One of the doors (rear left) of my '98 S70 had very hesitant power lock. When operated by power (switch on one of the front doors, key, remote) it would usually go down to lock, but unlocking was intermittent and after a while became completely non-functioning.

I looked through a lot of forum post both here and elsewhere and found numerous complaints about this, but in most cases the solution was to replace the whole latch assembly, which costs north of $100. There were some posts with people offering a repair or replacement service for $50. All good options, but if one does not wish to spend $50 to make the lock button pop up when needed there is a way to fix this DIY style. The information about what the problem is and the solution was not very easy to find (but it's out there if you look hard enough).

This is why I am writing this. Hopefully the search will be more fruitful for the next person looking to do this on a shoe string budget.

THE PROBLEM. The cause of the button not going up when operated vial the power means (the button will still operate by hand) is the motor which operates it. These motors have a built-in thermistor in series with the power feed to the motor. The purpose of the thermistor is to protect the motor from a stuck relay, which I guess used to be a problem in the old Volvo days of 240's. If the power is continuously applied, the motor will burn out in short order. The thermistor in series acts as a thermal fuse. When the motor is straining under continuous power it continuously draws current, which heats up the thermistor causing its resistance to rise dramatically. This effectively cuts the current off to the motor. Once the thermistor cools down it goes back to nominal resistance and the motor will operate again. In our cars this thermistor is getting old and its "nominal" resistance is going up with age. At some point it is up to the point that the current to the lock motor is severely limited. This causes the problem. The down motion has less mechanical force requirement and so the lock function continues to work while the unlock function requiring a little more power from the motor stops working due to current limitation of the old thermistor.

THE SOLUTION. The cheap DIY solution is to remove the thermistor and replace it with a solid conductor thereby removing the thermal protection to the motor and restoring the full power. Yes, the thermal protection will be forfeited using this method, but if you were going to replace the whole assembly anyway, why not get a few more years out of it even if you risk burning the motor out? In any case, the risk of motor burnout is low because the relays in our cars have very low rate of failure and we don't usually sit there and play with the power locks (if you do, STOP it!).

THE PROCEDURE. To get to the motor the door panel needs to be removed, the latch mechanism removed and the part of it containing the motor opened up. Door panel removal is accomplished by using a tool to lever the fasteners out of the sheet metal. This is kind of tedious and there is great risk of ripping the fastener anchors in the panel off the panel. This was the case for me. A few of the anchors came off the inside of the panel. I was able to glue them back without a problem, though. Once the panel is off, disconnect all the wires to it. The window switch, the warning light, etc. On my rear door I needed to undo the screw that goes into the glass guide to be able to remove the latch assembly. Once the assembly is out of the door I was able to take it to my bench. The part with the motor has a cover welded to the housing. I was able to pop that open by going around with a flat blade screw driver and twisting the blade in the seam. Go easy here and try various points. At some point it will give. Once it gives, continue going around patiently and the cover will eventually be worked off. Inside the housing are two cog gears and a motor with a worm drive gear on it. The motor lifts right out of its seat and the electrical contacts are two prongs in the housing that mate with the corresponding sockets on the motor, so there are no wires here. Once the motor is out of the housing the metal case can be separated from the plastic front of the motor by bending two metal fingers up and away from the plastic. Once the metal cover is removed you can see the motor armature and brushes. The brushes are connected to the contacts in the sockets that prongs go into. One brush is directly connected to the contact and the other has a rectangular piece of metal bridging the contact and the brush. Carefully remove this piece with needle nosed pliers or something small. This is the thermistor. Just pull it out of the plastic, it will slide right out with some friction. Once it's out, create a similarly shaped and sized piece out of some solid metal. A copper penny will work, I think. I used a metal fender washer that was very close in thickness. I traced the shape of the thermistor (simple rectangle) on the washer's solid part with a sharpie and cut it out with a rotary tool and a cutoff wheel. I filed and sanded the rectangle I produced and slid it back into the place of the thermistor making sure the contacts between the brush and the socket are bridged. Next, I reassembled the motor. Make sure the brushes a re properly seated on the commutator because you probably pulled them out of their place to get to the thermistor. Use a toothpick or something to pry apart the brushes while pushing the motor shaft down to where it belongs. The put the metal cover back on and gently tap the metal fingers back into their recesses in the plastic part. . Now pop the motor back in. I applied some fresh white lithium grease on the gears and glued the lid back on using a few drops of CA (Crazy Glue) around the perimeter of the lid. Went back to the car, plugged in the power lock harness and tested. Worked just fine. I reassembled the door and now I have a working power locks all around. When the others fail I will do the same and I am sure this time it will take me much less time. I would say it's an hour and a half job, probably much less when doing for the second time.

I hope this helps someone looking for this info.


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Vladimir. '98 S70 base, 5-speed manual - his, '93 945 - hers