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Old 10-23-2016, 08:53 AM   #1
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ISC no-go Sometimes problem???

1999 Cummins 8.3 ISC 330hp appx 75k miles on a Spartan chassis. For the first time ever after running the hills (55-63 with proper gear shifting) in 86 degree heat with the engine AC running and the coolant temp @180 (+10 / -0 while running the hills) we came to a stop at a rest area and idled for 8 min to do a potty stop. Upon trying to leave the electronic gas pedal had no effect and no forward motion or engine reving was possible. No fault lights and all service has been completed just no go but starts and idles fine. After sitting with engine off for 1/2 hr making calls we started and had throttle and forward motion. 1 week later similar situation after coming down from the mountains (90 degrees and 5.5k to 3.5k drop in elevation), exit to frontage road, come to the stop, try to go and nothing but idle for the second time. Wait 10 min with engine off and all works again. We have run similar/same routes last 3 yrs with no problems. Any help would be appreciated, including a reputable Cummins place in Tucson, AZ. Thanks for reading.
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Old 10-23-2016, 10:46 AM   #2
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Can't remember the name of the part but, there is a rheostat device under the accelerator peddle that sends an electronic signal to the engine to tell it how much fuel to give the engine. Mine failed on my 04 in just the same manner that you describe. Then shortly after buying my 08 I was messing around under the dash and accidentally unplugged mine. Would start but no go.
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Old 10-23-2016, 10:49 AM   #3
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TPS

I think you need to take a close look at your Throttle Positioning Sensor (TPS) as being the culprit in this problem.

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Old 10-23-2016, 10:58 AM   #4
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I had the exact same symptoms. Ended up being the HEUI pump. Of course I had a C7.
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Old 10-24-2016, 09:09 AM   #5
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Thanks everyone for the help so far, will update here after I replace the Williams Controls TPS. Off to get one once located.
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Old 10-24-2016, 10:16 AM   #6
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You have many TSB's on your 99 MADP may want to check them out, if this link does not work because of time out follow the directions in this link.
Some are recalls if they apply to your chassis.
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Old 10-24-2016, 10:24 AM   #7
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Had that kind of problem with the '02 DSDP 4090 Cummins ISC. Each time it was the exhaust brake sticking, But I did have some throttle control, just no power or forward motion.
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Old 10-24-2016, 04:30 PM   #8
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Thanks for the info MR_D more to consider as I do use the exhaust brake going down the hills and exit ramps. Even when I was in Neutral the pedal would not respond to any higher rpms. Was that you case as well?
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Old 10-27-2016, 01:27 PM   #9
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Update, rig is in the shop now unfortunately.
Prior to fault code reading (min $200 just to read the codes and didn't want to go out on the road yet) I replaced the throttle position sensor (Williams Controls part #131973 $79.00) in hopes to fix the problem. Yes it was a shot in the dark. Test run the next day left us hanging in the center turn lane of a 5 lane city street 1/4mile away from our start point and yes we were very stressed. 1/2 hr with flashers on and a cool off restart allowed us to pull off on to a safe side street while we called for mobile service as hoping the codes were present and yes they were.

Pulled (2) fault codes, (1) was for 2 relays that control the intake grid heater (this one has been there and admittedly a low priority, bad me) and (1) was for the idle validation / throttle position signals going Hi/LO to the engines ECM with 96 occurences starting 10 days ago, which makes sense.

My no-go problem is now defined as 1 of 2 things being an inconsistent short or open circuit in 1 or more of the 6 wire throttle harness wires going thru control boxes back to the engine ECM or the ECM has gone bad which is highly unlikely. Another slim possibility per a mechanic is slop in the sensor area of the pedal. This fault causes an engine de-rating back to idle thus no forward motion (my sometimes problem)

Coming out of the TPS plug at the pedal is a wire harness with 2 sets of 3 wires that terminate appx 2 feet away in the outside front firewall and each set of 2 goes to a 3 wire sealed plug and back thru the firewall inside the cab. Idle validation side is an on off type circuit and the TPS portion is a 5 volt feed idling at .65volts and wide open throttle should be 3.85volts.

Mobile tech and I reseated and dielectric greased the 4 sealed plug areas (which showed no corrosion). After this the fault had cleared but as before, after waiting 10min-1hr I could usually get going again so not convinced that fixed the problem.

This morning we took the risk of trying to drive the MH to the service shop (RWC Group in Tucson, AZ) 6 miles away from the camp ground by way of expressway. Did I mention we consider towing a very last resort and even hate to take it to a shop as I am very mechanical/electrical.

This mornings MH relocation produced no throttle fault codes and we made it to the shop under much stress of being stranded again. The shop is to push pull etc to attempt to locate the open/shorted wires with the aid of a code reader which we don't have.

Plan b if wire issue cannot be found will be to ask them to run 6 new wires between throttle and engine. Did I mention we are clean freaks and are still nervous about releasing our coach to a grease monkey while we wait in the lounge. Please wish us the best.
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