Received a P0123 before and after changing a dying VP44. Before and after VP truck would buck and surge violently after some time of driving. Approx 10 minutes and the throttle has a life of it's own. Continue to have problem after VP44 swap. Was able to swap the TPS/APPS with my buddy's truck and have the same problem with a good TPS/APPS. So I have ruled out the TPS for the moment.

Here is the funny thing, my temp gauge craps out at the same time my throttle gets possessed. IF the gauge is reading temp the throttle works fine. If the gauge is not reading, throttle is fubar'ed.


Questions --
Has anyone had a similiar problem and see it tied to the temp gauge?

Is there a schematic of all of the grounds on the truck somewhere? Seems like I should check all of those.
Any and all help is greatly appreicated.

1998.5 24V 5 speed with 150K, BD brake, 1.6 EDMs.
Jason

 

if im thinking correctly,the iat sensor and temperature sensor are what inputs the computer looks at to determine timing......it seems that you should try another temp sensor to me.when its wigging out,ill bet its changing timing erractically and causing this..........bama

 

OK, Problem has escalated and now when I turn the truck on it runs away (redline).

If I unplug APPS sensor, truck starts and holds idle fine.

Checked my grounds and I have 0 ohms with truck off on Pins 1 and 4 which are Battery voltage for the idle switch and APPS ground. Does this sound normal? Of course the APPS ground should be ground but what about pin 1?

When I turn the key on, but truck off, these see 46 and 45 ohms respectively. With key on and truck on they both see 85 ohms. Another dumb question but why the continuity change with truck on?

Also checked voltage on terminal #5 - APPS supply voltage and have a constant 5.07 +/- .01 volts. Key on truck off or truck on sees this same voltage.

Any insight greatly appreciated. My manual states to replace APPS at this point. Did that yesterday with a borrowed APPS (from a 99 24v Auto - note mine is 98.5 manual) and still had the same problems. Do not think it is the APPS.

 

Oh yeah, coolant temp sender is now acting normal...

 

Please help. Could this be an ECM issue??
Have I not done a good job of explaining what is happening? Is there something different I should be doing??
I was hoping for more of a response from DTR land...

 

My two cents. Do take it for face value becuase his is a unique incident. I’m looking at a 2000 Service Manual not a 98.5. I hope you have the time, patience, and tenacity, for troubleshooting this one. You’ll need good (factory) wiring diagrams.

I don't think it is the ECM. The truck starts, it records codes, and hell, it probably gives you the nice chimes when you turn the key. I think it is the engine wire harness around splice 165 or the ECM plug is loose or has been tamper with. I don’t know where S165 is on your motor but trace it down from the water temp sensor, and find where it join into the main loom on the way to the ECM plug. My book says S165 is a splicefor 12v distribution.


The DTC (code) indicates overvoltage on the APPS feedback P0123. The ECM tells it as it sees it. So ,pin 3 of the APPS has been detected as out of range. This feeds pin 25 on the ECM.

When the truck accelerated to WOT at start up it would mean the ECM saw that signal. It could have even saw more voltage than the 5.0V, thus the DTC. The thing is, the ECM thought it saw wide open throttle and commanded the VP to do it. Even though you rung out the APPS, the control signal could still be corrupted downstream. I think the bucking and phantom fueling is a transient short on the APPS sensor signal wire on the run to the ECM.

Both the APPS and the W/T sensor go directly to the ECM.

The water temp sensor also sends the ECM data directly. It also joins in at S165. Is it a coincidence the water temp gauge went to cold when the motor acted up? I’m doubting it. I thinking the wire harness. My WAG: What if the water temp sensor is sending a voltage to the APPS control voltage somehow inside the wire bundle, or that S165 is breaking down.
When you pulled the APPS the idle validate switches float high and probably defaults the ECM to an idle run. On the APPS pin#1 is the supply voltage for idle limit switches. Resistance value is probably meaningless off that pin. You could perhaps monitor voltage on pin #3 which is the APPS sensor signal. Do it with the APPS unplugged and motor running in idle and see what you get.

Check the entire wire run from the APPS and coolant sensor to the ECM look for any anomalies in the harness. Also check the security of the ECM plug. Everything has to be clean and tight.

 

Nmen -
Thank you for the time and the well thought out response. Impressive. I will report back.

I have given myself a window to try to figure this problem out and then it is off to the DRB at the, dare-I-say-it, dealer...

Again, thank you,
Jason

 

Thanks. Anybody can sit home in bed sick and purport all kinds of great answers. I even had a fever when I wrote it.

You're the one that will have to find the real answer. Let us know.

 

After some more reading I chose to have it taken to the dealer today. They said the ECM (P0606) looks to be toast.
I think I am going to change this myself as they are quoting a hell of a lot of money for one...
Thanks for the input here. Hope this solves the problem.
Jason

 

Bob Wagner overnighted an ECM to me. Hellava guy. I swapped it out in 30 minutes (dealer wanted to charge 3 hours labor) and it seems to have fixed the problem!!

 

Very interesting when I replaced my ecm it didn't change a thing still bucked at the 1100 to 1275 rpm and had little power. What I saw on an oscilloscope was very bad voltages , but not from the alt or batteries. Oh well you can't expect much from re&re boys anyway.

 

Neveragain -
Have you gone through all grounds on your truck?

 

Glad to hear the replacement worked. So what was the nominal cost of the repair when you did it yourself? (If you don't mind me asking)

And did the truck latch and post a P0606 code?

 

Truck threw P0606 for dealer. Dealer wanted 1700 for the ECM and another 3 hours labor to change it. Bob W overnighted one to me and he said if it works, he gets $500 for an ECM. Sounds like a great deal to me...
I owed the dealer for the 1 hr of their time ($85) plus a tow each way.

 

Thanks for the info Gies,

You've said it all.

Nmen