Throttle dropping out while under power

[DieselB59]

Just wanted to see if anyone has had this happen to them yet. My 2003 2500 HO with 48RE just started to do something funny tonight on the highway. This happens at any speed, light or heavy throttle. While just using the throttle to keep the truck at speed, the engine drops to idle instantly, almost like the truck just shuts off, but it returns to idle. I was thinking APPS, but then noticed that every time this happened, the square indicator on the D, 2, or 1 (whatever the shifter was selected into) would blink or just turn off all together, kind of like when you're in between gears, which is making me think it's the PRNDL/neutral safety switch intermittently dropping out. I have no codes whatsoever, and no check engine light. What does everyone think?

 

[rbattelle]

Yikes, that'll be a nightmare to diagnose! Gotta be an electrical bug in the transmission shift system, as you mention.



-Ryan

 

[DieselB59]

A while back, I had a 2003 3500 with 48RE come in with a transmission leak. Dealer had just replaced the recall transmission line. The PRND21 indicator would not register being in any gear. The switch on the trans was broken from bad use of a prybar by the dealer and we replaced it for the customer, upon restart, all indicators worked. I'm thinking that since we've had a whole lot of rain, and it was raining tonight on the ride home when this all started, possibly something got messed up in that switch. I think I'm gonna try and replace it tomorrow if one's available. I'm beyond 36,000 miles, so the famous 100. 00 deductible applies, so I'll try and hit that one at my cost.

 

[DieselB59]

I got under the truck tonight to replace the switch/sensor, and when I unplugged it, the electrical connection was full of transmission fluid. I decided that it would be best to do this along with a transmission service, so I dumped the fluid and changed the switch/sensor unit out. ATF was leaking out of the pins like a bad oil pressure switch leaks. That may very well be the culprit.

 

[rbattelle]

Glad to hear it was an easy find after all...



-Ryan

 

[DieselB59]

I did the trans service and took an extended test drive with it, and so far... so good!

 

[DieselB59]

The problem showed up again tonight. I noticed this time that the park and neutral indicators are the only ones that will light up when the problem is occurring. R, D, 2, 1 all do not exhibit an indicator when the respective gears are selected. Shut the truck off and restart and the problem showed its face for a split second then went away for the entire ride back to my shop where I left it for the night. Funny thing is... tonight was the first big rain we had since last time and although I only drove it three blocks before it went haywire, I think maybe the rain has something to do with it. This is starting to become extremely frustrating and is now bordering on unsafe to drive.

 

[jjdiesel]

Ecm?

I was thinking possibly ECM... but then you would likely get a code

 

[DieselB59]

Brought it in to the local dealer with a very detailed list of how and when and what I've looked at, just to see if they'd look at it with the DRBIII scanner and call the star hotline about this. Turns out, they called me later on that day to tell me that star wanted them to replace the ECM. So, I just got it back on Friday. We'll see what happens now.