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Trans running hot

bigalpha

bigalpha

Moderator
Location
Tucson, AZ
I have a 1988 MJ

  • 6 cyl
  • AW4
  • D35
  • stock gearing
  • B&M Trans cooler, bypassing the radiator
  • Trans fluid level is OK
  • Trans fluid color is good
  • ~2200 RPM at 70mph
I drove about 2 hours on the interstate between 70-80mph and the trans fluid temperature hovered about 200-210 most of the time. Going up hills made the temp go up and going down barely moved it.

It finally got to over 220 during the end of my journey which involved climbing this tall gravel hill.

Surely the temp shouldn't be this high, especially with an aux cooler at interstate speeds just cruising. What temps should I be expecting and how can I troubleshoot it?
 
1) What size tires are you running?
2) Are you measuring the temp before or after the tranny cooler?
 
1. I have stock sized tires.
2. The temp sensor is at the trans out (to the cooler).

As far as the converter - that's where you travel at 60mph+ and hit the brake, then the RPMs should shoot up by around 300, right?
 
bigalpha said:
As far as the converter - that's where you travel at 60mph+ and hit the brake, then the RPMs should shoot up by around 300, right?
Click to expand...


You must also have one foot on the gas pedal in am attempt to maintain speed ie: with one foot on the gas driving normal and above 45 MPH, lightly tap the breaks but hard enough to light up the break lights, the RPM should then increase by 300 +/-. When I do that, I could also hear the engine note change both to the higher RPM then back to the lower RPM when I release the breaks.
 
Does not sound to me like you have a Trans problem. Most of the HO guys are running around at 210 average on the engine coolant temp at the T-Stat.

I would suggest rerouting the hot Trans-fluid to the radiator cooler first, as liquid coolant there should be at about 180-190 F and it will cool much faster than air, then take the radiator Trans-fluid cooler outlet and route it to the air cooled cooler to knock it down a little more, then back to the transmission.
 
I will test the TC lock up.

What temp should the trans fluid run at? Don't a lot of people bypass the radiator completely and not have issues? If the coolant is at 180, then won't it keep the trans fluid at 180-190 before the aux cooler?
 
If the coolant on the cold side of the radiator is at 180 F, it will cool 220 F Trans-fluid to about 190 F (a 30 F drop, 10 degree approach temp), then the air cooled cooler would take it down maybe another 20 F or so, to maybe 170 F.

Since most vehicles have the Transmission fluid cooled (or heated) by passing it through a cooler (heater) placed in the colder side of the radiator, it is logical to assume that the Trans fluid is going back to the transmission at about 190- 210 F, and coming out of the transmission at about 200-220 F under the hottest, high load conditions.

Very few people have real data on their transmission fluid temps.

If your radiator is old, and the flow has slowed down due to some scaling in the tubes, the cold side of the radiator may be closer to 170-180 F while the T-stat coolant is at 220 F!!!!

I think normal exit temp on Transmission fluid is probably 200 to 220 F peak except under severe loads like mountain climbing. Lower on Renix jeeps, a little higher on HO engines.

Which fluid are you using? Dex III is more sensitive to temp than the new synthetics.

Climbing grades for a while always seems to test the cooling on the Trans fluid!!!
 
It's a new radiator and a brand new aux cooler. On the temp gauge for the cooler, at 220* it turns yellow so I figure that having the trans temp at 190 isn't a real good level. Not to mention, shouldnt it be cooler while cruising on the interstate since the trans isn't under a lot of stress?

Also, it's running Dex III.
 
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The temp gauge turns yellow at 220. I figured that was a 'don't run the temp into this range a lot' indicator.
 
At any rate, the temp still crept up past 220* and that's what concerned me.

If it running at 210 and getting up to ~230 or so is normal, then that's OK with me.
 
If you run Dex III, and frequently run it at 230F, be sure to go with frequent transfusions!
 
Well, it doesn't continuously run at 230, it stays under 220 except for longer moderate hills.

I'd like it to run much under 210, actually.
 
Running at 230F for say 10 minutes a day will shorten the life very quickly IMHO. The difference between operating at 210 and 230 F is steep, cuts the life nearly in half I think.
 
How is the gauge sender installed? How is it grounded? Single wire?

The reason I ask is I have the same set-up. I first mounted it on RUBBER hose...I still got a reading but it was sketchy and inaccurate. I then put it on the metal part and my results changed an are reliable.




trans001-1.jpg

  • AW4
  • 4.0
  • D35
  • stock gearing
  • B&M Trans cooler, bypassing the radiator (mines is a Haynes with an "auto meter" gauge)
  • Trans fluid level is OK
  • Trans fluid color is good
  • ~2200 RPM at 70mph
The radiator heat exchanger is also bypassed. The gauge is mounted on the FROM tranny line. My temps run like so:

Ambient temp betweeen 90-100. Coolant temp at 210.

Highway speed, no extra load = ~125
Highway speed, towing light trailer = ~140-150
Highway speed, towing, loaded up an 8 mile 6% grade = ~180-200

Dirt road speed, about 25-30, loaded, towing 6 miles uphill, aux fan on it has been as high as 260 for a short time. Past 250, but not pegged out on this gauge.

trans003.jpg


and then there is this table which doesn't really say what temp is "normal".
I would think that normal is the range somewhere near the low end of these temps AKA "low heat/long mileage"
TransLifeChartfromHECATInc.jpg


Then again I wouldn't call 70-80 "just cruising" in an 88 MJ. I "just cruise" at 65 or so.
 
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