Transmission overheating troubleshooting
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4wd Low
Joined: Nov 2024
Posts: 13
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Transmission overheating troubleshooting
My 1999 D2 seems to like overheating on long uphill sections (1500 ft of elevation at like 4-6% grade). No towing. Not too heavily loaded. Recently changed trans fluid and filter. 32" tires so not too bad. I keep reading that a properly maintained ZF should be bulletproof and have no overheating issues but apparently mine is the exception lol. Looking for some advice on order of operations for troubleshooting and options to fix it. Here's what my research has turned up so far.
Troubleshooting:
1. Check if the trans cooler is dirty/clogged with mud
2. Check that the viscous fan clutch is locking up. I checked this and I think it is working fine. There does seem to be a ~4in hole in the top of the fan shroud though but I doubt that would cause that many problems.
3. Check that the torque converter is locking up in 3rd. How do I check this? preferably without draining the trans fluid and playing with the solenoids.
If none of those are the issue, looks like options include:
1. Adding an Auxillary trans cooler
2. Replace the trans cooler with a P38 trans cooler
3. Replace the mechanical viscous fan with an electric fan. I think the fan off the 95 Mustang with proper shrouding should pull enough air but I've only seen one person seem to have success.
4. Re gear the transfer case or the diffs to match the bigger wheels. This seems like the nuclear option.
I also would like to install a trans temp sensor that actually gives the temperature instead of just being an on/off switch. Is there a drop in replacement that works? Or an adapter to add an extra sensor in the hoses?
Troubleshooting:
1. Check if the trans cooler is dirty/clogged with mud
2. Check that the viscous fan clutch is locking up. I checked this and I think it is working fine. There does seem to be a ~4in hole in the top of the fan shroud though but I doubt that would cause that many problems.
3. Check that the torque converter is locking up in 3rd. How do I check this? preferably without draining the trans fluid and playing with the solenoids.
If none of those are the issue, looks like options include:
1. Adding an Auxillary trans cooler
2. Replace the trans cooler with a P38 trans cooler
3. Replace the mechanical viscous fan with an electric fan. I think the fan off the 95 Mustang with proper shrouding should pull enough air but I've only seen one person seem to have success.
4. Re gear the transfer case or the diffs to match the bigger wheels. This seems like the nuclear option.
I also would like to install a trans temp sensor that actually gives the temperature instead of just being an on/off switch. Is there a drop in replacement that works? Or an adapter to add an extra sensor in the hoses?
Last edited by Ethan_mikel; Mar 3, 2026 at 12:14 AM.
TReK
Joined: Sep 2020
Posts: 2,956
Likes: 754
From: British Columbia Canada
I have the same question as Walt, how do you know the transmission temperature? The D2 doesn't have a means to monitor transmission temperature. Maybe you've added a means?
It's not difficult to tell if the Torque Converter is locking up. Select 3rd gear, your initial RPM will be higher (assuming you shifted down from 4th), then in a few seconds your RPM will drop slightly when it locks, sounding like it just shifted to a higher gear (also assuming you're driving on a relatively flat piece of road, if climbing it may remain unlocked).
It's not difficult to tell if the Torque Converter is locking up. Select 3rd gear, your initial RPM will be higher (assuming you shifted down from 4th), then in a few seconds your RPM will drop slightly when it locks, sounding like it just shifted to a higher gear (also assuming you're driving on a relatively flat piece of road, if climbing it may remain unlocked).
Thread Starter
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4wd Low
Joined: Nov 2024
Posts: 13
Likes: 1
Yes. Trans temp warning light comes on. The red gear icon on the top right of the dash. I’m not sure what temps I’m seeing because that sensor is just an on off switch and I don’t even know what temperature triggers it. Trans continues working fine. I pull over and leave it running in neutral for 15 min before continuing
Thread Starter
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4wd Low
Joined: Nov 2024
Posts: 13
Likes: 1
I have the same question as Walt, how do you know the transmission temperature? The D2 doesn't have a means to monitor transmission temperature. Maybe you've added a means?
It's not difficult to tell if the Torque Converter is locking up. Select 3rd gear, your initial RPM will be higher (assuming you shifted down from 4th), then in a few seconds your RPM will drop slightly when it locks, sounding like it just shifted to a higher gear (also assuming you're driving on a relatively flat piece of road, if climbing it may remain unlocked).
It's not difficult to tell if the Torque Converter is locking up. Select 3rd gear, your initial RPM will be higher (assuming you shifted down from 4th), then in a few seconds your RPM will drop slightly when it locks, sounding like it just shifted to a higher gear (also assuming you're driving on a relatively flat piece of road, if climbing it may remain unlocked).
TReK
Joined: Mar 2010
Posts: 2,781
Likes: 628
From: NYC
OK.
1. check trans fluid level
2. make sure the cooler is not clogged.
3. tape up the hole in the shroud. The fan is much less efficient if the suction effect is 'leaking' out that hole.
4. Try a different fan clutch.
Realize that air flow goes through the A/C condenser, then oil and trans coolers, then the engine radiator.
Something as simple as an A/C leak can clog your radiators. Here is a pick of the radiators on my P38 when I first bought it. I installed new.
1. check trans fluid level
2. make sure the cooler is not clogged.
3. tape up the hole in the shroud. The fan is much less efficient if the suction effect is 'leaking' out that hole.
4. Try a different fan clutch.
Realize that air flow goes through the A/C condenser, then oil and trans coolers, then the engine radiator.
Something as simple as an A/C leak can clog your radiators. Here is a pick of the radiators on my P38 when I first bought it. I installed new.
Thread Starter
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4wd Low
Joined: Nov 2024
Posts: 13
Likes: 1
OK.
1. check trans fluid level
2. make sure the cooler is not clogged.
3. tape up the hole in the shroud. The fan is much less efficient if the suction effect is 'leaking' out that hole.
4. Try a different fan clutch.
Realize that air flow goes through the A/C condenser, then oil and trans coolers, then the engine radiator.
Something as simple as an A/C leak can clog your radiators. Here is a pick of the radiators on my P38 when I first bought it. I installed new.
1. check trans fluid level
2. make sure the cooler is not clogged.
3. tape up the hole in the shroud. The fan is much less efficient if the suction effect is 'leaking' out that hole.
4. Try a different fan clutch.
Realize that air flow goes through the A/C condenser, then oil and trans coolers, then the engine radiator.
Something as simple as an A/C leak can clog your radiators. Here is a pick of the radiators on my P38 when I first bought it. I installed new.
TReK
Joined: Mar 2010
Posts: 2,781
Likes: 628
From: NYC
yes. the black sludge in/on the cooler in my picture is exactly that. a small refrigerant leak blocked a major portion of the cooling area. The A/C condenser is removed in my picture.
If you loosen the bolts to the mounting brackets you might be able to determine if they are clogged without dismantling the A/C system and need to recharge it.
If you loosen the bolts to the mounting brackets you might be able to determine if they are clogged without dismantling the A/C system and need to recharge it.
Last edited by WaltNYC; Mar 3, 2026 at 02:59 PM.
TReK
Joined: Sep 2020
Posts: 2,956
Likes: 754
From: British Columbia Canada
https://landroverforums.com/forum/di...-sensor-39339/
well I stand corrected. There is a temp sensor. The above post might be helpful.
well I stand corrected. There is a temp sensor. The above post might be helpful.
Nine times out of ten the sensor is bad, it happens. That said, the fan has almost no impact at highway speeds, nor does the radiator shroud (sure, some, but not that much) . An electric fan won't help cooling that much at highway speeds either, slow speeds pulling a grade off road, or idleing, but not highway speeds. https://extinctmotorsports.com/the-e...electric-fans/
Do you have an IR gun? You can shoot the trans pan and the cooler to see if it really is hot. Rave has the temperature setpoint where it flips.
Do you have an IR gun? You can shoot the trans pan and the cooler to see if it really is hot. Rave has the temperature setpoint where it flips.