Help with transmission diagnosis

[Xfingers]

02 XC70 with 130k miles.

When I first bought the car 4 years ago with 118k, after 30 minutes of driving the trans was spinning out of gear, slamming, clunk on downshifts, etc. Replaced the fluid, the car slowly adapted and the shifting smoothed out after @2500 mi. No problems for the last 7000 miles except when driving up a long hill to the ski resort. About 50% of the time, the car would get to the top and then throw a P0735 (5th gear incorrect ratio) and go into limp mode. I'd pull over and stop for about a minute, start the car up, my gears would come back, and I'd be on my way again. The transmission was shifting with no perceivable issues so I really didn't know what to make of it.

This summer/fall the trans was shifting better than ever, "almost like new" I told myself. Going to the ski resort, my XC flew up the hill no problem with power to spare. At the top, though, there was that pesky P0735 code and limp mode. When I turned the car off and started it back up, this time the gears weren't coming back and I was getting "-.-". To make matters worse, the P0735 code has now started coming on during regular driving and throwing me into limp mode. I'm also getting another code, P0700 (general transmission fault). Unfortunately, I've done a lot of driving in 3rd gear pulling away from stop lights, etc.

I decided to flush the trans fluid again (10k mi. later) but this time drained 3+ quarts from the pan. The top of the drain plug was covered with gray silt but it didn't seem like an alarming amount. There were no pieces or chunks of the trans in the oil. Although the oil was dark burgundy and needed to be replaced, the fluid change didn't stop the trans from going into limp mode. I should mention, every time the car goes into limp mode, if I wait several hours or overnight, when I start the car up again I have all my gears and the shifting is fine until the code returns.

So I've bought a VIDA/DiCE setup and started collecting info. First I reset the Trans Fluid Counter. I also checked the fluid level and at 80C it's right on the Hot Max mark. Now I'm driving around with VIDA running and monitoring various transmission parameters: Fluid Temp, Gear Ratio, Expected Gear Ratio, S2, SLU, SLT, SLS. Sure enough in 5th gear with an expected ratio of 1.03, the actual gear ratio is fluctuating much higher. The other 4 gears are not deviating much from their expected ratios. VIDA has also indicated that I need to upgrade the TCM software to 30677036.

Unfortunately, while dealing with this latest trans issue, another problem has reared its ugly head. Above a fluid temp of 55C, the trans starts slipping in all gears. This is something new I haven't experienced with this car before. Yesterday, the fluid temp was 70+C and I was barely able to pull away from the stop light. Had to use my flashers to warn the driver behind me! Took the car for a test drive today and the car shifted great with no problems. Held the speed to about 40mph to keep the car from going into 5th. Once the trans fluid temp hit 55C, I could feel the trans starting to slip again as it went through the gears.

Obviously I have no plans to replace the transmission ($5000+). These are the options I'm considering:
1. Install a HEAVYDUTY trans cooler with Magnafine filter. MY XC loves running at the lower fluid temps but is it reasonable to believe a cooler can keep the fluid in the low 50sC? For $100, I don't mind rolling the dice and see what happens. This one on Amazon is 12x12x3/4 in. so I'm thinking it will install in front of the radiator?
https://www.amazon.com/Hayden-Automo...mission+Cooler

2. Tackle the new GM valve body replacement procedure. To me, this is the real crux of the issue and reason for this post. Can my shifting problems be cured with a new valve body or is there significant damage to the transmission internals - bands, clutch plates, etc.? Again, the trans is slipping in all gears above a fluid temp of 55C.

3. TCM software upgrade. I don't have a relationship with a dealer and I think the nearest is about a 45 min. drive. There is an independent shop nearby who might have the ability. I'm sure the upgrade would be an improvement but I'd be skeptical it's going to solve my problem.

I want to thank everyone on this forum for all their help and support over the years. Without you guys, I would have abandoned this car by the side of the road years ago!

[tony1963]

I would have recommended new shift solenoids a long time ago, before you kept driving the car. New shift solenoids along with a drain and refill typically solves the problem.

With no idea of how many miles you have now, I can only suggest replacing the transmission. If you feel up to it, you can get a teardown manual and do it yourself.

[Xfingers]

tony1963, thanks for weighing in. I've got 130k mi. on the transmission. So the worn shift solenoids are causing the P0735 "5th gear incorrect ratio" code? What is the evidence leading you to figure the whole transmission is toast vs a valve body replacement? I'll search for a AW50 teardown manual, should be an interesting read...

[Astro14]

No software or fluid change is likely to fix this. When the temp gets above 55C, the fluid thins out enough for the line pressure in the transmission to drop below the level needed for it to work.

The first time you had issues was the time to address this. The car stopped doing it, but the problem (low line pressure due to worn valve body) was still there. It was masked by cold fluid, which is more viscous. Lots of throttle, climbing hills, but not warm weather, will warm up the fluid. That's when the transmission will act up again.

Change the fluid while you figure out what to do next. Resetting, reprogramming, aren't likely to help. Flush the trans completely, and it'll help, for a bit. Then, in a month, or perhaps sooner, you're going to have to source that rebuild. It's do-able to swap these. An all-day, 10 hour job. Rebuilding isn't easy, but buy the ATRA manual for this (it's also called the AF-33 in Nissan and GM cars) and you'll at least know what you're looking at.

[Astro14]

tony1963, thanks for weighing in. I've got 130k mi. on the transmission. So the worn shift solenoids are causing the P0735 "5th gear incorrect ratio" code? What is the evidence leading you to figure the whole transmission is toast vs a valve body replacement? I'll search for a AW50 teardown manual, should be an interesting read...

Most likely the SLT solenoid itself. The line pressure control.

The shift solenoids can be easily changed, but I wouldn't bother with them by themselves. A worn bore in the SLT is common in this vintage and causes all the issues you describe...and the five shift solenoids will do nothing to adresss that.

You can try a valve body. Do the whole thing. New or rebuild (there was a thread about a GM body costing only $550 a while back, read that thread) and if you're lucky, the soft parts aren't completely worn out, and the new valve body fixes everything.

But every time it slams, or misses a shift, you're doing damage.

And they don't "heal". The damage accumulates.

[Xfingers]

Thanks, Astro14. I just last week changed the fluid. Strange thing is, when I first started getting the P0735 code the trans was shifting well and wasn't exhibiting any noticeable problems. Back when I originally bought the car, after warming up the trans was spinning out and slamming into gear but not throwing codes. After a fluid change, the trans slowly adapted on its own and shifting vastly improved. The worst abuse this trans has seen (not including two previous owners) was when I first drove the car before the initial fluid change. The shifting was real smooth this past summer so I thought I'd tackled the trans bugaboo. I wonder if what that P0735 code was really trying to tell me was that my fluid was shot. I didn't consider it because it'd only been about 6k mi. since the change. Maybe going up the mountain pass stressed the weak fluid to the point where it couldn't sufficiently lubricate. The fluid color was dark burgundy when I replaced it.

When you say "the fluid thins out enough for the line pressure in the transmission to drop below the level needed for it to work" are you talking about the operation of the valve body? Due to wear in the valve body passages, only when the fluid is cold and thick can the various solenoids operate correctly to affect proper shifting? When the fluid warns up to 55C+, then the pressure of the flow thru valve body passages is insufficient and causes slipping during shifts? This would suggest valve body replacement as a possible remedy. I know its a coin flip but the $600 or so might be worth a shot before tossing in the keys. I'm familiar with the GM thread as well as your in-depth tutorial!

[Xfingers]

Based on the replies and digging around for info, here's what I'm guessing is going on inside my transmission:

When the trans fluid temp reaches 55C, the fluid has thinned to the point where the accumulated wear of the valvebody passages plays a factor in lowering the flow rate of the fluid through the valvebody. This reduction in line pressure doesn't allow the various solenoids to operate correctly, resulting in slipping, harsh engagements and wear on the transmission internals.
Simply stated: valvebody wear => solenoid malfunction => bad shifting. This has been mentioned many times before on this forum.

If my diagnosis is accurate, this would explain why my trans shifts fine when cold but acts up when the fluid gets warm. The unknown for me is how much damage has been done to the internal bands, clutches, TCC(?), etc. I'm willing to attempt a valvebody replacement to see if this corrects my situation or if I've got bigger problems. It's the middle of winter and I'm not sure how soon I'll have a chance to tackle the valvebody. First I'm planning to install a plate cooler with a magnefine filter ($75 in parts). I've come across posts suggesting hot fluid stirs up metallic particles which then gunk up the solenoids as another explanation why a trans can shift well when cold but poorly once warm.

If there's something I've overlooked or anyone has suggestions, I'm all ears.

Edit: Just spoke with a tech at Hayden Coolers. He didn't think that a worn valvebody was likely to affect line pressure. Seems to contradict what's been said on this forum. Maybe he's not familiar with the AW transmission?

[Astro14]

All oil thins with increasing temperature. Basic chemistry. There's no critical temperature at which it "thins too much" unless (and this is important) there is wear in the mechanism for which that viscosity was specified.

So, the SLT solenoid operates on a "duty cycle" to maintain the proper pressure. That's what it was designed to do. The on/off rate (duty cycle) can vary from roughly 5% to 95% to maintain proper pressure. If the bore of that solenoid is worn, then oil pressure can't be developed, even if the solenoid itself is up to 95% closed - the oil simply leaks around it...and you have low line (operating) pressure.

Low pressure to move band actuators/controls...they don't move as they should...so, shift timing is way off...and the transmission doesn't (or can't) select and engage a gear. You get "false neutrals" where the engine revs and never engages, "shift flare" where the engine raves in anticipation of the next gear engagement, but the gear engages slowly, and "slam shifts", which are self-explanatory.

So, given the combination of wear, and the reduction in viscosity with temperature, have resulted in a temperature of 55C which causes that pressure drop off in your particular car. In a health valve body, the SLT will be able to maintain pressure. In another car (like, say, my wife's XC) the drop in pressure might not happen until, I don't know, 85C?

So, will a cooler help? Yes, it keeps the viscosity higher that it would otherwise be, since the fluid is cooler. Will new fluid help? Sure, fresh fluid always has better viscosity than worn fluid (which drops as shear in the oil occurs).

So, a cooler, and fresh fluid, can stave off the inevitable need for mechanical repair.

But every time you have shift flare, late shifts, harsh engagement, or slam shifts, you're beating up the transmission. Too much of that and it's toast. There are friction materials in there...and quick engagement has low wear/impact while the malfunctions cause greatly increased wear. So, caught early, fixing the hydraulic issues will save the transmission.

But if the tranmission has been acting up, it's been wearing excessively. No way to tell which is the case in your transmission. You can try the valve body, but you have to be mentally (and financially) prepared for the latter case, and if so, the complete rebuild that is needed to replace those friction materials that were beat up, as well as fixing the hydraulic issues.

As a data point: my wife's XC was acting up, and I was able to fix it, for a thousand miles or so, with a fluid flush. After one of those every month, I dove into the valve body and now, with 240,000 miles on it, it still shifts perfectly. But the car experienced relatively few improper shifts, so the friction materials weren't too bad...and I couldn't tell that until, well, now, really, 120,000 miles later. It was caught early enough.

What worries me about your case is this: you stated that the problems have been there for over a year...and I can't judge what is "harsh" to you...and a year is enough time for hundreds of thousands of bad shifts...so, what felt normal to you might have been bad shifts that caused wear...and it could be completely shot.

Or, you might be as picky as I am, and it was only dozens of shifts that were bad...so, it might be OK.

[Xfingers]

Thanks, Astro. I placed the order for the new GM valve body. That and a used wiring harness off Ebay came a little shy of $500. Along with my own labor to do the replacement, it's a reasonable investment to see if I can get the car shifting properly. I feel you on the damage to the trans internals. This car sees about 3000 mi. per year.

Although I've been getting the P0735 code (5th gear incorrect ratio) for a few years, it was only maybe 5 times each winter when driving uphill long distances and I wasn't aware of any bad shifts. (JRL has good advice; put the car in 3rd.) This summer/fall the car was shifting better than ever (no harsh engagements, clunks or flares) so I was a little surprised when the P0735 code returned this winter going up that mountain pass and throwing the trans into limp mode. Now when I turn car off, wait a minute, and restart, car goes back into limp mode whereas last season I'd get the gears back. To make matters worse, car started throwing code/limp mode during regular driving on flat terrain. The trans would make a little bump noise (3-4?, 4-5?, 5-4?) and sure enough, 3rd gear only. Obviously the trans needed attention.

I replaced the fluid which was definitely dirty (deep burgundy). Bought VIDA/DiCE and put car in Adapt Mode thinking maybe the computer needs to adapt 4-5 shift to fix "5th gear incorrect ratio" code. VIDA adaptation procedure for upshifts says work thru 1000/1200/1400/1800/2200 rpms. 1000 RPM? Are you nuts?!!! I tried to keep the tachometer steady at 1000 and my trans DID NOT LIKE IT!!! CEL and limp mode!

As I've noted, the car shifts flawlessly when cold so I've been driving around with a VIDA laptop watching the trans oil temp. I've been driving at low speeds to stay out of 5th gear and avoid limp mode. As temp hits 55C, I start to feel a slight slipping in all gears and this is what has me worried. With the new valve body, will I still get slipping at 55C+ temps? Time will tell.

The short answer to your question: Yes, while diagnosing this issue I've had a few ugly moments where the trans has slammed into gear. I've been nursing the tranny since the day I started driving the car so I haven't been rough on it at all but as you say, the damage from these mis-shifts is cumulative.

[FirstVolvo]

If you haven't already done so be sure to get the 274470 Volvo gasket kit (FCP Euro, Amazon…) so you have two new sealing rings that are used between the valve body and transmission case. The GM valve body part includes the small cover gasket but does not include the sealing rings. It's also a good idea to have some tacky transmission assembly lube on hand to hold the seals in place during assembly.