Code P0753 troubleshooting

[edcasey]

This is a writeup I did on another forum last year. I've pretty much abandoned other diesel forums so I thought I would post it here.

A few weeks ago my truck suddenly decided to stop shifting and got stuck in 3rd gear (limp mode). It wouldn't up-shift or down-shift. I was able to manually down shift to second but it wont shift automatically and it couldn't be manually shifted to first.

I checked for codes and got a P0753 which according to the manual is:

Trans 3-4 Shift Sol/Trans Relay Circuits - An open or shorted condition detected in the overdrive solenoid control circuit or Trans Relay Circuit.

I started threads on two forums asking for help troubleshooting the problem. I got several responses including replacing the overdrive solenoid, the governor pressure solenoid, pressure sensor, PCM.... I really hate throwing parts at a problem so I decided to do some research and start troubleshooting. I'm definitely not an expert at automatic transmissions but I have been repairing electrical problems for 20+ years. Using the 2001 service manual and information I found through Google searches I was able to fix the problem myself.

What the P0753 is basically telling you is the electrical circuit responsible for shifting the transmission into overdrive isn't working. The circuit is actually very simple. Battery power goes through a 20 amp fuse to a relay than to the overdrive solenoid in the transmission and than to the PCM. The PCM controls the overdrive solenoid by turning on and off the ground allowing current to flow.

You, of course, want to check the fuse first. Than swap the transmission relay and the horn relay (they are identical) and hit your horn. If it honks, than the relay is good. The next step is to check the circuit after the relay. You just pull out the relay and insert one probe of your multimeter into the slot shown in the picture. This is the slot that sends power to the transmission.

Next, you want to remove the center (white) connect on the PCM. The PCM is located on the firewall behind your air cleaner. Just push down on the tabs that are located on top of and under the connector and pull the connector out.

Turn the connector over to see the pins (holes). They are marked with small number. Touch the probe to pin 21 but be careful not to jam the probe in and damage the pin.

Set your multimeter to Ohms and get a reading. It should read about 31 ohms. It shouldn't read open like mine does.

This tells me my overdrive solenoid circuit is open some where. It's more than likely the solenoid itself but could be the wires or the connector on the transmission. My next step is to remove the transmission pan and filter. The overdrive solenoid is located under the filter. The filter is held on with two T-25 screws.

The govenor solenoid body needs to be removed first. Its held on with four 11mm bolts and two T-25 bolts.

The overdrive solenoid is actually part the of the transmission control solenoid assembly which also contains the torque converter clutch solenoid. The transmission control solenoid is held on with three T-25 bolts. Also, disconnect the wire that goes to the connector. There are also two zip-ties that need to be cut off.

I wanted to see exactly what was wrong with the overdrive solenoid so I removed the black plastic cover that's over the solenoid coil and the problem was obvious. The coil wire was broken.

I got a new transmission control solenoid from a local parts store for $111.00. I'd hate to see what a Dodge dealer would want for it. The new one also comes with the wires.

I tested the new part just to be sure. I'd hate to get it back together and find I have a bad part. The overdrive solenoid and the torque converter clutch solenoid are identical and both tested good at 31.4 ohms.

I put everything back together. I used the old wiring harness because replacing it requires removing the valve body. I couldn't see doing the extra work to replace a harness that is good. The final steps included a new filter and 8 quarts of fluid. I took it out for a test drive and it works perfectly. Total cost was $153.87.

Just a few additional notes on this problem:

When the PCM detects a short or open circuit in the overdrive solenoid circuit it shows a P0753 code and places the transmission into "limp home mode". What that means is the PCM shuts off the transmission relay so no power goes to the transmission. This is actually what causes the transmission to stick in one gear. This caused me some headaches while troubleshooting this problem. If you look at the wiring diagram below, you'll see the transmission relay secondary circuit receives power from the PCM (through the same wire that provides power to the alternator field winding) and than the relay power returns to the PCM for a ground. When the PCM detects a P0753 it shuts off the ground for the secondary circuit. This stops the transmission relay from closing and the transmission goes into "limp home mode'.

The other thing that caused me some confusion is the 2001 service manual shows pin 86 on the transmission relay as receiving power from the PCM and pin 85 being the ground back to the PCM. This is backwards from my 2000. I don't know if it's a mistake in the manual or if they switched it for 2001.

Good luck with your repair. I hope this helps if you ever have a P0753. __________________

Edited February 15, 2010 by edcasey

[gassernomore]

Great write up! :thumbup2:Hope Mike puts this in wih the tips section!

[Mopar1973Man]

Great write up! :thumbup2:Hope Mike puts this in wih the tips section!

It got my vote for the tip section... Very well done with the pictures and write up! Give me some time and I should have it up soon!

[Guateman]

Here in central america hard to find parts we will repair the solinoide. with new coils. Just the wire cost...any way congratulation nice work

[Mopar1973Man]

Thank you edCasey... Your great write up has been added to the archives of the M73M.com site... http://mopar.mopar1973man.com/cummins/2ndgen24v/trans/trans.htm Hopefully see many more great write ups from you!

[edcasey]

Thanks Mike! I'm about half way done with a write-up on installing a Walbro 392 fuel system including the pluses and minuses of such a system and the parts necessary to put it together. Unfortunately, it's been slow going because I have been very busy at work and haven't had much free time. I'm also thinking of doing a write-up on the physics of a fuel system or more specifically the flow of a fluid. I studied fluid dynamics many years ago and with a short review I should remember the math involved.

[dually]

THANKS ! Your hard work will save everyone time and money.I love hanging out where the smart people are.Hope it all rubs off and stays.

[Mopar1973Man]

Thanks Mike! I'm about half way done with a write-up on installing a Walbro 392 fuel system including the pluses and minuses of such a system and the parts necessary to put it together. Unfortunately, it's been slow going because I have been very busy at work and haven't had much free time. I'm also thinking of doing a write-up on the physics of a fuel system or more specifically the flow of a fluid. I studied fluid dynamics many years ago and with a short review I should remember the math involved.

I'm all ears...

THANKS ! Your hard work will save everyone time and money.I love hanging out where the smart people are.

Smack the tanks on EdCasey post... I'm sure he'd enjoy that...

[BluePine]

Thank you edCasey... Your great write up has been added to the archives of the M73M.com site... http://mopar.mopar1973man.com/cummins/2ndgen24v/trans/trans.htm Hopefully see many more great write ups from you!

These are some of the best transmission pics I have seen so far! The close up detail and labeling saved me a ton of time trying to get a "visual" of what is actually going on... I have been reading up on transmission issues, but since we don't have a garage at our house and it has been SOOO cold I haven't been able to even take a look under the truck, and since I have never messed with any transmission it is a lot easier to understand when I can look at your photos to "connect the dots"! Honestly I really wish there was a picture database, where you would be able to swap/ post photos that were taken similarly so that people like myself could get a better idea of what exactly is under all the "covers & case" in order to more accurately asess & plan a troubleshooting/ repair project.... This forum is a great start, it's the best resource I have found! Just hoping that more people will continue to photo document like this... THANK YOU, THANK YOU, THANK YOU:thumb1:

[Utah Dually]

If you do a search on the Walbro 392 you find a lots of pros and cons on this pump. There is one thread on Diesel Bombers about how long did the pump last… The answers ranged anywhere from one day to a couple of years. Sounds like the upside is that this is a lower cost alternative to AirDog and Raptor pumps. They are cheap enough you could buy two and easly change them out. The down side this there is no kit yet by an supplier. My question is how do you hook up the momentary running of the pump when you engage the starter? Or do you have to hard wire this pump to an on/off switch? You would have to buy a fuel regulator by pass which is no big deal. Put a 80 dollar real filter in front of it, use the stock filter behind it and you might have a low cost alternative if the reliability is there.

[ButterBean]

Hey Mike do you remember the pins you tested on the solenoid? I cant tell from picim guessing this is the same problem im having with the P0753 code. i have no 1st or over drive. I have to take off in 2nd then manually shift up into 3rd.but something tls me its before the trany like a short im not getting but 0.05v at the red wire on the 8 pin wring harness at transmission. im guessing I should be getting 12v herethe relay is good.