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Engine will drop to idle when driving at highway speed. Accel pedal is unresponsive. After short time everything is back to normal. CEL is on and code P0121 is stored. This is intermittent and follows no pattern. Any help would be apprieciated. Thanks Donald
 

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A cousin had that problem and it was the injection pump, It happend to him a few times far from home but it would eventually make it home...
 

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2nd Gen Guru
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In my case the P0121 error code was my APPS and it had failed... So I ponied up the $400 bucks and got a new one from the dealer...

Problem gone...
 

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gone fishin!
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im not a 12 valve guy in any way, but a buddy of mine had that same problem and he replaced his tps and apps and nuthing changed, he replaced his vp44 and nuthing changed. he got his computer reflashed and nuthing changed. someone told him to un hook his electrcal conection to the altonator and see if it helped. low and behold it ran like a top so he replaced his altnator and it solved all his problems
 

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That PO121 code is usually a good indicator that it is the APPS(Accelerator Pedal Position Sensor). When the electronics on the VP burn out it acts the same way, but doesn't throw that code and usually doesn't throw any code. The APPS will always throw a code.

As for the alternator comment, that could be because the alternator was running too low and everything else was draining the system and nothing was seeing proper voltage.
Sean
 

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it actualy was charging fine. it was a very bight electrical engeneere that works for johndeer that told him to do that. he sayed there is a reverse power relay in the altenator that keeps the noise generated by the alternator from feeding into the ecu. i think the ecu actualy controls the charging and voltage regulating of the alternator. its that noise that kinda scrambles the computer kinda like holding a magnet against a computer screen. i guess thats what makes it mess up. all i know is thats what solved the problem.
 

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its worth a shot i think. it doesnt cost anything to unhook the the alternator and try it.


oh by the way my buddys truck was throing the apps code to. it was also throwing a ecm failer code.
 

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Thats interesting. I've heard of that alternator thing messing with lock up control on the automatics, but never with an APPS issue. Very good to know.
Sean
 

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yea his would be going down the road at like 60 65 mph and all of a suddon it would just go to idle and after a few seconds it would come back. and would keep doing it randomly. then it started idleing rough and missing realy bad when lugging when the tq converter was locked in od. he replaced everything and spent a couple grand trying to figure it out. he said the older JD powertec's with the bosch vp44 injector pumps would do the same thing. espetialy the ones that are exposed to extream sunlight and rain. like in a statinary engine. and thats all it is is the alternator.
 

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If floridaboy's theory is true then I would suggest getting a ScanGauge II and setting on gauge for Volts and check the voltage as you traveled to see if the alternator is keeping a 13.5 to 14.5V range. You will be able to check for code on the fly too to see if the voltage and CEL light are together... You might even check to see if there is a weak connection at the ECM and APPs that might be marginal and when the voltage falls that the APPS code is thrown...

I got to admit the grounds and wild highs and lows voltages can do some really weird thing with a Dodge Cummins (or even a Dodge gasser)

Good tool to get...
http://mopar.mopar1973man.com/2002 Dodge/BOMBS/scangauge2/scangauge2.htm
 

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mopar man, im not sure if its even wild voltage spikes. its more like interferience. i think it would be something that would show up more on a ocilascope than a volt meter. i think its more like static feed back that gets it. i know that all rotating stator devices like motors or generators or alternators will create electronic noise. there is a way to eliminate or kinda quiet it down with like shoky diodes and ceramic suppresors. im not sure what they use in the 24 valve alternator, but what ever it is it failed and was alowing some sort of feed back to come back to the ecu and scramble it.
 
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