No Go~!

[badtruck]

Hey there, I've been building mud chuckers for a number of years now, but I'm totally new to the Diesel world. I've just purchased my next project it's a 91 F-250 ,4x4 ,7.3 L ,non-powerstroke, automatic. I am having some problems while driving. When starting out the engine revs but the rpm gauge does not go up, and it takes some time to get the truck up to speed, then on the highway once up to speed the gauge will occasionally jump, along with a burst of power, and also while on the highway when the gauge is up if I hit a bump it hits zero and the engine noise drops. Being that it's December, having these power fluctuations on snowy roads could prove problematic.

Any help or suggestions are certainly appreciated, Thanks in advance

Badtruck
( Jeremy )

[zspear1]

Does that truck have the E4OD (Electronic 4 speed overdrive automatic tranmission). years and years and years and years ago, back when there were no powerstrokes........ I had a guy bring me a 91 f-250 4wd with automatic and 7.3 IDI N/A diesel, doing the same exact thing your is doing IT turned out to be chaffed/burnt wiring harness that ran from the computer down to the transmission which sent vital data about engine dynamics to the tranny so it can shift properly his kept hitting passing gear when the tach would go out or move around while running down the road causing engine RPMS to go up becuase the trany was in a lower gear. It would shift ok driving around slow but could not keep itself in OD. Could also be a fualt in what ever driver feeds the tachometer its signal. Without that Tach the E4OD won,t shift correctly. Just my .2 cents worth. hope this helps

[badtruck]

I think you nailed it, thanks. where exactly would you start to chase this problem

[zspear1]

I found that one by tracing the wire harness from where it plugs into the tranny, and following it till I found the burnt and chaffed wires. That Harness will be in a loom so you may have to look for signs of loom damage, I think the root cuase of this one's failure was becuase it had new tranny put in and the shop techs that did it did'nt check to make sure the harness was not touching the exhaust manifold. On these older trucks the wiring is almost always an issue in odd problems like this. Another way would be to unplug the Harness and check each circuit for resistance with an ohms meter.

[badtruck]

Well, I found the general area of the problem by having a buddy wiggle wires while I watched the Tach for any movement. Turns out the break or short is under the hood just on the other side of the firewall from the steering wheel. There are three harnesses located here as well as a relay ( I think for the lights ) and what looks like some type of sensor.

Would I be best taking the truck to an auto/electric shop.

Thanks,
Jeremy

[zspear1]

Yea you might do that they will test those circuits and find which one is the culprit, and fix the short. ON the one I fixed I just replaced the harness, but I knew it was bad. Your short could be loose fitting or loose or coroded ground as well. A good auto electric shop should know how to fix.