Volvo 940 d24TIC downshifts at high speeds

[Lars G]

Hi.

Just bought my first d24tic and its an automatic. I somewhat know my way around a manual transmission such as the m47 and m90, but am quite a greeny on the automatic.

Anyhow the case is:

When i accelerate to about 100km/h + and try to decelerate the gearbox downshifts to the lowest gear possible at that speed so either second or third which makes the revs hit the limiter. As soon as I break and it goes below 100 it goes back to normal. The transmission has been flushed about 3000km ago. What can the problem be?

Keep in mind this only happens once i start to let pressure off the gas pedal. If it shifts down and i continue to accelerate it goes right back to normal. Seems like it wont let me cruise at that high speed because it works perfectly in any other speed below 100km/h.

Thanks for any help

[ngoma]

Zf 4HP22L?

[Lars G]

Originally Posted by ngoma

Zf 4HP22L?

Its an AW71 pretty sure.

[v8volvo]

That is a very strange issue.

Sounds like a problem either in the valve body or the governor. Either the valve body section that is controlled by the kickdown cable is incorrectly going to a setting that looks like full throttle causing the downshift, or the governor faulty and is simulating a lower road speed than the car is actually doing, causing it to call for a lower gear?

Does the engine actually overrev, like beyond the redline? Or does it still choose a gear that is lower than requested, but within the engine's speed parameters? If it still keeps revs below ~4500 then that would suggest the governor is OK perhaps, since I don't think this trans has any reference of crankshaft speed to control revs otherwise. But if it's shooting the revs to 5000 or more then maybe a governor fault is a possibility.

The ZF trans sometimes can have a governor problem that results in delayed shifts out of first gear, usually caused by buildup of clutch material in the governor from a problem elsewhere in the trans. But I don't think I have ever heard of that issue causing problems at highway speed. Normally with this issue, once it shifts out of first on the first upshift after a cold start, things work as they should.

I'm trying to think what else could cause this to be related to higher road speeds. Could the pickup screen inside the transmission pan be loose and allowing the pump to suck in some air when spinning the pump at high speeds for extended time (ie highway travel)? Maybe combined with slightly lower than correct fluid level? Aerated fluid could cause all sorts of misbehavior in the valve body I would imagine. Or if the pickup screen is clogged and restricted perhaps similar issues could surface.

Maybe next time you travel on the highway you could pull over after approaching the speed where it acts up, and pull out the dipstick and see if the fluid looks foamy or has air bubbles in it? And also of course, have you confirmed the fluid is full to the correct level and looks/smells normal? No water milkshake, not extremely dark, etc?

Beyond those checks I think my first steps would be to do a complete normal transmission service, changing the fluid and dropping the pan to replace the pickup screen and look for problems for how the old one was installed/broken O-ring or clogged screen, ensure new screen is installed successfully, and inspect inside pan and on the magnets for foreign material or other signs of trouble before cleaning it up well and reassembling. Many automatic trans misbehaviors can be solved just with a fluid refresh if the fluid has not been serviced in a long time. At least that has been my own experience. You often never know when it was last done, whether the correct fluid was used, etc. The transfusion method is my favorite for getting all the old stuff out but even just draining and filling can help.

I very much doubt it has an AW71 as those to my knowledge were never used behind the diesel engines. Diesels in 7/9 series only had ZF 4HP trans. If you want to confirm, look at the transmission pan at the bottom of the trans. If it has a pan shape that tapers narrower towards the rear and is bolted to the trans with 6 securing clamps and a rubber gasket, then it is the ZF which is what you should expect. If instead you see a square shaped pan that is installed with bolts going through a flange around the perimeter and a cork gasket, then that would be an AW trans and we will be very interested in how it is mated up to the engine.

[v8volvo]

Another question: do you observe that the torque converter lockup function is working normally? Assuming the trans is a ZF, the TC should lock up when the car reaches around 45mph (70 km/h) then should unlock again as you slow down to around that speed or a little less (hysteresis).

If lockup isn't working correctly and/or if you notice this problem is closely related to when the converter is or isn't locked, then that might suggest there is a valve body issue related to the TC lockup pressure circuit that is somehow triggering the kickdown circuit when the TC is commanded to lock up. That could perhaps be from a leaky valve body gasket or something else like it, speculating.

That's the only other idea I can imagine that could create such a clear speed threshold for when the trans screws up vs when it acts normally. It comes to mind because the TC behavior does work exactly in that way. And on some cars with taller geared rear axles like I believe the later Euro market diesels had, the speed threshold might in fact be higher like 100 km/h rather than the lower speed where our older US model cars here do it, with their short geared axles intended for 55mph speed limits that were the rule when the cars were new.

[v8volvo]

Oh yeah, one final question: do you know if this issue started to occur all of a sudden one day, or did it gradually become noticeable and get progressively worse over time?

I realize the car is new to you and you probably got it in this condition so maybe you don't know this but maybe you can ask the previous owner. This would give some clues as to whether it's something like a sudden internal valve body failure, or more related to gradually deteriorating trans fluid condition or a slowly clogging pickup screen etc.

[Lars G]

Dont know if i answered all your questions but here we go:

The problem only happen when i go 100-110 or above. I tried taking it to 130 once and nothing happened until i took my foot of the throttle. When it downshifts it revs up to 5000-5500, but when i take my foot off really slowly it sort of shifts up and down between 4000 ich and 5500.

I took a look under it and from what you described it looks like I have the ZF tranny.

The Oil is nice and red on the stick and just below max both when it is hot and cold. It wqs flushed and changed about 3000km ago.

It happened to me on my way driving it home, but has not gotten any worse. The previous owner Said he never experienced it because he had never driven it past 90.

If you mean the OD when you say the torque converter it kicks in between 70-90 depending on how much pressure i have on the throttle and if I am driving up hill or not.

Think that covers it all��

[v8volvo]

Never driven past 90 km/h? Must have been used for picking up groceries only eh?

Does the transmission respond correctly the rest of the time, at slower speeds? I.e. downshifts when pedal is floored and upshifts when pedal is released?

Is there any possibility the kickdown cable is wound backwards on the throttle pulley? Can you post a picture of what the pulley setup on top of the injection pump looks like?

[Lars G]

Originally Posted by v8volvo

Never driven past 90 km/h? Must have been used for picking up groceries only eh?

Does the transmission respond correctly the rest of the time, at slower speeds? I.e. downshifts when pedal is floored and upshifts when pedal is released?

Is there any possibility the kickdown cable is wound backwards on the throttle pulley? Can you post a picture of what the pulley setup on top of the injection pump looks like?

It responds just as it should on everything else. I have realized now though that it doesnt go into OD before 90 exactly. Noticed when i paid attention to it. I can take a picture of the cable one of the next days. I am not home at the moment.

[Lars G]

Ive done some thinking for the last few days.

I assume the shifting is controlled by some kind of brain. A combination of speed, rpm and throttle which then decides what gear is best. What if this assumed brain resets when I go beyond 110. I measured my speed by GPS and realized that 110 is 100.

So lets say it "resets" at 100kmh. When I am then driving 110 the brain thinks I am driving 10kmh and downshifts to second gear when i release the throttle because OD in 10kmh doesnt make any sense. Anyways. Might sound stupid but I have no idea what I should do.