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Highway downshifts going downhill

My mechanics are in flat Philadelphia and the problem happens regularly when I’m traveling through the Pocanos or the Catskills. At around 70 mph when I am going down a hill the automatic transmission will shift down from overdrive into 3rd or even 2nd gear. It should stay in Overdrive. I have to tap the gas pedal to keep it in overdrive. They already looked at the TPS.

Also, around 75 mph my ABS light goes on and stays on until I restart the car. Not sure if this is related. Just had a complete brake job.

Are you sure that it shouldn’t downshift on its own?
You did not reveal the model year of your truck, but I can tell you that on many recently-manufactured vehicles, the “fuzzy logic” and grade logic built into the ECM/TCM do cause the transmission to downshift. So, this may be normal for your truck, or it might not be.

As to the ABS problem, I doubt that this is related to the transmission situation.

There is a good chance that its related. I don’t know the specifics on this truck either in terms of ABS or transmission. But, in general, vehicle speed info is critical to the control of both of these systems. Is seems likely that you may have a speed sensor problem of some kind.

I would say that your best bet would be to have someone with the right kind of scanner (probably a Tech II, though you never said what year your truck is) find out what kind of error is triggering the ABS light.

Give folks the trucks model year (other things like mileage wouldn’t hurt either) and maybe you won’t get conflicting answers.

Sorry. I thought it would automatically add that to the post. It is a 2000 Chevy S-10 with a 2.2 liter engine. I am sure it should not shift because the downslope is usually quite gradual. If anything, it should stay in overdrive and just lower the rpm to 2400.

It’s got 115,000 miles.

I’m pretty sure that this ABS system does use info from the vehicle speed sensor. I still do not know the exact set up. You will have wheel speed sensors at each front wheel. You may or may not have them on the rear. Then there is a vehicle speed sensor on the transmission. I’d say that someone who knows what they are doing needs to do a thorough check of your speed sensors.

its an s-10 just shoot it

No, no, sell it to a Mexican. They take those old beaters, more than ten years old, and keep them running for a very long time. They can import the older small pickups if they live in the country. The Ranger is also very popular. For the costs, they don’t mind working on them.

I’ve never noticed this behavior on my 2000 S-10 Blazer in the 8 years I’ve driven it. As for the ABS, I would check the connector at each wheel sensor.

Different size tires front and rear would set off the ABS, if the size difference is slight, the ABS light would not come on at lower speeds.

Ed B.

I really don’t like car payments and I don’t mind what it is as long as it gets me and my stuff to where I want to go. But if it is a serious transmission problem I might have to shoot something.

If the front and rear tires were different sizes (160 on the front, 165 on the rear)could that cause the problem?

The 160/165 is about the width of the tire. That won’t matter. What might matter is differences in tire diameter.

You’re probably just going to end up with a new vehicle speed sensor. This is very minor work - it is external to the transmission and not an expensive thing. I just said to have someone who knows what they are doing check it all out b/c guessing about repairs gets really expensive.

So they can test even when the car seems to run fine? This is part of the original question. It only happens when I am far away from my regular mechanics on the highway. I am thinking that I might have to guess. Or ask somebody knowledgeable like yourselves.

You need someone who is serious about figuring it out. They can put it on a scanner and drive it. They’ll obviously have to get it out onto the highway and get it up to highway speeds.

Or take it to someone who can pull the ABS codes since that doesn’t require them being there when the codes are triggered. This may or may not answer the question, but you certainly want to know what is triggering the ABS light either way.

You are correct. The truck uses the info from the VSS in the trans for the rear wheel ABS.

Someone serviced the transmission and replaced the TPS and then declared it fixed. The other mechanic likes to keep the truck a few days before getting to it. I’m reluctant to go through that again. Maybe I should just keep replacing sensors until something works. There is the shift solenoid, the downshift sensor, wheel speed sensor, vehicle speed sensor (?), maybe something in the transmission is clogged so fluid is not getting where it is needed going downhill.

I’m not the type of person who can sell the thing knowing there is a strange problem.

Is the down shifting occurring when the cruise control is on?

It is worse when cruise control is on. Sometimes when cruise control is off I feel like I can stop it from happening by very slowly letting off the accelerator as I come over the top of the hill. Slowly letting off until I am just slowing down slightly going downhill. Then it does not downshift.

or accelerating downhill but then I end up going to fast. If I accelerate it does not downshift.

I think the engine cools down while you are heading downhill for a while, and the transmission downshifts in response. This is not only normal but desirable. You want your automatic transmission to downshift during long declines. The idea is that you don’t have to brake as often to slow the vehicle, and so the brakes are cool in case you need to stop quickly. If you need to stop quickly, you don’t want hot brakes.

Your car should not stay in overdrive on hills in any event. This gear should only engage and stay engaged on relatively flat ground.

I been in other chevy s10s and they don’t do it to the extent that mine does, or at all. Mine will drop into second gear and hit above 4000 rpm on a very slight slope. It does not make sense.

This is going to sound like a stupid question, but so what. You said “If anything, it should stay in overdrive and just lower the rpm to 2400.”

Have you tried just taking your foot off the gas?