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  1. #1
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    Radiator (internal) transmission cooler bypass?

    So one of the first things I was told to do when I bought my T was to install an external transmission cooler, and bypass the internal one in the radiator, due to it commonly failing and taking out the transmission in the process. So itís been two years now, and I still havenít gotten around to it. However, after recently reading about several H3/H3T owners with relatively low miles (80K) having their internal transmission coolers recently fail, itís back on my radar.

    On one of the social media H3 groups, a member indicated that most of the trans cooler failures were with the original radiators that came in our trucks (quality issue GM didnít own up to and issue a recall for), and that later OEM radiators donít seem to exhibit that same issue. Does anyone know if thereís any truth to that? Multiple automobile manufactures run trans coolers in the radiator (liquid more efficient cooling agent than air), and in fact most manufactures of external trans coolers recommend that you run them in parallel with the internal cooler. Iím just wondering why you hear about the H3/H3T failing so often.

    For you folks running aluminum CSF radiators, are you utilizing the internal transmission cooler as designed, or are you also bypassing it with an external trans cooler? I havenít heard of an aluminum CSF radiator failing for this, but I guess that could be because there are less of them in service.

    When I took ownership of my T at 63K miles, the radiator looked relatively new. However I doubt the original owner would have already replaced the radiator at that mileage. Iím currently at 74K miles, and want to get out ahead of anything catastrophic. Iím debating on whether to 1) buy a new OEM radiator for a manual transmission (no internal trans cooler, more radiator cooling surface) and adding an external cooler with thermal bypass, 2) buying a new OEM radiator and running an external trans cooler in parallel with the internal cooler (under the assumption that it was a quality control issue with the original radiator), or 3) buying a CSF radiator and running an external trans cooler in parallel with the internal trans cooler.

    Thereís something to be said for peace of mind though. For you folks that are running an external trans cooler and bypassing the one in the radiator, any issues with your current set-ups?

    Thank you in advance for any and all feedback.
    2009 H3T Alpha OffRoad w/ Tuff Country 4" lift

  2. #2
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    The radiator can crack at any moment, just bypass for now and replace it only when necessary.

    I run a larger radiator for towing and swapped out for a manual radiator.





    Sent from my iPhone using Tapatalk

  3. #3
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    My internal cooler failed a few months ago. It was a new AC Delco radiator I had purchased from a Chevy dealer that failed. It was only a few months old. I havenít posted much about my experience, but I was going to put together a thread. Long story short I installed another new radiator from a H3 manual transmission truck, which does not have an internal cooler. I flushed the trans really well. Flushed the engine really well. Installed a Hayden 677 external trans cooler in the upper passenger side area in front of the radiator. The trans has been working fine so far. I would encourage everyone to bypass the internal cooler so the $250 rad doesnít ruin a $2500 trans.
    10 T

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  5. #4
    Bald Tire
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    In my 2008 Alpha I have ran the CSF radiator for the last year and use the internal cooler. I havenít had any problems with it. Done some good wheeling with it and havenít had any issue with cracking or leaking.


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    DFW Spartan (10-14-2018)

  7. #5
    El Diablo
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    I have option #1 (OEM manual trans, full bypass) and I've had no cooling issues here in AZ even with our heat, but I admit I do not have a temp gauge on the transmission. Current transmission has about 63,000 miles on it and fluid has been changed once or twice and has not exhibited any issues. If you go this route, remember to upsize the external cooler to make up for the loss of the internal cooler.

    I don't the the internal trans cooler failures are frequent by any means but it's just that the consequences can be so catastrophic. Like you said, it may be for more for peace of mind than actual necessity.

  8. #6
    Bald Tire
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    My setup for the transmission cooler runs in series with the OEM cooler in the Radiator. I did this because of the cold temps we see in the winter here in Flagstaff, Arizona.

    I tested my setup mid July in 110 degree heat (itís a dry heat) and ran it up and down the I-17 at the heat of the day and never rose above 200 F.

    But since Arizona decided to only have one week of fall and immediately go into winter, my setup is currently biting me in the ass. The transmission temp will not rise above ~120 F. So I will need to add a 180F bypass so that the fluid rises up to operating temperature.

    I have pics and part numbers if your interested.

  9. #7
    Bogger
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    I have the CSF with no issues. OEM failures were usually from the welded connection joining the tube fitting to the internal finned tank.
    2006 H3 Adventure & modified Rancho with SOA

  10. #8
    El Diablo
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    Quote Originally Posted by EndeavoredH3 View Post
    But since Arizona decided to only have one week of fall and immediately go into winter, my setup is currently biting me in the ass.
    We're having a beautiful, if wet, fall in Phoenix!

  11. #9
    Bald Tire
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    I just installed a Derale 13613 Series cooler with a 25792 Fluid Control Thermostat bypassing the factory radiator cooler. The thermostat fully opens at 180 but so far the transmission hasn't gotten over 162. At highway speeds, my water temp stays at 178-182 while the trans is at 162. During my morning drive at 40 degrees, the engine reached it's operating temp in about 5 mins of stop and go driving where as the trans took about 10 minutes to reach 160. I'll have to wait until Texas decides to warm back up to see how it cools in the heat.
    2010 Silver H3 Adventure/Luxury, 285 K02s, T-bar crank, Trail Duty ext bump stops, Bilstein 5100 shocks, Schwarttzy Skids, Chassis Unlimited rear diff cover, Air Doc Intake, Outfitter Design steering rack reinforcement kit, N-Fab RKR Rails w/ removable steps, Fumoto oil drain valve, Derale transmission cooler & thermostat, Scangauge ii.


 

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