Driving down the interstate between 55 and 75 mph the engine will begin to howl. I'm not nailing the gas, just driving along. This happens quickly, but not abruptly. It's as if I hit the gas to pass someone and the tranny is shifting down into a passing gear.

If I had OD, I would swear that the tranny had downshifted or was in a passing gear. Of course I do not have OD.

I have slowed down (it sticks in the "passing gear") and shifted into 2nd, which works, so I don't think that the tranny is shifting down. There is a noticeable drag (almost like being in 2nd) when this happens.

Nothing else seems wrong, although this did start after my last run on the rocks. I have driven the truck about 5,000 miles since then, but never noticed it before so it may or may not be because of something done on that run.

'90 Burb. 350/TH400. 100,000 miles. All original drive train. No smoke/burning smell.

WTH??????

so is the transmission slipping? if so, i'd check the fluid level first. if not, i'd suspect the clutch fan.

I vote for the fan clutch

I suspect that it may be a clutch fan also. when it does the noise does it sound more like "roar"?
very loud and seams to sap the power from the gas pedal?

the only other thing I could recomend to check is the torque converter lock up soliniod, it could be fried and the engine is over reving on occation

Sounds like the direct clutches are smoked. Check your fluid to see that it's not darkened and burnt. If the 3rd clutches narrow up enough, the trans won't have enough pressure to keep them engaged and shifts to second.
Get it to show the symptoms, and shift back and forth between second and third. If you don't feel the shifts, you're in for a rebuild.

If it is indeed kicking down (rpms change) then I would check the transmission ,odulator valve first thing. Easy fix. Do this after checking fluid level and checking to see if the fluid is burnt

I suspect that it may be a clutch fan also. when it does the noise does it sound more like "roar"?
very loud and seams to sap the power from the gas pedal?

the only other thing I could recomend to check is the torque converter lock up soliniod, it could be fried and the engine is over reving on occation

th400s do not have lockup.

this is true no lock up for a Th400

could have sworn I have seen lock soleniods on them. the olds ones didn't interchange internally with the newer ones. they were pushed out of service by the 4L80E.

I wonder if its because I worked on so many P chassies, they had alot of stuff you didn't see on the smaller trucks.

could have sworn I have seen lock soleniods on them. the olds ones didn't interchange internally with the newer ones.


Pre-'68 TH400s came with switch-pitch converters, when behind big engines in big cars. Long tailshaft versions in Buick, Olds, and Cadillacs had a solenoid on the pump and pressurized the converter to change stator pitch. I tried to find someone who would build a lockup converter for it, but nobody thought the market was there for a 3000 stall converter with lockup. Oh well.

mine in my cadd did the same thing, I know exactly what your saying. I could down shift into second and you could tell it, but back into drive and you could hear the change now its parked in back because it want come out of drive! I think mine is smoked wish I could help other then saying dont drive it ! when mine started doing it it went quick

I had the exact same problem, near the same speeds etc. I replaced lines, torque convertor, and a bunch of other crap the TH400 manual told me to before realizing the internals in the trans were spent. I had it rebuilt and it worked great afterwards.

I am not sure my experience is a lot of help to you but I thought I would chime in as my symptoms were exactly the same as yours.