Automatic transmissions like to operate at a certain temperature – which is about 200’ F (93’ C).
Any cooler than that, and the transmission fluid is a bit too thick for transmission to operate correctly. The transmission cooler is not named very well and is more of a transmission fluid heater than a cooler, but the radiator (cooler) does keep it around 195-200’ F mark.
The primary item in transmission that produces heat, is the torque converter. When at a stop and you stand on both the gas and brake, the converter will swell in size (a bit) and fluid temp will skyrocket to 300-400’ F (150 to 205’ C) due to the fluid vortex inside the converter. The converter is about the only thing that will produce any major heat. The bearings, pump and clutch packs can generate some heat – but not much.
Transmission fluid will start to break down once it gets to a certain temperature (not sure what temp that is, exactly). Also, the rubber seals inside of transmission will harden if exposed to a lot of heat. When those clutch pack seals (look like a large O-ring, but rubber is “L” shaped) turn hard, the clutches will start to slip because fluid flows past the seals. The clutch plates are primarily made of paper and glue. Yes, out of paper. When those clutch packs start to slip under a load, you now created a new heat source with a material that can actually burn (the clutch disks).
When you hear someone say they burnt the transmission up, they are referring to the clutch disks catching fire (or worse, catching the vehicle on fire). I have seen some transmissions that are burnt to a crisp and to the point you cannot dissemble the transmission. Bearings/bushings seize onto shafts, aluminum will distort and warp and other things. It is not a pretty picture and IF you can get the transmission apart, generally, most parts are damaged beyond reuse.
Typically, I only see this on vehicles towing something or when four-wheeling off-road for a long period of time.
Usually the transmissions that do
need an external cooler (in addition to the radiator cooler) is are those vehicles that tow, that have the gas petal pointed down for lengthy periods of time (more than a ¼ mile at a time) like police pursuit or racing, or who drive uphill for lengthy periods (life in the mountains).
Occasional periods of those three is nothing to worry about.
Vehicles with a lock-up torque converter (which most Chryslers made mid-year ’78 and newer have) is likely to produce less heat from torque converter than non-lockup converters (mid-year ’78 and older) will – but either type can produce heat, in cases described above.
- For drag race usage (only) – if you can afford the extra weight, I recommend keeping the stock radiator/cooler in place to keep the trans fluid at the 195-200’ (F) mark, for consistent shifting, sense consistency is sometimes more important than random times are (to some). I would at least run an air cooler if you don’t want to use the radiator.
- For real road racing or for real police pursuit (where gas petal will be pushed to floor, more than not), for full time towing or for those who live in the mountains, I do recommend adding a
large external transmission cooler (much like what the factory police cars use) as well as using the radiator.
- For normal street usage (the other 99% of us), using the radiator cooler (only) is all you need.
Now with that said, some transmission coolers can not be cleaned out very well (typically with other brand cars) and for those cars, after a transmission failure has occurred, it is recommended to replace the trans cooler (and/or add an in-line filter) just because of possible metal contamination.
Most of all Chrysler vehicles, even those made today, the trans cooler uses brass or aluminum tubing and are easy to clean out.
Note: always flush the cooler lines after any transmission failure (no matter what brand vehicle)!
In my case, I do want to take my cars road racing every year and even drive on Texas Motor Speedway – so adding an external transmission cooler is in my plans.
Now getting back to the transmission oil temp gauge. By far, the best location for the sensor it is in the pan. The cooler lines will not give you an accurate reading (either too hot or too cold, depending on which line you tap into). The pan will give you the best result.
I took a Mopar Performance Deep sump transmission pan and had an adaptor welded onto it for a mechanical temp gauge. I don’t recommend doing this to a normal transmission pan – for the sensor most likely will interfere with the valve body or something else inside (unless placed in an exact position). Using a deep sump pan will help with that. Even better, I have seen some aftermarket transmission that are already pre-drilled for a mechanical temp sensor – so they did the homework for you.
On my garage wall, I have an A727 Mopar Performance deep pan, with a mechanical sensor fitting welded in, for when I build the A727/A518 for my Fifth Ave.
It is highly recommended to use a deep sump oil pan for any vehicle that will see more than “old lady” driving. Not picking on anyone, only trying to give a “vision” as to my thinking.
If you looking for an inexpensive deep sump pan, I recommend getting one for the 4-speed overdrive pans like this one for A727
46RH 46RE 47RH 47RE Transmission Pan A518 618 A727 Chrysler Dodge Jeep (99976) | eBay
A904/A998/A999
A500 40RH 42RH 42RE 44RE Oil Pan 1988 & Up fits Jeep Chrysler Dodge NEW | eBay
I “think” these pans are made by Dorman, but not sure. Most of these pans use extra thick metal – which is a good thing sense they sit a bit lower in the car than a stock pan.
I like working with these painted pans for they look better, tend not to rust and cleanup is faster. The overdrive transmissions have an extra layer to the valve body – which requires a deeper pan for the extra parts. These pans can be used as is, but I do recommend using a filter extender – so filter is closer to the bottom of pan.
A727 kit, like the one I mentioned above.
Aftermarket kit for A904/A998/A999
Most people can get by without a need for a transmission temp gauge.
Personally, I would prefer a mechanical sensor with a bright warning light (or buzzer) if temperature gets to a preset reading (like say at 275’ F (135’ C) - but not sure if someone makes one that way.
I would
STOP if transmission temp got to 325’ F (163’ C) and would consider a larger cooler if temps get to 300’ F (150’ C) often.
I can’t remember where I welded my sensor fitting to, but I “believe” it to be on the rear of pan because I remember routing the cable (not sure what you call the sensor wire) along with the speedometer cable.
BudW