Paul,

The Park Neutral Position Switch is actually a very simple mecahnical contact switch.

On these Volvo's it serves several purposes.

It lets the TCM (transmission control module) know what gear you have selected, the TCM then can determine whether the backup lights are supposed to come on/off, it then knows how to control the solenoids for shifting purposes based on what gear you selected, if your in Park or Neutral, it can then tell the ECM (engine control module) to let you start the car.

If it doesn't get the proper signal from the PNP switch, it doesn't know what you want to do so it can't react properly and goes into DEFAULT/Limp Home/PIA mode which bacsically gives you 3rd and 4th gears to get home or to the shop.

I don't know why the PNP fails so much. Even though it is located within a couple of iches of VERY hot exhaust pipes and sits bolted to a hot transmission. There are NO electronics in this part. When I took mine apart I was impressed how amazingly simple this switch was and then immediately wondered why it fails so much.

The conclusion I came to after looking at this one switch was : That the grease they use in the switch (as a lubricant for the moving arm) appears to get burned (perhaps from arcing from the contacts when they are moved). This charred grease turns black and has a very different consistency from the clear yellow grease you can see on parts of the switch. Once this grease gets carbonized and brittle, it appears to get on the contacts themselves and creates the problem. My PNP only had 62,900 miles on it and most of the grease inside was carbon black. There's no reason for that grease to get so bad unless it's getting burned from the arcing.

This is just my guess based on only one switch taken apart.

I'm considering cleaning and regreasing the switch with Synthetic grease and testing it to see if it will work and perhaps keep it as a spare.

I will be changing the PNP on our 95 960 soon and will dissassemble that switch as well and see if it has the same problem.

This is just my guess based on only one switch taken apart.

I hope I answered your questions.

DEWFPO
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1998 S90 062,925 1995 964 145,750