DTC P0121 Testing I dont understand could someone help/explain

[codie2379]

1. Turn ignition off. Disconnect APPS harness connector. APPS is located on a bracket, just above fuel injection pump. Start engine and allow the engine to idle. Using DVOM, measure voltage at terminal No. 5 (Dark Blue/White wire) on APPS harness connector. If the voltage is greater than 5.5 volts, go to next step. If the voltage is 5.5 volts or less, go to step 3.  I get 5.6

 

2. Turn ignition off. Turn ignition on with engine off. Using DVOM, measure voltage at terminal No. 5 (Dark Blue/White wire) on APPS harness connector. If the voltage is greater than 5 volts, repair short to voltage on Dark Blue/White wire between APPS and ECM when the ignition is on. ECM is located on driver's side of the engine, just in front of fuel transfer pump and contains a 50-pin connector. If the voltage is 5 volts or less, repair short to voltage on Dark Blue/White wire between APPS and ECM when the engine is running. I still get 5.6 and I dont under stand what to do? I don't understand this statement " repair short to voltage on Dark Blue/White wire between APPS and ECM when the ignition is on."  Repair short to voltage? I don't get  what this means to do please helps

 

 

3. Turn ignition off. Using DVOM, measure resistance between ground and terminal No. 4 (Black/Yellow wire) on APPS connector. If resistance is 10 ohms or greater, go to next step. If resistance is less than 10 ohms, replace APPS. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. I get 39 ohms

 

4. Start engine and allow the engine to idle. Turn ignition off. Turn ignition on with engine off. Using DVOM, measure voltage at terminal No. 2 (Light Green/Dark Blue wire) on APPS harness connector. If the voltage is 5.5 volts or less, go to next step. If the voltage is greater than 5.5 volts, repair short to voltage on Light Green/Dark Blue wire between APPS and ECM. ECM is located on driver's side of the engine, just in front of fuel transfer pump and contains a 50-pin connector.    I get 5.2

 

5. Turn ignition off. Disconnect ECM harness connector. ECM is located on driver's side of the engine, just in front of fuel transfer pump and contains a 50-pin connector. Using DVOM, measure the resistance of Black/Yellow wire between terminal No. 4 on APPS harness connector and terminal No. 32 on ECM connector. If resistance is 10 ohms or greater, repair open in Black/Yellow wire between APPS and ECM. If resistance is less than 10

[Guest 04Mach1]

@codie2379 What was the result of step 5 of the troubleshooting?

 

Also I've seen idle validation issues like this on the Cummins CM871 ISX engines and the fix was a new ECM. 

[codie2379]

#4 APPS to #32 ECM has hardly any resistance but I don't believe that the ECM is bad yet!!! although it very well maybe!  if some could explain to me what it means to repair short to voltage on Dark Blue/White wire between APPS and ECM when the ignition is on."  Repair short to voltage? I don't get  what this means to do please helps if I could complete this step then I could have confirmation that the ECM is indeed bad. maybe mopar1973man could explain  

[Guest 04Mach1]

@codie2379 it will involve taking the harness loose from the engine and taking the loom off the wiring then look for rubbed, cut, or corroded wires. Of course you will make the repairs with the ignition off and preferably batteries disconnected. If you happen to find extensive damage like multiple wires to the harness once the loom is off I strongly urge a new harness as the plastic loom greatly damages the wire coating due the wires vibrating against the inside of the loom leading to chafing all the way through the wiring. Manufacturers like Cummins, Detroit, and any heavy duty diesel no longer loom bare wiring, instead they wrap a very strong cloth tape around the wires that is abrasion resistant then will sometimes loom the harness over the tape.

 

I usaully skip all the trying to find the short "BS" and overlay a new wire of the same gauge from pin to pin, in your case ECM to APPS.

Edited April 20, 2019 by 04Mach1

[Royal Squire]

Anything over 5 volts is too high?

[codie2379]

 I get what you are saying and understand, but Step 2 in  the procedure says to repair short to voltage on the same wire, repair if its above 5, repair if its below 5, only difference is whether the ignition switch is on or truck is running. What supplies that 5 volts or where is it regulated

 

1. Turn ignition off. Disconnect APPS harness connector. APPS is located on a bracket, just above fuel injection pump. Start engine and allow the engine to idle. Using DVOM, measure voltage at terminal No. 5 (Dark Blue/White wire) on APPS harness connector. If the voltage is greater than 5.5 volts, go to next step. If the voltage is 5.5 volts or less, go to step 3.  I get 5.6

 

2. Turn ignition off. Turn ignition on with engine off. Using DVOM, measure voltage at terminal No. 5 (Dark Blue/White wire) on APPS harness connector. If the voltage is greater than 5 volts, repair short to voltage on Dark Blue/White wire between APPS and ECM when the ignition is on.  ECM is located on driver's side of the engine, just in front of fuel transfer pump and contains a 50-pin connector. If the voltage is 5 volts or less, repair short to voltage on Dark Blue/White wire between APPS and ECM when the engine is running.

[Guest 04Mach1]

26 minutes ago, codie2379 said:

 I get what you are saying and understand, but Step 2 in  the procedure says to repair short to voltage on the same wire, repair if its above 5, repair if its below 5, only difference is whether the ignition switch is on or truck is running. What supplies that 5 volts or where is it regulated

 

1. Turn ignition off. Disconnect APPS harness connector. APPS is located on a bracket, just above fuel injection pump. Start engine and allow the engine to idle. Using DVOM, measure voltage at terminal No. 5 (Dark Blue/White wire) on APPS harness connector. If the voltage is greater than 5.5 volts, go to next step. If the voltage is 5.5 volts or less, go to step 3.  I get 5.6

 

2. Turn ignition off. Turn ignition on with engine off. Using DVOM, measure voltage at terminal No. 5 (Dark Blue/White wire) on APPS harness connector. If the voltage is greater than 5 volts, repair short to voltage on Dark Blue/White wire between APPS and ECM when the ignition is on.  ECM is located on driver's side of the engine, just in front of fuel transfer pump and contains a 50-pin connector. If the voltage is 5 volts or less, repair short to voltage on Dark Blue/White wire between APPS and ECM when the engine is running.

Your 0.6 volts over the threshold of 5 volts so the repair will be the dark blue / white wire between APPS IF there is indeed a wiring issue. My curiosity is what is the dark blue / white wire voltage at the ECM?

 

My bet is a bad ECM or possibly a bad APPS. Rarely have I seen a short cause over voltage I've replaced countless Cummins ECM's due to internal voltage problems and incurable fault codes and the replacement ECM always cured the faults. Do you know anyone with a truck near the same year that you could borrow their ECM to test. I personally wouldn't throw an ECM at it without confirming that the fault would still exist with a known good ECM.

[codie2379]

When I get home from church I’ll look at voltage coming out of ecm, what was confusing me is something you committed on, I have never seen a short cause over voltage. Thanks for you help haller back with results