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Old 04-08-2009, 12:52 AM   #1
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Wink Bypassing Radiator to only B&M tranny cooler?

I was wondering if its OK to bypass the radiator and have Tranny feed and return going through only B&M 70268 Tranny Super Cooler??
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Old 04-08-2009, 01:46 AM   #2
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I've done it that way with no significant ill effects.

Best if you put a thermal bypass valve inline with your cooler - it routes fluid right back into the transmission below operating temperature, or routes it to the cooler once it gets there. It's true that heat is the enemy of automatic transmissions, but if they run too cool for too long, it can also cause trouble.

I think B&M invented the valve, but you can probably also find it now from outfits like Trans-Dapt or Flow-Kooler.
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Old 04-08-2009, 03:25 PM   #3
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Hmmm....

Would this work with a factory cooler as well?

The reason I ask is the hard line that goes into the top of the radiator from the tranny will not seal up, I think the brass fitting on the radiator is FUBAR. So it continues to leak no matter what.

The brass fitting is permanantly fixed to the radiator so the only way to fix is to replace the radiator again (just replaced).

If I could bypass the radiator and go directly to the cooler that would fix my problem.
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Old 04-08-2009, 04:15 PM   #4
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I have bypassed my in radiator one and put a cooler in front of the radiator. I also used a thermal bypass valve, for the reasons stated above.

Here's the cooler
Click the image to open in full size.
Here's the bypass valve
Click the image to open in full size.
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Old 04-08-2009, 04:18 PM   #5
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Old 04-08-2009, 07:50 PM   #6
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i wonder How much cooler this keeps the fluid vs. then the in radiator cooler.
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Old 04-08-2009, 07:54 PM   #7
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Quote:
Originally Posted by xj_mike View Post
I have bypassed my in radiator one and put a cooler in front of the radiator. I also used a thermal bypass valve, for the reasons stated above.

Here's the cooler
Click the image to open in full size.
Here's the bypass valve
Click the image to open in full size.
what did that set-up cost you not including the guage and sender?
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Old 04-08-2009, 08:51 PM   #8
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Quote:
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i wonder How much cooler this keeps the fluid vs. then the in radiator cooler.
At an offhand guess, I'd say 10-15*F or so.

Think about it - the spec temp of the XJ is around 210*F. So, the average coolant temperature runs about 205-215*F. The transmission "cooler" may be in the "cold" radiator tank, so let's say it gets to around 207*F or so. Won't really get any colder - since thermodynamics gets in the way.

The thermal bypass valve is usually set for opening/closing around 190*F. Rejecting heat into the atmosphere is easy enough - you can reject heat to get right down to ambient temperature (thermodynamics again - you can reject heat from one medium to another and get cooler than the ambient temperature of the receiving medium,) which usually runs about 100*F lower than operating temperature of the transmission fluid. From what I recall, you can get your transmission temperature down to right about 190*F (or whatever the valve is set for,) plus/minus a couple degrees. It's been a while since I've set up one of these systems, but I recall them working rather well. Mike, can you confirm? Or do you have a gage inline?
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Old 04-09-2009, 04:14 PM   #9
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As for price, I think I spent about $40 on the thermo bypass and my friend gave me the cooler (new) from an unused project. I think it was only about $50 or so.

As for cooling, I find my water temp stablized about 210 and doesn't climb as fast under load like it did before. It will still hit the 219 mark which turns on the electric fan and cools down faily quick, since the tranny fluid doesn't go thru the radiator.

As for the tranny cooling, I screwed up and ended up putting my gauge in the output line on the tranny so the temps I see are coming directly out of the tranny. Normal freeway driving, it runs about 160-180. I find if I can keep the converter locked up (either in 3rd or 4th) the temps stay low. If the converter starts slipping (like passing or climbing a hill, shifter in 4th but it kicks down into 3rd) it will start heating up quickly. I normally will then drop the shifter into 3rd and can feel the converter locking up, then the temps start to come down.

The highest I have seen is about 260 but that is in 1st gear, high RPM's in the sand dunes. Keep in mind this is the temp coming OUT of the tranny, not the cooler. I have been thinking of adding a second temp sender and using a switch so I can see input and output temps.

Hope that answers the questions.
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Old 04-09-2009, 04:28 PM   #10
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run it through the radiator then the cooler... i did it that way. just figure out which line is the return to tranny line, and cut it and splice your cooler in
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Old 04-09-2009, 04:40 PM   #11
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What does the bypass valve do and is it necessary?
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Old 04-09-2009, 11:31 PM   #12
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Quote:
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What does the bypass valve do and is it necessary?
It allows the temp to come up to a preset point (190* IIRC) before opening and allowing the fluid into the cooler. This way you don't have to worry about the fluid being too cold in cold climates.

Kind of does the same thing as the in radiator cooler does, it helps to heat the fluid up quicker. Down side is the tranny temp can get much higher then the coolent temp, so you end up ADDING heat to the water coolent. Since XJ's already are marginal with their cooling systems, why add more heat to it.
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Old 04-09-2009, 11:54 PM   #13
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Quote:
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i wonder How much cooler this keeps the fluid vs. then the in radiator cooler.
Radiator coolers can only cool the fluid to the temperature of the coolant, while aftermarket auxillary coolers can cool to ambient air temp, in theory, at least.

So, if the auxillary is down stream from the radiator, or bypasses the radiator, your fluid will be cooler. If upstream of the radiator, you will not cool below radiator coolant temperature

I don't agree that trans fluid can be too cool. Otherwise, the trans wouldn't work well in temperatures below freezing in the winter until you had let the engine warm the fluid up.

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Old 04-10-2009, 04:55 PM   #14
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I already have the stock tranny cooler, but it looks pretty small compared to the aftermaret stuff. Will this still work with the factory tranny cooler?
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Old 04-11-2009, 09:48 AM   #15
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Quote:
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I already have the stock tranny cooler, but it looks pretty small compared to the aftermaret stuff. Will this still work with the factory tranny cooler?
Stock, OEM coolers are adequate for most "normal" driving, BUT if you off-road much, tow a trailer, or drive highway speeds a lot (80%+ of your daily driving) then I suggest an aftermarket cooler mounted ahead of the AC condenser with as much space between as possible.

If you go with one sized for RV use (over 16,000 lb vehicle) then you can probably bypass the OEM completely in any driving situation.

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