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Context: I have an old (1994) diesel van. It has a transmission computer, and not an engine computer. It's not OBD2 compatible, so it's mostly just simple sensors and whatnot.

From my recollection, I think this happens in all gears.. (auto trans)

When I'm driving and actively accelerating, everything is normal, but when the acceleration and engine speed dips below wheel speed the rpm doesn't stay with the wheel speed.

A couple examples: At 60mph, if I let off gas, rpm will drop to idle, and I don't have any engine braking

At 30 mph (idk let's say 3k rpm) I can lower rpm to 1.5k with no consequence and speed will remain the same, no engine braking.

I also have an overdrive problem, I should be at 2500-2800 rpm at 60mph, but I think I'm not getting my last gear (or lockup). I'm mentioning this because it's probably related.

So far in my troubleshooting, it's pointing to either a solenoid problem (there's 3 of them inside the oil pan), or a transmission issue. I've checked all the easy sensors, and they seem to be operating normally.

Does this make more sense as a bad solenoid or bad torque converter, etc? Trying to wrap my head around this with my limited knowledge.

Thanks,

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  • What is the make/model of the vehicle in question? Do you know what the model of the transmission is you have?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 14, 2023 at 12:46
  • Did you buy it recently, or is it recent behaviour of a vehicle you are familiar with?
    – Weather Vane
    Commented Dec 14, 2023 at 19:09
  • @MattStrom you should be able to test how many gears there are by a smooth acceleration from rest to cruising on a level road, and keeping an eye on the tachometer.
    – Weather Vane
    Commented Dec 14, 2023 at 19:18
  • I didn't want to post the make/model since I thought it would be better to keep it vehicle agnostic. There are forums for my vehicle I'm on to get more specific answers. I bought it about a year ago, and to my knowledge this has been an existing issue.
    – Matt Strom
    Commented Dec 22, 2023 at 18:17

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From my experience with automatics it's completely normal for them to go out of gear when you take your foot off the accelerator, coasting instead of staying in gear saves fuel. Engine braking in an automatic transmission is only something I'd expect to see with sports models as it gives a more manual feel to the driving experience. So that sounds like it's by design. If you put your van into L it should stay in gear and give you engine braking, if it doesn't then you have a problem. So far it's expected behavior.

Keep in mind that diesels cruise at lower RPM than gasoline engines, they have higher torque so don't need to spin as fast to give you the same power. You may see 1700rpm on the gauge and still be in gear.

Regarding your overdrive, solenoids are a common issue, so you may as well replace them if you've dropped the pan and have access. Before going deeper I'd do some thorough research on your particular transmission to make sure you know what's expected, that way you'll know if you have an actual problem.

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  • Modern autos (1994 included) usually have torque converter lockup in top gear which will provide engine braking.
    – HandyHowie
    Commented Dec 14, 2023 at 14:14
  • The OP's problem too is that the gear doesn't seem to be shifting up when cruising.
    – Weather Vane
    Commented Dec 14, 2023 at 19:07

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