No Bus code

Tiny
JOHN CARLTON2
  • MEMBER
  • 1999 DODGE RAM
  • 5.9L
  • V8
  • 4WD
  • AUTOMATIC
  • 112,500 MILES
So, I recently replaced a bad transmission with one out of another truck and I took the PCM for that transmission and put in. Truck ran fine for about three days no issues but then it stalled and didn’t want to start back after about 30 minutes it start up no issues then the next day it stalls again and read no bus and the codes read on the dash 920 and 921. Could I swap the old PCM back on to fix the problem?
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Tuesday, December 14th, 2021 AT 3:50 PM

5 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

I need to know if it is a 5.9L diesel or gas engine. If it is diesel, the first thing I'm going to suggest is to check the connector at the transmission governor solenoid. They are known to get loose and cause a communication error no bus 920/921. The solenoid is in the transmission right next to the filter. (See pic 1 below) If it is a gas engine, I need to know if the codes also include 999.

Let me know as much as you can. Also, if the PCM you switch has the same part number as the original one, switch them to see if that changes anything.

Let me know.

Joe
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Tuesday, December 14th, 2021 AT 7:05 PM
Tiny
JOHN CARLTON2
  • MEMBER
It’s the gas and it also says 999. I’ll try to switch them today and see if anything changes.
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Thursday, December 16th, 2021 AT 6:08 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

Now that I know it is a gas engine, I can tell you the issue appears to be between the PCM and the instrument cluster. Since the cluster was likely not touched, the PCM is my first suspect.

Take a look at the code description.

920 -- Cluster not receiving a signal from PCM

921 -- Cluster is not receiving a distance pulse from the PCM

999 -- Simply indicates codes are stored. If you hold the trip button down, it should go through a self-test.

Codes 920 and 921 are likely being caused by the PCM. It could need to be re-flashed or reprogrammed with updated information.

Let me know what you find when you switch the PCM.

Joe
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Thursday, December 16th, 2021 AT 4:57 PM
Tiny
JOHN CARLTON2
  • MEMBER
So, I put the old one back in and it’s not dying anymore but the overdrive off light came on and won’t go off and the transmission temperature came on as well but trans temperature is at normal temperature about 196-200. Could it be a sensor or a ground issue?
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Friday, December 17th, 2021 AT 3:38 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

If I had to guess, it is something with the communication between the PCM and transmission, but anything is possible.

If you look below, I attached the wiring schematic for the transmission circuit. First, note the PCM is grounded at the front of the engine (black wire with a tan tracer). Make sure it is clean, tight, and not damaged.

Next, the temperature issue and OD issue are separated by different grounds. The OD light switch is grounded via ground 200 in the left kick panel. The temperature signal is grounded via a black wire with a light blue tracer wire through the PCM.

The idea that there is a splice in that ground would be something to check. According to the schematic, it is at the rear of the engine, center, and near the injector 5 break out. Check there.

Let me know what you find. If there are no problems found with the wiring, you may need to have the computer flashed.

Joe

See pics below.
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Friday, December 17th, 2021 AT 5:03 PM

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