This topic is a 'hot' one on many forums. Should I, shouldn't I (answer is yes you should), where physically to place it, where in the system to place it, etc.
A friend with a 4wd T-100 noted that his truck has a light that comes on when the trans temp is high. Might check to see if that is an option or if it can be added simply.
The trans is cooled by the fluid that it pulls up out of the pan. Not all of that fluid is sent to the cooler after going thru the trans, only that which was pumped into the converter goes thru the cooler. The fluid that wasn't sent to the cooler simply rains down back into the pan. The fluid that has come back to the trans from the cooler is dumped into the pan, mixing with the fluid that has rained down out of the trans. At least this is how the GM & Ford autos that I've worked on were arranged and I've seen nothing to indicate that other trans' are done differently.
For a gauge sender putting it in the cooler line means that you're only seeing either the fluid temp coming out of the torque converter or you're seeing the temperature of the fluid coming back from the cooler. Neither is a real indicator of what the actual trans internal temps are, but it is a good place to monitor if you want to know what the hottest that the fluid has gotten is. If you tend to spike the fluid temps a lot this might be a good thing as the fluid's chemistry changes when over-heated and it should be replaced if it has gotten excessively hot or has been spiked several times.
While I prefer putting the sender in the trans pan, it too has some downsides. Complexity of install is the first. Unless you're a semi-talented fabricator, know one, or are near-by to me your options for doing this are not well engineered and can be leakage points. If you do put the sender in the pan, add a drain too. Sender in the pan is essentially the same as monitoring the outlet from the radiator, it won't give you an exact temperature - just an idea of the range that the trans is operating in. Some guys monitor both, either with two gauges or with a switch between two senders. I use a grounding switch type sender to turn on a light. A search thru various catalogs, like VDO, will turn up temperature switches with just about any temperature calibration the you might want.
For me personally I consider the trans to be a permanent item and the fluid to be a consumable. Annual replacement of the fluid in heavy service is more than acceptable to me.
Keep in mind that as designed the trans is expected to run at the same temperature as the engine because the radiator cooler ties the two temps together. This isn't necessarily the ideal arrangement, it just would cost the OE too much to make it differently and it is close enough to work well and live. The trans does need to reach at least 160*f to boil off any condensation in the fluid. Ideally it would run between 160*f and 180*f.
So, coolers. What to get and where to place it depends a lot on where you live and under what circumstances you operate the truck in. There are 3 relatively simple ways and one not quite as simple way to install an auxiliary trans cooler.
A: Ahead of the cooler in the radiator.
B: After the radiator cooler.
C: Instead of the radiator cooler.
D: Before or after the radiator cooler, but with a thermostat.
A: There are proponents of this plumbing configuration who claim that in doing it this way that you reduce the thermal load on the radiator. I'm in this camp. What this also does is allow the radiator cooler to warm the fluid back up if the aux cooler did too good of a job. Makes the system semi-self temp regulating. Absolutely no doubt that this will work the fluid harder, but again I consider the fluid the consumable and not the trans. I see this configuration as being more flexible with regard to ambient temperatures.
B: For years this was considered the only way to plumb an aux. cooler. Doing it this way does not reduce the thermal load on the radiator, though it does continue to cool the trans fluid. I see no advantage to this configuration, and even though I'm trying to present the options unbiased I'm having a hard time finding a valid argument in favor of this configuration.
C: I would only suggest this for a vehicle that only sees high ambient temperatures and does not ever need to operate in cold weather. If this is done the cooler needs to be the biggest unit that can be made to fit.
D: Probably the most ideal would be this configuration though it is certainly not the least expensive option. Before or after the radiator cooler will have the same sets of pros and cons as A & B. Can also plumb it to be in parallel to the radiator cooler. Not sure that I would do this, but it's a possibility. Adding the thermostat means that the aux cooler only works when it is needed. These are available from
BAT-Mocal,
Pegasus Racing, and possibly
Baker Precision though I didn't easily find one there.
A note on the hoses and fittings usually supplied with "trans cooler kits", they're junk. Don't waste your money as that type of hose fitting will leak. Maybe not right away, but it will leak. Mess factor aside, there is the inconvenience, expense, and the potential danger of a failed transmission.
The better solution that doesn't mean going to the "race car" AN stuff is called "Push-Loc" or Barb-tite" etc. depending on who the mfg is. Can buy lightweight aluminum versions of this type of hose end from places like Summit Racing, but can also get steel and brass hose ends from industrial vendors. For those near the 93003/4 zip I'll highly recommend
Don's Industrial in Ventura. The hose used on these fittings is specific to the fittings and I strongly recommend using the Aeroquip FC33 hose. I have had much experience with this hose, and though it's not cheap, it will last and work and seal far past your interest in the vehicle. The crucial key to understand is that this fitting and hose design does NOT want or need a hose clamp. In fact, putting a hose clamp on these fittings will compromise the joint and make it fail prematurely.
The racing vendor links above also have coolers as does
Summit Racing and
Speedway Motors. The stacked plate design is more robust in rough service and reportedly has a smaller pressure drop than does the tube and fin type of coolers though Fluidyne, a very good name in coolers, does make tube & fin type coolers so there must be an advantage to them in some applications. If you go with the thermostat configuration then I would get the largest cooler that you can make fit. Some late model trans' need a lot of cooling so the biggest cooler that you can find may not be a bad thing.
BTW, I've seen a plot of the trans life vs. temperature curve in a trade publication (wish that I'd have kept it). the curve is an inverse logarithmic curve. That is to say that as the temperature increases linearly the trans lifespan decreases logarithmically. 10* increase from 180* means not a lot. 10* increase from 250* is a huge loss in lifespan.
