strange transmission behavior

[jstark]

Howdy guys, I've been lurking for I while, and I really dig the website...

I have an '86 Ford B700 skoolie with a 370c.i.d and an AT-540. My band has put about 18k miles on it in the last couple summers with no major problems (it's at about 150k mileage).

One thing that concerns me, though, is some weird transmission behavior. On hot days, when driven for any considerable length of time, it sounds like the transmission revs up for about 5-15 minutes at a time. By this, I mean it sounds like the transmission's RPM seems to go up by 1k or so.

The frequency and length of the occurances seems to be directly correlated to temperature. If we run at night, I can go for 10 hours without it happening. Driving through AZ in July during the middle of the day, it was wound up pretty much the entire time. I've installed a tach, and my engine-RPM stays rock solid during the "rev-up". Anyone know what 's going on? Am I in danger of a transmission failure?

Thanks in advance; I really appreciate all the help this site brings.

David
mailto:dquacken@engineering.uiowa.edu

[Steve]

You can drain the ATF out and rub it between your fingers for any grit or ditryness. You probably should consider getting a trans cooler if you dont already have one.

[lapeer20m]

Originally Posted by jstark

. Driving through AZ in July during the middle of the day, it was wound up pretty much the entire time. David
mailto:dquacken@engineering.uiowa.edu

what makes you believe it is related to the transmission? I'm not really sure how the auto trans would speed up by 1K rpm while you're still driving at the same speed......

Are you sure it's not the engine fan? Many engines come with a viscous clutch that doesn't really engage fully until the engine gets hot. When engaged, the fan can make twice the amount of noise that the engine is making at highway speeds.

[jstark]

You can drain the ATF out and rub it between your fingers for any grit or ditryness.

I replaced the filter before the last trip, and the ATF appeared fine.

I'm not really sure how the auto trans would speed up by 1K rpm while you're still driving at the same speed......

Neither do I...that's why I'm so confused. Basically this is what I'm enquiring about--is there any sort of mechanism in the AT540 that intentionally causes this behavior?

Are you sure it's not the engine fan?

Yeah, it's definately not the fan; I'm familiar with that sound. The sound is coming directly from just right of the driver seat, on the raised metal section, where the doghouse meets the floor. I should have mentioned that when the "revving" is occurring, this metal heats up wicked bad. Like "way-too-hot-to-sit-on" hot.

Thanks again guys,

David
mailto:dquacken@engineering.uiowa.edu

[frank-id]

Bus trans are a special kind of transmission. The units are very strong and long lasting with just a little care. To determine what problem the trans may have, some more information is needed. A few guages are needed for monitoring the performance and condition of the trans. The guages needed are a 320 degree temp guage and an engine tachometer. If this noisey bus were in my shop for a check over, I would remove the trans pan and check the internal oil filter. The filter is large and a bit spendy at about $30. I would replace the external trans oil filter and look for any signs the filter had begun to collapse and any water. Some times a trans can be full of ATF but the filters can be so restrictive the trans will loose lockup. By observing the engine tach with every shift a pattern can be established. When the trans makes the odd sound the tach should be onserved to notice if the RPM raises with no more forward speed. I'm not a big fan of having the trans cooling circuit inside the radiator. I like a seperate trans cooler. The best coolers are called stacked plate design. The not good coolers are known as tube and fin design. It would be impossible to have too big a cooler. Checking the trans fluid for color, feel and smell would be a good test. Standard ATF can be made worthless is about 1 hour. Synthetic ATF cannot be destroyed but costs about $5 per quart. Frank