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S10 Pickup Downshifing on highway

I have a 2001 chevy S10 pickup with a 4 cyl. engine and auto transmission. I’ve had this problem since I bought it used in 2006. What happens is when I am driving on the highway (55mph and up), there are times when the engine will downshift from 4th to 3rd gear while I am driving at a constant speed. I’m not pressing on the gas to accelerate faster and it happens even when I am driving on a flat road. This usually happens during the hotter months, from about April to October and hardly happens if at all during the colder months of winter making me think it is a temperature related problem.

What I’ve noticed over the years is that it happens more frequently if I drive in the city first (doing a lot of stopping and going) and then get on the highway. I’ve noticed it more when I am driving on the highway and let’s say for instance the car in front of me starts to slow down, and when I start to lift my foot off the gas, that’s when it will downshift from 4th to 3rd. I usually have to let my foot completely off the gas so that it shifts back up to 4th and then start to press on the gas again. If I have been driving for a while, it keeps happening, and there have been times where I drove slower on the highway to keep it form acting up so frequently.

I took it to a Chevy dealer this past summer to have it looked at. The repair man said that he did not know what would be causing it and that they couldn’t replicate the problem so I took it on a drive, with him riding with me, and got it to act up once on the drive. He observed what it was doing and would talk to another mechanic about it. He later concluded that it was just how the truck operates when driving at high speeds like that with a hot engine. I told him that I am mainly concerned about over-reving my engine, but he said that is not a concern. However, neither him nor anybody that I have talked to has ever heard about this happening to any other vehicles. I even had the transmission fluid replaced two summers ago, but it didn’t seem to make an impact.

Is the Chevy mechanic right about it just being a normal operating feature of the engine/vehicle, or could it be something else?

It sounds like the torque converter clutch is randomly disengaging. How’s the condition of the transmission fluid?

If the tranny fluid is okay, then look at something as simple as a defective brake pedal switch. As soon as you step on the brakes, the torque converter clutch disengages.

Tester

It may be normal. That motor only had 120 ponies when new, was very unareodynamic, and tacked to a 3-speed (4-speed?) automatic it may need the extra leverage occasionally to keep its speed up on the highway. A headwind, 9even in gusts), a very slight incline, even a slight variation in the road surface can be enough to make a difference.

This used to be a common behavior of small engines with limited automatic trannys 30-40 years ago. And as they crept up in mileage and a few of the limited ponies went to pasture, they’d do it more often. By all means have it checked, but it might truely be normal.

I agree that it just sounds like the torque converter clutch disengaging. If you are driving along at a fairly constant pace and pull your foot off of the gas it will disengage. Its supposed to. Then after the throttle position & speed stabilize it will lock back up. And, as mountainbike implied, if you’re just going along maintaining speed a good gust of head wind or a little grade can also disengage it.

Thank you very much for the input. I checked the tranny fluid and all seems well. I’ve been busy these past couple of weeks but I will hopefully be able to get it in to have the torque converter clutch looked at within the next month or so.

A friend of mine had a Buick with a 4 cyl years ago that acted very similar. I changed the thermostat and never had a problem again. Apparently the transmission was not staying at proper operating temperature. The thermostat was staying open and not letting the engine and transmission run at a normal temperature.

I had a similar problem when I had a Plymouth minivan that I bought uded that I thought had 50,000 mile spark plugs and it turned out to have 30,000 mile ones. It stopped downshifting when I changed the plugs.

As Tester said, it might be a faulty brakelight switch.