Engine/Transmission (1994 - 1998) Random kickdowns and temp light

[Howard Durand]

According to the service manual, the transmission temperature sensor is used to help control transmission fluid overheating. If the sensor sees a temperature of approximately 280 degrees F, an input is sent to the power train control module. The PCM will then force a 4-3 downshift. Once transmission temperature has cooled below alarm temperature, a 3-4 upshift will be allowed.

As the fluid temperature rises, the sensor resistance goes lower. If the sensor circuit becomes grounded, that would indicate a very high temperature.

Unplug the connector at the sensor and check both leads against ground with an ohm meter. The black wire should be grounded. The violet wire goes to pin 42 on the PCM and should not be grounded.

Since yours acts the same even with the sensor unplugged, the sensor is eliminated as the problem. Try it without the resistor and see if it still does the same. If so it would appear that the wire to the PCM is grounded somewhere or the PCM has a problem.

Pin 31 on the PCM controls the transmission temperature lamp. I wouldn't think that it could be part of the problem but with these things you really don't know which comes first, the chicken or the egg.

If nothing can be found I would try to get the PCM tested.

 

[Howard Durand]

One other thing to try. It won't cost anything. Disconnect both bateries and unplug the big cable at the PCM. Loosen the screw holding the plug in place. As you back out the screw, wiggle the plug and it will come loose. Locate pin 42 and mark the wire attached to it. You may yet want to cut it and tie it to ground through a resistor but first re-install the plug, re-connect the batteries and take it for a run. Let us know what happens.

 

[DavidWhite]

Any new news on this thread. I have the same problem; 4-3 downshift, then the trans temp light. I am going to go to a lock-up sw, synthetic fluid, and an aux trans cooler. Hopefully this helps. Already have a TCI converter & a valve body kit. I am interested in the hydro fluid thing. Also, anyone try synthetic 5w-20 in their xfer case?

 

[Howard Durand]

David,



Do you know for sure if the transmission is overheating or not? What year is your truck and how do you use it?

you need to determine if there is really an overheating problem or if you have an electrical problem. If you don't have a temperature gauge, it is difficult to tell.

If the transmission is actually overheating, the fluid will probably become discolored and have a burnt smell.

 

[DavidWhite]

Howard,

Yes the transmission got very hot. Also I have a DTT noise filter & a new TPS as of 2000 miles ago. My truck is a 2000 Dually Quad, 4:10's, edge comp, BHAF, 4" ex, & it only has 68000 miles on it. I use it only to pull boats.

 

[Howard Durand]

There are not too many things that affect transmission temperatures. Low fluid levels or poor fluid circulation through the cooling system are two things that come to mind.

By the way, there are boats and then there are BOATS. How heavy are these boats you are towing? And what kind of terrain? Towing heavy loads in the wrong gears can cause excessive heating. Try running with the OD locked out which allows 3rd gear with TC lockup which will give better hauling capabilities although, I would think you should have less problems with the 4. 10 gears.

 

[Ahopper]

joe G. could you explain to my why you run hydralic fluid also?

 

[DavidWhite]

My larger boat is not really that heavy. Only 10K or so. 7% grades don't help. I want to know about the transhydraulic fluid too.

 

[Joe G.]

joe G. could you explain to my why you run hydralic fluid also?

First of all it was recommended by someone who's judgement I trust.



It runs cooler. This can be a nuisance if you have a 47RH transmission because running empty on the highway it may not get warm enough for the PCM to allow OD. It lubes better. Shifts are better. It does not break down from heat nearly as bad as ATF. There is no scum in the pan at oil change time. You can go a very long time without having to change it. It is designed for hydraulic farm equipment that uses clutches very much like automatic transmissions.



The only drawback I have been able to find is that it is so clear that it is hard to check the oil level. Have to be in good light to do so. It is a light tan or gold color that appears to be brown in a bucket. John Deere Hy-Gard® is one brand of this type of oil. There are others.



E-mail me if you want a copy of one of the e-mails I received when I was deciding on switching to it.

 

[DavidWhite]

Well... . Looks like I smoked the trans comming home from the boat trip. Underneath of the truck and front of trailer are dripping with ATF, and trans is slipping bad. Will look@ it when I get home. Thank god I have a company truck..... mines DOA

 

[Howard Durand]

Don't give up on the transmission yet, one of your cooler line fittings may have gotten too hot and come apart. There is a problem with the earlier fittings, they have a plastic sleeve in them and have a history of failing under high heat. If the transmission is still living, you really need to add more cooling but a performance re-build would probably be better. That 10k boat is a pretty hefty load for a stock transmission.

 

[DavidWhite]

Is my trans (2000) a 47RE or a 48RE?