My Mechanic has given up

[thomashill1]

I have a 2012 Winnebago Class C (ford f450 w/6.8L gas. I bought it used about 8 months ago and have had an ongoing issue with it. When driving at highway speeds while pressing the accelerator if you go over a really rough patch of road the wrench light will come on and the the coach will go into limp mode. (no power, just enough to pull to the side of the road). I can than turn the engine off, wait 5 seconds and turn it back on and all works again.

If I see a rough patch coming up, (big potholes, or bridge expansion joints will normally cause this) than I can pull my foot off the gas pedal and glide over the rough road and then reapply gas and all works fine.

this issue has happened to me at least 20 times. During that time, there are never any codes to read, (once you restart the rig the wrench light goes away and so do the codes). However, there were two times the the engine light also came on and both times it indicated that there was a issue with the throttle body.

I have replaced the throttle body twice- the first time was an aftermarket, it did it again, and they put an original ford part on it the second time. but the issue still happens.

Just for other information, we have already done a complete tune up, Plugs and coil packs and the transmission was replaced with a Jasper rebuilt transmission. In addition, we have already wiggled every ground wire we can find looking for a bad ground.

I am at a loss at this point, any other ideas what can be done?

[Flyer15015]

Throttle Position Sensor ?
The bumps are jiggling your foot, and it does not know what to do ?

Mike in Colorado

[winnbrave1]

IMHO, it has to be a pedal/throttlebody issue. Did the mechanic eliminate them?

[winnbrave1]

Looked at your profile to see where you are located, but nothing there.
I know of one authenticated case where the crank sensor/harness was bad and done virtually what you decribe.

[SergioP]

Originally Posted by winnbrave1

IMHO, it has to be a pedal/throttlebody issue. Did the mechanic eliminate them?

That’s what I’m thinking. Either a bad connection at the pedal or in the pedal electrics. With a good scan tool you should be able to see the pedal voltage and at the throttle body. The wrench light is for the electric throttle system only, if the TB has been changed twice, the pedal or wiring are the only things left.

That’s assuming it has throttle by wire instead of a cable.

[thomashill1]

Thanks. Let me investigate the pedal and it's harness.

[sibe]

Be care full. Grab Harness and start to wiggle up/down as a bumpy road..

Try the main harness from ECM, then the harness to engine etc.. It can be a crack also on ECM board..


my old rant..

I had a tranny shift in and out of OD and then go into 2nd gear limp. Drove me nuts for months,, Almost replaced the tranny, after 4 mechanics looked at it..

My sons motorsports/auto tech instructor said tell you dad check the vehicle speed sensor circuit.. Sure enough.. I wigged the harness up and down the frame.. I got the speedo to then not work and shifting was poor..
I spend some time and found the sensor wires frayed in harness and shorted together and one stray copper strand would hit another frayed ground wire...

My point was we had ABS codes and fault,, PSOM (speed module) errors then besides the shift solenoids/OD codes.. ALL from bad wires,,

[Mark_K5LXP]

Do you have a code reader? Leave it plugged in and when it happens, pull codes before you turn it off and restart. I use torque running on a tablet anytime I'm driving as a gauge display and have observed numerous pending codes while underway that disappear with a restart.

Mark B.
Albuquerque, NM

[Unplanned Tourist]

I have a W24 Workhorse with the 8.1 Chevy, but I'm sure a ScanGauge II would work the same for your Ford.

I can set it to display throttle position in real time. If you could see what the engine control module is seeing, then you can look for irregular inputs as you drive. Like others have said, possibly the TPS or wiring problem.

[bigb56]

Any wiring harnesses that cross the engine and are bolted down? If so they can chafe and cause a short. This is normally on fairly higher mileage engines though. The hole in the wire insulation can be very tiny, almost microscopic, I have seen it on the Ford/Navistar 7.3 engine harness where it crosses the valve cover.

[77Travco]

Originally Posted by thomashill1

During that time, there are never any codes to read, (once you restart the rig the wrench light goes away and so do the codes).

A cheap code reader can be more aggravating than helpful. Stored codes do not go away that fast. You need a better diagnostic tool such as ForScan on a laptop so that you can actually see the individual modules.
You can get a Forscan license for cheap... I think I paid less than $10 last time. It's a very powerful tool on a laptop but be sure to create a backup file before changing parameters within the logic.
Either that or find a competent shop.

[L-M-L]

Something to try is to take a screwdriver and use the handle end to tap around at the throttle body and at the accelerator pedal sensors. Another thing to tap on would be the PCM. You want to give it a hard enough tap to jar it some but not hard enough to beat it up. Also have the engine running when doing this so if a problem shows up you will know it. In the early 80s when they first started using computers in cars and trucks, we had a lot of problems with them, and this was one of the ways we tested them. It could be a lose connection at the throttle body or accelerator pedal sensor. Lightly tap at the connector or try shaking the wiring harness, and grab the connector itself and wiggle it, then push and pull on the connectors. Problems like these can be very hard to trace down and sometimes it's just a matter of trying to tighten the wiring connectors so they fit tighter to the sensors or PCM.

[thomashill1]

Update. Went back to shacking cable harnesses. Found that a thick 12v supply line that feeds the main fuse box in the engine compartment was cutting into metal frame and actually sparked when I wiggled it hard . It did not throw the wrench light when it sparked, but I have to think it has something to do with it. I have covered the broken insulation in electrical tape. And pulled the cable up in a different direction so it no longer comes in contact with that piece of metal. Have not had a chance to drive it on a bumpy road, but here's hoping this takes care of it.

[SergioP]

Wow. That’s a great find! Being a 12v feed to the fuse box it can affect a bunch of circuits causing all finds of weird malfunctions.