47RH Tranny Troubleshooting - Dodge Cummins Diesel Forum

94-98 Drivetrain (except engine) Tech articles dealing with the transmission, transfer case, axles, driveshafts, etc.

 
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post #1 of 1 (permalink) Old 12-14-2009, 01:25 PM Thread Starter
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47RH Tranny Troubleshooting

Courtesy of Art

The 47RH transmission was used on 1994 and 1995 Dodge Rams behind the Cummins and V10 engines, and is hydraulically controlled - for the most part. The transmission overdrive (OD) and torque converter lockup (LU) are actually electrically controlled by the Powertrain Control Module (PCM).

This troubleshooting guide assumes the transmission is still in healthy mechanical condition. Unhealthy symptoms include fluid that is no longer a clear red but now cloudy, brown or black, and/or having a burnt smell.

No computer can make a good decision with bad input. Bad input = bad output.

The OD and LU on the 47RH transmission are PCM outputs. The PCM looks at several sensor signals (inputs) in order to decide whether or not to engage OD and LU:

1.) Throttle position
2.) Brakelight switch
3.) Transmission temperature
4.) Engine speed
5.) Vehicle speed

The Throttle Position Sensor (TPS) is usually the the culprit, but not always. Perform this first: TPS testing and adjustment

If the brakelight switch was not working properly, it could also prevent LU. If the PCM thinks the brakes are being applied, it turns off LU. If lifting the brake pedal with your foot allows OD, then the switch and/or its adjustment are the culprit.

Transmission temperature... If the PCM thinks the transmission is either cold or too hot, it will not allow OD. The temperature sensor is located on the torque converter outlet (front cooler line) of the transmission. Its electrical resistance (Ohms) limits at the various temperatures are published in the service manual. With a warmed-up transmission (>176degF), the resistance should measure less than 1340 Ohms. Many people just connect a 1k Ohm resistor in place of the sensor to allow OD even when the transmission is cold. It does defeat the overheating transmission protection feature though, so keep that in mind if you do not have a trans temp gauge installed.

Engine speed - the Engine Speed Sensor is a magnetic pickup located at the front of the engine at the engines vibration damper. This sensor tells the PCM if the engine is running and just how fast. That input is used to control several things in addition to OD and LU. For example, the PCM also uses that signal to decide how to control the alternator, the cruise control, and the tachometer. Any malfunction of the ESS will cause several problems besides no OD or LU.

The Vehicle Speed Sensor could also be a cause, but then the speedometer and cruise control (if equipped) would act up as well.

If each of these components checks out fine, there is still yet another possibility. Keep in mind the PCM only sees what electrical voltages are on its connector pins. Any bad connection or short along the entire electrical circuit is just as bad as a defective sensor. Look for rubbing, frayed, or pinched wires. Check the connectors to make sure they are free of corrosion and any bent or pushed back pins. Dielectric grease is your friend, it keeps moisture and the resulting corrosion out.

Another troubleshooting tool is the Diagnostic Trouble codes stored by the PCM. Cycling the ignition key on-off-on-off-on within three seconds and then monitoring the Check Engine lamp will allow you to access the codes. These codes are identified and described in the Service Manual.

If you still made it this far and OD and/or LU are not working, try installing a PCM out of a truck that is known to be good. This should be the last resort.

Personally, I prefer to troubleshoot and replace only the bad parts -instead of blindly purchasing and installing parts hoping to stumble across the cure.


Bill
'95 2500 4wd auto
'95 3500 5 speed heavy hauler
Stock for one day

Last edited by illflem; 12-14-2009 at 01:53 PM. Reason: fix link
illflem is offline  
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