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How to debug not working AWD

Help, Advice, Owners' Discussion and DIY Tutorials on Volvo XC90s. The XC90 proved to be very popular, and very good for Volvo's sales numbers, since its introduction in model year 2003 (North America).
bogdanp
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How to debug not working AWD

Post by bogdanp » Mon Mar 30, 2020 11:39 am

I have a 2007 XC90 with not working AWD

haldex filter and oil has been changed but I don't how to go further. Haldex pump and DEM module what fuse do they use? what number?

please advice

I do have the possibility to read vida info from the car

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wanderdüne

Re: How to debug not working AWD

Post by wanderdüne » Mon Mar 30, 2020 4:59 pm

Here is the manual from the USA, I don't know if it is the same for you:
https://volvo.custhelp.com/app/manuals/ ... /om_id/460
Go to the INDEX, click on FUSES (121) , scroll down to page (125) It says it is fuse #23 (7.5A) in the rear cargo fuse block. On the drivers side in the back of the XC90 behind the panel on that side.

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Re: How to debug not working AWD

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Re: How to debug not working AWD

Post by tjmotter » Fri Apr 03, 2020 7:20 pm

Here is an overview of how the AWD works in the XC90. I gained most of this through research to figure out why mine wasn't working. This may help you source the reason for it not working.

Mechanical overview
- a gearbox (called the "Angle Gear") is bolted to the transmission on the passengers side of the output. This gear is much like a differential and is responsible for transferring the rotation from the transmission to the driveshaft.
- a driveshaft connects the "Angle Gear" to the rear differential assembly
- the rear differential assembly consists of two parts even though it is one physical unit. The front half of this assembly contains the "Haldex". The rear half of this assembly is the "differential"unit". The "Haldex" contains a controller, a pump and a couple of sensors (and a filter). When turned on (by the main computer), the pump pushes fluid through the system which locks the clutch pak and (essentially) locks the driveshaft to the rear differential unit which in turn, spins the rear wheels.
- In normal operation, the transmission is always providing power to the "angle gear" and the "angle gear" passes this to the Haldex but there it stops because in normal operation, the XC90 is a front wheel drive vehicle. It becomes an AWD vehicle only when the main computer sends a command to the Differential Electronic Module (on the Haldex) telling it to turn the pump on to engage the rear wheels.

Electrical overview
-As said, the Haldex connects/disconnects the driveshaft to the "differential unit" based on commands from the main computer to a device called a "Differential Electronic Module (DEM)" that sits on the side of the haldex. This module has a pressure sensor connected to it and has two electrical connections. One of those connections goes to the pump while the other connects to the Main computer. Once instructed by the main computer to turn on, it applies power to the pump while monitoring the fluid pressure in the haldex.

Areas that fail
- The "Angle Gear" connects to the transmission with a sleeve. This sleeve is a weak part in the design and it can become stripped. The easy test for this is to crawl under the vehicle while it is in park and try to spin the driveshaft. In a properly working system, the driveshaft will not turn. This is because it would be physically connected to the transmission which is in park. Remember, this driveshaft is not yet connected to the rear wheels (because the Haldex isn't on) so if it rotates easily while the system is in park, it means that this sleeve and/or the angle gear has failed. This proved to be the issue with my XC90. Brand new parts to fix this would total about $2000 but to get at it, the engine needs to be braced and the front subframe has to be removed as well as most of the exhaust so expect a bill of up to $6000 with labor. Note that this type of failure is "blind" to the Main computer so it will not throw any codes.

- Haldex pump - this can fail as well. The easy test is available through a search on YouTube and requires you to simply disconnect the pump from the DEM module and apply 12V to the pins (the video tells you which one is ground and which is +12V). If the pump fires up and sounds normal (no grinding, no lugging) it is probably ok. If not, it is a PITA to change since the drive shaft and its yoke need to be removed. Removal isn't too hard with the correct tools but retorquing the bolts can be very difficult. The pump itself isn't terribly expensive and even companies like Dorman offer replacement pumps.

- Pressure sensor - this device sits within the DEM module and can tell the computer that the fluid is not flowing properly. This type of failure will issue a code so if you have a Volvo aware OBD scanner you can connect to the DEM module which should describe the fault and point to this device. If you have a pressure issue it is often a clogged filter so first try replacing the filter in the haldex to see if that corrects the problem. If not, this sensor costs about $160.

- in some extreme cases, the DEM module can fail. This is often caused by a failing pump which pulls much more current than the DEM module can handle causing it to blow. There are companies on eBay that can repair these for you for about $100.

- the last item is the fuse for the DEM module. It is located in the rear fuse box (drivers side behind the removable panel) but note that if this fuse is blown, it also typically means that the pump is failing. If you are lucky, this fuse will blow before the pump damages the DEM module.

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ggleavitt
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ggleavitt

Re: How to debug not working AWD

Post by ggleavitt » Sat Apr 04, 2020 11:34 am

This the vehicle with the swapped ABS (BCM?)- viewtopic.php?f=10&t=93468&p=537819#p537819?

ABS/DSTC (BCM) and DEM functionality is intertwined, both way communications over the CAN bus. Since you suggest that you have VIDA, there's a description in the Product Specifications/Design and Function/System Overview for 465: Differential electronic module (DEM) on what connects to what and how.

If you were able to get your module reloaded (cloned and/or code modified) by the Dreamland guy and and it's now working in the vehicle, do the mechanical test as above (excellent write-up BTW) and if that passes follow the Xemodex workflow- https://xemodex.com/us/diagnostic-flow-chart

I might like to see your codes out of curiosity if you're offering to post them.

040420- fix ABS, I suspect it was BCM that got swapped.
2005 2.5T AWD-200k
2008 V8 AWD Sport-130k

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