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Q: transmission heats up at low speeds on 1999 Ford F-350 Super Duty

When pulling our horse trailer at slower rates of speed like high traffic with lots of stopping or climbing hills on gravel roads where you can't drive as fast our transmission wants to heat up. What can we do to solve this problem. If we can keep our speed up were fine. Otherwise love this truck
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Which engine do you have in this truck? I'm assuming the 7.3L Diesel...

Did you mention that your truck is equipped with the external oil to air cooler for the transmission also?

And - does your truck have any programmer or "chip" installed to enhance performance?

The 4R100 transmission is notorious to being quite a bit at its limit with a stock stock 7.3L Diesel. Add anything to the mix and you exceed its capabilities. Once this transmission overheats, it will again and again until it is replaced. Some folks replace the front pump and seal to try and fix things but mostly to no avail.
You are not getting enough air flow across the trans cooler either because is clogged and needs to be replaced or flushed or if is in the radiator instead of remote cooler then coolant rad is not getting good air flow for same reasons. You can add an electric cooling fan that can be switched on in the low speed conditions to help cool.
we have already added the electric fan,flushed the transmission and replaced the filter. The engine does not overheat just the transmission. The original fan does not kick in soon enough to cool the transmission. Once it does kick in it cools it right down.
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