Chevy K3500 auto tranny slips out of overdrive when coasting

Maintenance/Repairs
1


I have a 2000 K3500 Chevy 5.7L (I think, it's not a 424) with an automatic transmission that will slip out of overdrive when 'coasting' (i.e., When there's little load on the motor just as you crest a hill or complete a coast down a hill, you can feel the transmission 'lose itself', and then downshift into 3rd.)

Once it slips out of overdrive, nothing I've tried will get it back into overdrive, except for shutting down the engine for 5+ seconds. Simply turning the engine off and then restarting doesn't clear the problem. You have to leave it off for 5+ seconds.

At first this happened once or twice in a hundred miles. Now it's becoming more frequent. Obviously I rather not pull to the shoulder on some roads, but running it in 3rd gear (about 1500 rpm higher) only makes Big Oil richer. And shifting into neutral at 60 and turning off the engine should probably be avoided on a regular basis.

Is this a common problem with these vehicles? What's to fix?
2

I have had 2 Chevy S-10s with automatic and it is normal for them to coast when you let off the gas, but it is not normal for them to downshift to 3rd, unless you are going too slow for overdrive.

3

Have you tried scanning it for codes yet?? What is concerning me is that O/D will not apply again until you cycle the ignition switch, this is telling me that most likely you are looking at an electrical issue. How many miles on the vehicle?? Have you serviced the trans lately? How often has it been serviced?? Have you checked the fluid level/condition??

transman

4

Haven’t scanned the codes yet. The truck’s been babied since it was new. Odometer is at 55,xxx, all fluids were flushed (not drained) approx 5,000 miles ago. Tranny fluid level is currently on the full mark and not discolored at all (clear read), and there’s no resistance when you rub it between your fingers.

I was also suspecting an electrical issue - probably computer related - because it’s more than just cycling the ignition switch. You have to leave it turned off for at least 5 seconds (one-thousand one, … one-thousand ten) for it to clear.