I recently replaced the S1 solenoid in my 1993 850 transmission [that had itself been replaced even though out of warranty in 1997]. I was able to find out a little via this forum, but not really as much as I would have liked, so I thought I would post some tips here. Replacing the solenoids is a little fiddly, but not difficult.

I will not cover diagnosis as that has been done before, however it is worth firstly describing how the solenoid failed.

Solenoid S1 failure: Several years ago, I started getting an S1 solenoid open circuit fault very intermittently. It happened once per year until a few months ago when it started happening every few weeks. I found that the S1 resistance was 35ohms instead of the spec which is around 16ohms. The S2 solenoid was 16ohms. So, I bought a new S1 and waited a couple of months. During that time, the solenoid went to 52 ohms and was failing to operate every couple of days. Not surprising, as it would have only been operating with about one third the force of a good one.

How to replace the solenoid (this would also apply to the S2 solenoid):

a. Put the front of the car on stands or over a pit to allow access underneath.

b. Drain the transmission fluid through the drain hole. Note, you WILL have more come out later, so select an area where you can spread cloths or similar underneath or so AT fluid spillage does not matter.

c. Remove air filter, air inlet hose, MAF or other cables, air filter carrier, battery, battery carrier, AT dipstick. Unbolt clamp on battery -ve cable and move it out of the way. Unplug the large cable which goes on the top of the tranny [from the AT computer].

d. Undo the pipes from the radiator to the tranny where they enter the tranny. Cover them carefully and tuck them out of the way.

e. Now comes the fun part. There is a black cover on the front of the tranny, near the dipstick. It is held by 9 T40 torxx screws. Remove all the screws. I used a Torxx set with 3/8" square drive plus a universal joint, half inch adaptor, half inch universal joint, extension bars and a ratchet. All the screws are MODERATELY easy to remove, but I found two of them really tough due to the subframe assembly. Indeed, the half inch ocket accessories could not fit in the available space for a couple of them hence the use of 3/8" socket drive [or smaller].

I also had to jack up the tranmission/engine by about 1.5" so that I could get out one of the screws and also because the cover plate only just makes it past the subframe and even then will require some wiggling.

f. Remove the cover plate. DO NOT slide anything between the cover and the AT body to prose off the cover or you risk distorting or damaging it. You can tap parts of the cover flange which protrude past the tranny body in a few places.

g. The gasket is chemical gasket, which is fiendishly expensive. Instead, I used 0.4mm oil jointing gaskey which I cut out using the cover as a template. This worked fine with no leaks and I have used the same material on a tranny before with no problems. Before making the gasket, clean off all the chemical gasket. Take care on the tranny that pieces of the pink goo do not go inside the tranny!

h. The S1 and S2 solenoids are at the top. The S1 is closer to the engine. Unplug the connector and the reatining bolt beneath. This also holds a cable clip. I suggest you check the cable which runs inside the tranny cover as there is a slight chance it is damaged rather than the solenoid. Replace the solenoid and remember to plug it in. It is worth checking the resistance at the tranny electrical contactor before re-assembling the tranny cover.

i. Carefully assemble the new gasket and the cover, re-securing it with the T40 screws. Putting the rest of it back together again si straightforward. Don't forget to refill the transmission and then check the level when the fluid is hot.

The whole job took me about 5hrs, including manufacturing the gasket. I could probably do it in 3.5hrs now I know what to do.

Cheers,

Dave