Unusual Transmission Problem...

[polychromeugand]

One of my two 1992 Legacy wagons has a transmission problem that has me stumped. After its started cold (well, it summer here) it operates well for a couple of minutes and then ... it just seems to disengage the clutch. It doesn't re-enagage while the engine is on no matter how long its left in Neutral or Park. Turn the engine off, not even long enough for it to come to a complete stop, and turn it on again and all is well again for about 20 seconds, and then it disengages the clutch packs again. This can be repeated just about endlessly as far as aI can tell. Oh, and its exactly the same for forward D 3 2 1 and Reverse and with or without Manual engaged.

 

Before this began there had been some badly delayed enagement on shifting into gear. The valve body was removed disassembled and cleaned and the solenoid and accumulator o-rings were replaced as were the upper and lower seperator gaskets. The transmission was flushed using the cooler lines. The delayed engagement problem disappeared and it seemed to be running very well and then about 10 miles later this new problem started.

 

The only description I could find on-line of a simiilar problem is purportedly from a transmission tech training manual and it says that the problem is the fluid thinning as it heats and the solenoid valve strainer being clogged and unable to dump enough fluid. So out came the control valve assembly again and all the solenoids got cleaned and tested. There was some sludge but now they work great. I also double checked the 9 check valve balls and the micro strainer and the whaterever it is on the tiny spring as I reassembled it. But all that didn't solve the problem. Neither did a bottle of TransX. (I'm not proud of trying "hope in a bottle" but I'll fess up since someone was bound to suggest it.)

 

I did the shift-lever macarena to get the TCU codes and the transmission control unit has neither any current error codes nor any stored eeror codes. I also tested the wiring harness and the resistance values for all the sensors and the solenoids, the ATF temp sensor and the throttle position sensor and the idle switch. Everything was at spec.

 

I could put the transmission gauge on it and watch the clutch pressure drop off as it disengages but that won't put me any closer to solving the problem. I can't think of a mechanical problem that would be reset by turning the key off for a second.

 

I have no ideas for how to test the TCU outputs while its operating. I don't think its likely the solenoids since they have plenty of "OFF" time in N and P and the very briefest off/on power cycle resets the problem... for 20 seconds. It also doesn't make sense that the a defective TCU would do this as opposed to staying dead.

 

I'm not a huge fan of the idea of grabbing a transmission from a scrap yard. There's one available about an hour away for $350, but I understand seperating the transmission from the engine involves muscling it around for an hour or so. I have a pair of torn rotator cuffs, so that sounds like more pain than I'm willing to sign up for. (anyone have an idea what a shop in the Boston, MA area would charge to do the work?)

 

So... I'm completely stumped... has anyone seen anything like this before... anyone have any ideas? ... ANY ideas ....

[WoodsWagon]

If it's reset with a key cycle, it's unlikely to be hydralic failure. If you unplug the main harness connectors to the transmission, I believe it defaults to 2nd gear, so if you unplug it and drive it around and it works fine in 2nd, the tranmission is probably not at fault.

 

Electronic modules like the TCU can fail in bizarre ways, so just because it works breifly doesn't mean it's not at fault. I've got a ECU that runs the car fine for a couple days, then switches into a default mode where it holds the injectors on at near full duty cycle as long as the key is on but turns the fuel pump off. Leave it parked for a month, it runs fine when you come back, then craps out 2 days later.

[Fairtax4me]

Sounds like an electronics problem for sure.

You might try to get a pressure gauge on it and see what you get for running pressure and if it changes for the worse when it kicks out of gear. But I'm gonna go the new TCU will probably fix it route.

[nipper]

It has to be properly diagnosed with pressure gauges. i can see this being a bad acumilator, piston in the transmission.

 

If you thnk it is an TCU issues find another to swap out.

 

It can be the pressure valve (sorry forget the exact name) in the tranny too.

[idosubaru]

i've seen this exact same issue. the only difference on mine was it would last longer than 20 seconds. it also had the symptom you describe where at first it wouldn't shift...just hung in 2nd gear. unfortunately i had a spare trans so i just swapped it out.

 

he's right - unplugging the transmission will force it to run mechanically in 3rd gear and locked 4WD. i've unplugged a transmission and manually installed a switch to control the 4WD (it's only one wire, really easy) solenoid so you don't have to run around in locked 4WD all the time. i've driven like this for a long time in a daily driver a few years ago. a little slow getting going in 3rd and high rpm's highway but who cares at least the car moves. you could easily do this and have a drivable car while you continue to diagnose.

 

the TCU uses the TPS signal from the engine, is it worth swapping in another TPS just to see? or testing yours? i would think the trans is toast but you seem intent on figuring this out.

 

that's pretty awesome you're totally fine ripping into the guts of an auto trans.

[bstone]

Check engine light possibly? Sounds like an electrical problem, likely computer controlled solenoid or similar. I say that as you said if you turn off the car for even a second but try again it works. Just sounds like a computer issue. It can also be a hydrolic issue, tho I would start with electrical.

[polychromeugand]

If it's reset with a key cycle, it's unlikely to be hydralic failure. If you unplug the main harness connectors to the transmission, I believe it defaults to 2nd gear, so if you unplug it and drive it around and it works fine in 2nd, the tranmission is probably not at fault.

 

Well, I took it for a couple of rides just to be certain of what is happening. It takes a bit longer to begin disengaging, bit it still disengages. Without the TCU there is a prolonged tail-off as it disengages the clutches. It also still comes back after the briefest of key-off cycles. There is never any engine braking. There may be some information in how it behaved.

 

If I understand Section 3.2.M.24 Fail Safe Function;

 

Solenoid 1 & 2 off should put the hydraulic circuit in 3rd gear hold, but it drove like 2nd gear.

 

Solenoid 3 off should lock up the overrunning clutch. The overrunning clutch was wide open - there was no engine braking. This is an unexpected result because the manual's description is just the opposite. The manual's description of how the function works doesn't seem to agree with the hydraulic schematic.

 

Duty Solenoid A (Line Pressure) Off should be max line pressure - I didn't put the guage on it.

 

Duty Solenoid B (Lock Up) Off should release the lock-up clutch leaving the torque converter to transfer engine power at all times.

 

Duty Solenoid C (Transfer) Off should leave it in direct coupled AWD at all times. I didn't notice any unpleasantness while making a U turn at minimum steering radius. Right now there is about as little sand on the roads as there ever is. I expected something like when the differential gear oil has over-ripened, only more so.

 

So... I figured I would try the stall RPM. Its 2700, (book says 2600 to 3000 is OK.) The test can't be run for more than a few seconds, but it stays good that long. Obviously when "disengaged" the stall RPM woul be very high.

 

Well, its still confusing, but I suppose this lets the TCU off the hook for the moment.

[WoodsWagon]

Key cycle shouldn't affect it if you unplugged the harness from the tranny. Did you just unplug the TCU?

[polychromeugand]

Key cycle shouldn't affect it if you unplugged the harness from the tranny. Did you just unplug the TCU?

 

Yes... I unplugged a square connector with 16 contact cavities and 13 loaded contacts on top of the transmission near the firewall that probes out with all the correct resistances and grounded pins as the transmission and after unplugging it the transmission was stuck in a single forward gear. (The other connector near there is rectangular and has 12 contact cavities.)

 

At this point I don't have any useful ideas. This transmission has a low enough number to have the oil pump housing flexes and blows out the seal problem. (and all the other housing related problems later revised). I don't know if it ever got the the various band-aids, but whether it did or didn't it might take a lot of time disassembling this transmission and figuring it out, and applying more band-aids and at the end of the process it will still have almost 200,000 miles on it.

 

Unless anyone has had any similar experience it seems like its time to either put in another transmission or scrap the vehicle. Admittedly anything I can get from the scrap yard is going to be old as the hills, but if I can get a '93 or '94 it won't have half of the design bugs.

[WoodsWagon]

What I'm trying to get at is if there is no electrical connection to the transmission, all connectors unplugged from it, so it's stuck in 3rd gear, and driven, and it still disengages, then it's 100% a hydraulic problem. If it holds 3rd fine with everything unplugged and you can drive it around that way, then the hydraulics are fine.

 

By key cycle, are you just quickly clicking the key off and back on while moving, or are you letting the engine come to a full stop, then starting it again? Letting it stop would also stop the transmission front pump.

[polychromeugand]

Sorry for the delay. Yes, with the TCU disconnected nothing electronic can change the behavior and it can only be a hydraulic problem. The key off time is as brief as possible, but allowing for the tachnometer needle lagging the engine I'd say the engine is close enough to stopped. The transmission does have a low enough seriel number to have the oil pump housing flex and a number of other issues that were later fixed. I don't know if the metalized gasket and other band-aids were applied without checking the old dealer service paperwork.

 

At this point my guess is that when I cleaned the control vavle assembly it restored the pressure regulation, and ended the delayed engagement, but there's another hydraulic circuit, possibly in the oil pump, that hangs up at full pressure. If I connected the pressure gauge to each circuit and recorded the behavior I might be able to isolate the current problem, but...

 

Since its become certain that any repair will involve removing the transmission its become more expedient to swap it than to do any more work on this one. It seems to have reached the point where if you try to fix one thing at a time something else will fail so it needs to be rebuilt or replaced. Since I've torn my rotator cuffs I should have someone else remove and install it and for $350 for a used transmission they can do it all at once.

[edrach]

Sorry for the delay. Yes, with the TCU disconnected nothing electronic can change the behavior and it can only be a hydraulic problem. The key off time is as brief as possible, but allowing for the tachnometer needle lagging the engine I'd say the engine is close enough to stopped. The transmission does have a low enough seriel number to have the oil pump housing flex and a number of other issues that were later fixed. I don't know if the metalized gasket and other band-aids were applied without checking the old dealer service paperwork.

 

At this point my guess is that when I cleaned the control vavle assembly it restored the pressure regulation, and ended the delayed engagement, but there's another hydraulic circuit, possibly in the oil pump, that hangs up at full pressure. If I connected the pressure gauge to each circuit and recorded the behavior I might be able to isolate the current problem, but...

 

Since its become certain that any repair will involve removing the transmission its become more expedient to swap it than to do any more work on this one. It seems to have reached the point where if you try to fix one thing at a time something else will fail so it needs to be rebuilt or replaced. Since I've torn my rotator cuffs I should have someone else remove and install it and for $350 for a used transmission they can do it all at once.

Yikes, transmission issues AND a torn rotator cuff. Talk about double bad luck. You're wise to have someone else do that transmission swap for you. Best of luck with the repair.