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Discussion Starter · #1 · (Edited)
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.
 

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Okay so I replaced my speed sensor on the back of the t case, also replaced the sensor on the rear axle, speedo still says I’m doing 90mph when I’m actually doing 55-60 an wont shift to over drive or lock up. I replaced the solenoids inside the trans am nothing, what would u suggest I try next?
 

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Okay so I replaced my speed sensor on the back of the t case, also replaced the sensor on the rear axle, speedo still says I’m doing 90mph when I’m actually doing 55-60 an wont shift to over drive or lock up. I replaced the solenoids inside the trans am nothing, what would u suggest I try next?
You may have already figured this out, but for future readers of this post, here's some help:

Sounds like you need a different number of teeth on your speedo gear, to slow it down a bit. You or the previous owner may have changed gear ratios or tire size and thrown it off. The most accurate way is to figure out mathmatically how far it's off by counting mileposts versus your actual odometer reading, over a 10-mile section of highway. I'm guessing your ODO is going to read something like 13 or 14 miles (30% or 40% over) in a 10-mile section...so you'd need to slow down the speedo gear by getting one with more teeth. Once you figure out how far it's off (let's say 35% for instance), you'll want to pull your current speedo gear, look for the stamp on it that says how many teeth it has (33 for instance), and then add 35% to that number. (35% of 33 is 11, and add that to the original number of teeth...33 + 11 = 44 teeth needed). Order one up online or MAYBE at the dealership if they have not discontinued them (should be about $20), pull the old one straight out of the housing it's in, and snap the new one back in again. Here's some images to help you out and make it make far more sense than my blatherings: speedo housing dodge ram at DuckDuckGo

I needed 37.5 teeth, so I rounded DOWN to make the speedometer read slightly high, rather than slightly low, so that I don't get pulled over for speeding. (more teeth slow it down, less teeth make it revolve quicker). I do a fair amount of offroading, and this ALSO makes the odometer turn slightly quicker as well...meaning that if I'm looking for trail mile 4.3 to make a turn...it comes up slightly sooner on my ODO so that I don't have to backtrack.

Hope that gets you taken care of!
=KOR=
 
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