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jlb27537 · 04-24-2010, 05:33 PM

Hello, New here.

First, I tried the link to the Allison Publications page, but can not figure out how to download a publication. They want me to order a hard copy???

I have a '96 Sahara, 300 CAT, MD3060. Problem: When I start driving from a cold start, the Trans oil temp gauge, (sender is installed in the top of the oil-to-water cooler on cold side of radiator) starts climbing toward 200 degrees in about 10-20 miles of driving. Normally it will not be much above 140-160 in 20 miles. With the gauge reading 190-200, it will suddenly drop to 150-160 and stay there for the rest of the trip.

I have changed the sender, checked the wiring best I can. It acts just like there is a thermostat that is sticking, it finally opens and the temp drops to the reading it should me.

I have been thru about 10-15 cold start cycles (sit overnight) and sometimes on a cold start it will act properly all day long.

If someone knows if there is a thermostat in the transmission oil ckt, where is it?

Or can someone e mail me a copy of the oil ckt of the MD3060. This is a 1996 model.

This is NOT a engine cooling problem.

Thanks Jim
jandlbrooks at yahoo dot com

RJay · 04-25-2010, 10:29 AM

Hi jib, welcome.

Unless I'm reading your post wrong, what you're experiencing is normal. The transmission cooler is integral to your radiator and while the thermostat is closed no water will circulate around the oil cooler and you oil temp will climb. Once the engine thermostat opens water will circulate around the oil cooler and bring the oil temp down.

samert111 · 04-25-2010, 10:43 AM

Jim,
My temp sender is located in the housing going out to the cooler on the back of the transmission just below the driveshaft yoke and not in the cooler itself, and like yours, my temp will rise to 200 degress but it stays there always. However, yours dropping to 160 degrees once the engine comes up to operating temperature sounds more like the way it should operate and I've often wondered why mine doesn't do that. When I shoot the temperature of the sender housing with my IR gun after fully warming up it checks at 160-170 degrees so I'm thinking my sending unit may not be accurate.

I have the entire Allison troublshooting manual for your transmission I can send you if you drop me an Email to samert111@yahoo.com It's a big PDF file at a little over 5 mb but I've sent it to others before. It has some diagrams of the hydo circuits like your looking for. Not sure if there is a thermostat in there anywhere though.

jlb27537 · 04-25-2010, 06:10 PM

Steve, Thanks for the file. It shows all the oil schematic's for each gear, but I am still unsure about the ckt that feeds the cooler. And which line from the tranny is the output and input.

The thought about the engine thermostat makes sense, however in the 4 years of owner ship it has never done what it is doing now. The first time it did it, was after driving about 3 hours, flat, 80 degrees, Texas. Slowed down to go thru a town and trans went to 200 in about a mile. Pulled over and let it idle for about 10 minutes. Normally that would drop the trans oil to 140, it dropped to about 180 and stayed there. From there was in the Texas hill country and no way thermo was closed. Drove on in to campground, 20 miles with engine 180-190 and trans at 200.

Sat for winter, did change the sender in top of cooler, no change. On a cold start, engine will come up to temp in about 3-5 miles of driving. Trans temp will start climbing in about 10 miles and go to 200 in 10 miles. Will stay at 190-200 for sometimes 30-50 miles, then in a matter of feet it will drop to 150-160 and stay there for the rest of the day. I can not believe the engine thermostat is still closed causing this issue, however, it is easy enough to change, so that may be next.

I'm still thinking of a thermostat of some sort in the tranny. I have no codes in the key pad when this is happening. I've stopped and checked. Fluid is fine.

Steve, if you have any more oil ckts, IE external cooler flow, shoot it to me.

Thanks Jim

jlb27537 · 04-25-2010, 07:10 PM

Hello,

I have looked again at the oil ckts. It appears the green is the cooler line off the lube filter and there is no thermostat in the tranny. Going on another trip this week. Will try and get a IR temp gun before I leave to check it and get more clues.

Thanks for the thoughts.

Jim

jlb27537 · 05-06-2010, 08:23 PM

My transmission oil temp gauge has a light in it that if the fluid reaches 240 it will turn on. The ckt for the light was failing intermittently causing the gauge to read high by about 40-50 degrees. The indicated temps were never over 200, when the actual temps were about 160.

The oil ckt for the cooler is: from the torque converter pressure regulator, hot oil from the converter goes to the cooler, from the cooler to the lube filter and back to the sump.

Has been a fun problem, but I won. Never had a Allison issue, a dash gauge issue. After talking with a experienced Allison tech these things a bullet proof IF you keep them serviced, clean fluid and filters. IF running Dextron III, keep it below 180 at the sump. Sam, get a IR gun and check the sump, 200 is a bit high for Dextron. So I have been told.

Jim
'96 Sahara, 300 CAT, MD3060