PCM chipped or stock? I'm wondering if something is getting jostled or pinched when you are working on the dash...

 

The OP's truck wouldn't have had an electric fuel pump; all the OBS PSDs were mechanical.

Re. the chip, yes, FIRST thing to do if it's chipped is to REMOVE it (not just set it to "stock", but physically disconnect it to take it out of the picture).

 

Right, but my understanding of the project was that it is a 7.3 PSD from an OBS swapped into a '79 chassis? Am I incorrect in that understanding?

Maybe a little more background on the project and more detail on what parts were used from each vehicle will help us pinpoint the issue.

 

OP says it's a '97.

 

Everything relating to this thread is related to a 1997 F-Superduty. The project part of this changed nothing as far as cutting or hacking into anything. It worked a week ago then something changed. Took a fuel line off the filter housing and I'm sure it had a vacuum in it. Might not be related to the original problem but now it's a problem.

 

No chip. All stock. For now.

 

Well then. I was Waaaay off, lol. I thought this was a PSD swap into a highboy chassis. If that's the case, then I'm still curious as to what fuel tank you're using and whether it was modded to have the PSD draw the fuel from it. Did I imagine that or did I just read that into the situation based upon your screen name?

If it's just a straight F-Super duty that may simplify things then. I'd suggest that rather than focusing on the dash, we start at the beginning troubleshooting the engine for a no-start issue.

Use this Link

 

K I'll clarify a bit. The 79 body is on the F-Superduty chassis. Nothing was altered from the diesel to accept the body. By this I mean the harness minus the wipers and heater was pulled from the original cab and installed into the 79. Nothing has been cut or grafted together between them at this point. It was running a week ago without dash. After installing dash it had a no start. Just a fluke I pulled dash and it fired. It did run before with the dash. Diesel tanks... Nothing changed there.

Seriously thinking whatever the pulled dash bypasses is enough to compensate for another problem but I'm completely open to suggestions.

I did install a new crank position sensor just because. No change.

 

Okay, well it makes me feel better to know that I'm not losing my mind then.

I would start stepping your way through the troubleshooting steps in the chart I linked to above. That will help narrow the search for a solution. Knowing whether it is a HPO, fuel, or electronic issue will definitely help us pinpoint the issue regardless of whether it is related to the dash or not.

 

Back to - is this happening with removing / installing the WHOLE dash, or just the instrument CLUSTER?

 

Yes, yes, what exactly are you unplugging to get it to run? The connectors under the dash in the firewall (if so, which one(s)) or just the instrument cluster / gauges.

 

Removing the three plugs from behind the dash cluster, sorry didn't see that question earlier.

 

So these are the three plugs that come through the firewall?

 

Hmm, one remote possibility is the charging system warning light (battery icon). It's basically a series connection with the alternator trigger wire. So maybe, MAYBE the alternator is putting a seriously heavy load on the engine, and when the cluster is disconnected, no alternator op, no load, so it starts better. Just as a test, you could try disconnecting the alternator trigger wire right at the alternator (make sure batteries are fully charged), then reconnect the cluster, and see what happens when you try to start it again.