I remember something about these trucks and needing the dash to run right or something, don't remember what it was or if it actually existed but.... If my dash is installed, it cranks over and half fires like it's starving for fuel, but unplug the dash and it'll start up like it was out of fuel then run. Dash in, it'll start for a couple seconds maybe then stall, dash unplugged, it'll run all day although not as smooth as it should. Does anyone have an idea of what's going on?

97 F-Superduty 7.3L 5 speed

 

Hmmm, that's an odd one. One thing that definitely could affect the way the truck runs is the Baro sensor that is part of the dash assembly. It is located more or less directly below the steering wheel under the dash. It looks like a square relay with a round connector on it. That would be one thing to check anyway.

 

It's in a project truck, Have had it running well for a couple days, today I'm tidying up loose ends and installed the dash, push truck outside and go to start it and this happened..... took a while to suspect the dash but pulled the plugs out and it fired up, but now it's grumpy. Yesterday it fired up and ran nice with no dash.

Been reading about no starts due to cam sensors..... and computer not seeing a tach signal and not fueling engine. If removing dash enables a "limp mode" or something to bypass the CPS, it would be where I start. Just can't figure out the rough running today vs yesterday all due to installing dash.

 

The CPS wires directly to the PCM so I can't see that being the issue. The only things I know of for sure that you eliminate by unplugging the dash are the BARO sensor and the 2 sensors on the throttle pedal (TPS and IVS).

Did you install the whole late model dash and wiring harness or was it a cut and splice type of job? If the VREF signal was incorrectly spliced and isn't at the full voltage that would cause running issues.

 

Took the whole harness, laid it into new (old) truck. I'll try find the Baro sensor and take a good look at throttle pedal. Thank you and any more input is MUCH appreciated.

 

Are all the dash lights and things from the OBS donor connected to the harness? Ford has a bad habit of using the filament for a given bulb as part of the sensor circuit for a sensor. I would have to look at a schematic again to see if that may have been the case for something here.

 

Yup, everything OBS is in the project truck, found the Baro sensor, is there any way to test it? Also, peeled wiring cover away from throttle pedal, and noticed all the wires had been tapped- there's like an inch of wire sticking off of each wire, not sure what that was all about.

 

I'm not sure whether you can test the baro sensor or not, but you can just unplug it and the PCM will default to a standard value and the truck will run without it. Id unplug it and give that a try.

As for the wires tapped into the throttle wiring, I would wager that someone had set up a high idle on it. I'd clean that wiring up and tape things up good while you're in there, but I doubt that was your issue. I'm not home this weekend or I would have access to a lot better information.

 

If the Baro is unplugged, would that default the same as having the dash unplugged? I'm trying to get a buddies truck over so i can borrow his sensor. The wires on the pedal confuse me because the truck has the factory high idle box under the dash, although it was a work truck with a winch so who knows.

When you say the dash lights are part of the circuit, are you saying that if a dash light burnt out that it could actually cause it to run differently? Or that say.... a cheap bulb could cause a different ohm or voltage reading in a circuit kind of thing?

 

For example, if the battery light is burned out, it will keep the alternator from charging.

 

Lmao... it's a serious question given what I read......

 

Agree ... ^^^ ... VREF is + 5 volts from the PCM. It supplies power to all sensors. In your case you can measure it at the TPS, IVS or any other sensor. If it's missing the truck won't start or run.

 

5.03V at the pedal

 

With the dash connected, correct?

Yes, unplugging the BARO would likely give the same default reading to the PCM as having the dash unplugged. I would definitely take a look at that. Just unplug the thing and see how the truck runs.

 

Yes, with dash installed, just repeated to be sure, 5.05V at pedal, had it on charger all night so don't know if that changes anything (0.02V). Changed the Baro sensor with one from a friends truck last night, no change in my truck with his, and no change in his truck with mine..... If you take the dash in/out part out of the equation, truck seems like it's starving for fuel. So much actually that I'm pulling the box and pulling the sender out and taking a look. It's actually gotten worse now and the dash no dash thing isn't having the same effect, BUT the other day it was the difference between starting and not starting, no questions. It's still getting enough fuel to smoke and half fire but even when it ran, it had a lack of power like less fuel than it wanted, do not understand what the difference with the dash was tho.