This is one of those obvious, but easily missed type things. I've had this gauge for years,
& have always watched the tranny temperature. Not that it's kept any from actually breaking,
but... oh well. Oil will start to break down & 'coke' after it gets hot. It will end up being
a sludge in the bottom of the case, & won't lubricate worth a (expletive omitted). Towing a
trailer will heat up a manual just like an automatic transmission, although not nearly as much.
But, if they aren't watched, running lower gears while pulling that 25' prowler up Monarch Pass NM,
(worst one I've ever towed anything up) may cook it for good. The friction of the reduction gearing
will create quite a bit of extra heat. They will usually cool back down when in direct again though.
If the temperature runs high while in direct, start looking for a replacement tranny.
The gauge on my dash. (the round black one) It's a SunPro oil temperature gauge with integrated
sending unit. Reads from 100 to 340 degrees.
This is the sending unit. I made an adapter from brass fittings & replaced the tranny's oil fill
plug with it. To add oil, remove the temp probe, then the brass plug assembly. The probe only protrudes
into the transmission about 1/4". The probe measures the oil's temperature as it's flung around inside
the case. If a bearing starts failing, you'll know it from watching the gauge on the dash.
I've always figured it's time to pull over & let it rest if it gets much over 250 degrees. The SM-420
4-speed runs about 130 degrees when running long periods at 70 MPH on the freeway.
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