2003 XC70 - Loss of rear wheel drive (FWD only, AWD not working)

[Brandon_K]

Hey guys.

Compared to many of you I'm a newer Volvo guy. I bought a 2003 XC70 (built 11-2002) last winter as a work / winter vehicle and quickly fell in love with it. I put 40k miles on it between December 2016 and April this year with little issue, before I parked it mostly for the summer. I did some minor work to it as things popped up; coil packs, ball joints, strut bearings. I even attempted to get the climate control working properly. It had always been dealer serviced prior to me owning it (I bought it from my step Dad and he gave me every service invoice that was ever done to it, 10's of thousands of dollars of work, not cheap vehicles to own when the dealer is doing the work!)

Anyhow, moving on. I got it back out last week and we had a decent snow fall in Cleveland. I noticed immediately upon pulling out of our snow covered parking lot that I was not getting any sort of traction and that the traction control light (which I had never seen before) started coming on. I didn't even know it had a traction control light! I had a coworker watch as I floored it in the parking lot and he confirmed that only the front tires were spinning.

At this point I put it up on the lift. With the wheels in the air, engine running, transmission in drive, the passenger front and drivers rear wheels spin. If I slow the driver side rear down with my hands, the passenger rear starts spinning as expected. I was unable to attempt to stop both rear wheels as I didn't have a second set of hands at the time. With the engine off, transmission in park, I can freely spin either rear wheel. The drive shaft (prop shaft?) will not spin in park, it seems locked with the front as I believe is expected.

I've always done my own vehicle work, anything from basic maintenance all the way to completely rebuilding my 4.0L Jeep motor into a 4.6L stroker motor. I'm mechanically inclined, the issue is simply that I have zero experience with AWD systems. Most of my cars are RWD or part time 4wd so they're incredibly easy to diagnose. There has been no odd noises coming from the drivetrain, not now, not since I've owned it. No bearing howls, no grinding. HOWEVER, when I went to move it for the first time this summer the rear wheels were locked up, to the point that it skid the rear tires through my gravel driveway until it hit the street, at which point I had to rock it between drive and reverse a few times. It finally let go with a solid pop and I was on my way. At the time I thought the parking brake had frozen as I've experienced this a few times in my life, but now I'm not so sure that was the case.

I've done a bunch of searching on the matter and it seems that it's a semi-common failure on these cars with a number of various failure modes. The bevel or angle gear seems to be the worst possible failure mode (is this basically what I would normally refer to as a transfer case? This one just being full time and a single gear ratio?). If I'm correct that the bevel gear is right off of the transmission output, I don't think that is my failure as the rear driveshaft remains locked when in park and spins when in drive.

That leads me to believe there is something going on in the rear differential or associated control systems at the rear diff. During my searches I've learned that there is a Differential Electronic Module that contains a oil pressure and temp sensor, as well as a "choke valve" or oil sensor, depending on who's video you watch and what they refer to it as. It appears that this part is actually software coupled to the car and if I get a junkyard module to swap it with, I have to have that done at a dealer? Past that there is a oil pump in a small can off on the other side of the Haldex DEM and those are common problems.

Before I go pushing parts in to it and hoping I get the right one, are there any diagnostics I can do to help narrow it down? No codes are being thrown, at least not with OBD2. I don't have VIDA to know if that will tell me anything else. I don't know how the rear differential on these work, but it's clearly some sort of variable system as it's not 100% driven all of the time. I also don't want to go throwing a $200 pump and $200 DEM into it when I can get the entire rear differential assembly, including the DEM and pump for less than $500 shipped from a donor car.

I'd like to get into a P3 in the next year (though I hear the 3.2 is a dog of a motor) so I'm going to put a $500'ish limit on the repair. If it's more than that, I'll buy another P2 XC, pull the drive shaft on this one and sell it as a FWD XC (of course being clear to the buyer that it's an AWD car with a busted whatever).

Any help would be appreciated. Hopefully I've given enough details to help point me in the right direction.

[jonesg]

I would check the haldex fuse #17 in the rear panel.
Then get a vida scan, buying a vida on a laptop was essential for me.
If moving to p3 I wouldn't bother fix it.

[oragex]

As above, several parts can be wrong I'd start perhaps checking the PEM (google "pem Xemodex"), it gets corroded inside. Used ones must match a serial number. But there are other components that may fail at tge Haldex

[850Squared]

Xemodex has a pdf called DEM Flow Chart that will walk you through diagnosing your DEM issue. There are several issues it could be...from your Electronic Module, fuses, wiring harnesses, Haldex pump etc. Most likely it's your DEM....but you'll need to follow the steps in that pdf to be sure.

[vtl]

Read the codes, it may say DEM-0006 or 7 - Haldex pump or pressure sensor. Mostly like the pump is gone. Not too cheap, but you can get one for VW for 1/2 of price.

Also driveshaft is bad too because of age and mileage, it needs a new CV joints and at least a new rubber around center bearing.

P3 is worse than P2 all around, honestly.

[Brandon_K]

Update;

Thank you all for pointing me in the right direction. After the responses I did some reading up on the DEM module as well as failure conditions. I also found the flowchart on Xemodeex's site.

I went out tonight and got under the car thanks to our unseasonably warm weather. I'm unable to pull the codes as I don't have VIDA. There is a local indy shop not to far away, I may run over there tomorrow and have them pull codes.

No blown fuses at the REM. The pump definitely wasn't running, not now nor when I had it on the lift. Connector "A" at the DEM gets +14v and a solid ground at the appropriate pins. I fed the pump +v and ground directly from my PowerProbe on connector B (not connected to the DEM obviously) and the pump came to life and it pretty easy to determine if it's running or not, even if you don't know what you're listening for.. Upon plugging it back in to the DEM, the pump stayed running, but at a noticeably lower RPM. From what I can tell the DEM is using PWM for the pump control which makes sense to why it sounds like it's running slower. I took it for a test drive and it's still not getting any power to the rear (very wet roads made it very easy to light up the fronts on command).

So;

DEM appears to be powering the pump
Pump runs
No blown fuses

I'm going to try to take some time tomorrow to pull the DEM off and take a look inside. My background is electronics, so it's not out of the question for me to open it up, clean up any corrosion and replace anything at the component level that may need replacing. From what I read last night, it looks like there is a MOSFET that controls the pump that tends to blow up. But since my DEM appears to be working, I'm at a loss right now. I'll probably replace the fluid and the filter at the minimum as this car has 164k miles on it now and it looks like there is a variable clutch in the diff that uses friction material. I'll have to look through the work orders to see if the Haldex unit was ever serviced. Past all of that, assuming the pump is getting fed from the DEM, it looks like the next step on the flowchart is to replace the temp and pressure sensors. Maybe simply removing them and cleaning them up will help and at least narrow down the issues.

Read the codes, it may say DEM-0006 or 7 - Haldex pump or pressure sensor. Mostly like the pump is gone. Not too cheap, but you can get one for VW for 1/2 of price.

Also driveshaft is bad too because of age and mileage, it needs a new CV joints and at least a new rubber around center bearing.

Driveshaft and carrier bearing look to be in great condition. I'm certainly not sinking any money into those unless I can get power to the rear. None of that matters if the diff isn't engaging.

The flowchart says DEM-0001 and/or DEM-0002. Are your codes different or was that a mistake?

Do you have a part number on that VW pump? From my research today the VW pump is in fact the same Haldex pump, but it's significantly more expensive than the Volvo P/N and the VW guys are actually buying Volvo pumps. I found a Dorman unit for $200 which isn't horrible. I also found a russian rebuild kit with new winding, bearings, brushes and oil seal for $50 shipped.

P3 is worse than P2 all around, honestly.

Mind sharing your reasons? I've never driven a P3, but it's really hard to find a low mileage P2 and I really love the body style on the P3. Plus I drive at night a TON so I'd like to find a model with HID headlamps.

[jonesg]

I pulled mine this summer and had a look. If you know your way around electronics it could be a simple project for you.
Maybe get some haldex fluid before removing the unit , there will be some minor fluid loss.
Lots of YouTube's on these units but no hack vids.

[Brandon_K]

I got to thinking tonight, just out of curiosity what is the failure mode of the bevel gear?

I used some logic to determine that the bevel gear isn't bad (that is, the rear driveshaft stays locked up when the tranny is in park and the driveshaft spins when it's in drive, which leads me to believe nothing is sheared internally). But after looking at some pictures of the bevel gear unit and seeing that it doesn't seem to be much more than a ring and pinion that drives a common shaft. I've read that the bevel gear going will still allow FWD to operate, but I'm at a complete loss as to how it doesn't fail in a catastrophic manner and leave the car dead in the water. The only thing I can see happening is the bolts that hold the ring gear to the shaft shearing, which would kill the car (and I've actually had this happen on my Wrangler to do a incorrect R&P setup), or the input shaft and pinion gear shearing, which would also lead to a very bad situation.

Past that, I got some great news today, one of my local salvage yards has the entire rear differential, inclusive of all of the Haldex parts out of a 04 XC70 available. $75! This will allow me to salvage the pump and both sensors, clean them up and swap them over. I'm going to stop at the dealer for fluid and a new filter. That also leaves me with a good differential in the event that the clutches in the Haldex system are smoked.

[vtl]

Bevel gear, AOC and rear differential far outperform torque capabilities of this engine. Mechanically they run forever if the oil is properly changed.

If the BG broke inside, you'd hear it for sure. If you can't rotate the driveshaft by hand that means the collar sleeve is not stripped. So the only thing left is AOC: either pump of pressure sensor. Both known to fail within 10ish years.

[Brandon_K]

If the BG broke inside, you'd hear it for sure. If you can't rotate the driveshaft by hand that means the collar sleeve is not stripped. So the only thing left is AOC: either pump of pressure sensor. Both known to fail within 10ish years.

This is exactly what I needed to know. Thanks!