I was going to replace my thermostat (didn't realize that they throttle body needs to be removed first) the other day. Can someone tell me what this is and why it'* not plugged in (the radiator is in the upper left and the air intake coming into the air box is in the bottom of the picture. It'* a 1990 SSE)?

It'* the air compressor for the rear load leveling.

You have to remove the tb to get to the thermo? I wasn't aware of that on 90'*.

i didnt, it was a very tight fit with the socket and extension and it goes in at an angle but i pulled it off with no probs, and is your rear end sagging at all if not then the rear shocks were probably replaced with ones that dont have the rear load leveling which would be why the compressor is disconnected

I have a 1990 model also and I didn't need to pull the throttle body to change my thermostat. It was a tight fit, but all I needed was a 1/4 drive ratchet with 4 inch extension to reach the water neck bolt. The only difficult part was getting the water neck out of the intake. I used a sturdy piece of wood to work it side to side until it came off. Otherwise it was one of the easiest thermostats I've changed. I disconnected the MAF and the IAC just to get them out of the way, otherwise, it'* a straight shot.

Your ELC compressor is probably disconnected because:

Your rear ELC shocks leak like a shotgun on a balloon, and it pissed off the previous owner when the compressor kept trying to lift the back of the car to the proper ride height.

That system goes a long way towards ride comfort, handling, and performance. If you can afford to, I'd check the system out and get it working.

http://www.williamwren.com/struts/elc.html

When I picked up my 90 SSE, the fuse was removed for it. Found out the reason why. The rubber boots on the shocks were finished so the air went out faster than could be pumped in. I was going to replace the shocks with regular ones, but found out the ones that are supposed to be on the car were about the same price as the non air ones.

I did also have to get a kit from the dealer for the small plastic air hose that fills them. Wasn't that bad $$$ wise to get fixed, and I am glad I did.

If you buy the Monroe shocks listed on my link above, the OEM air fittings will mate to them without a kit.

I think I needed the kit since the hoses weren't in the best of shape, plus it was at the same time that I had the tank replaced, so we figured while we were in there, for the cost of it, it would save $$$ later on.

I have almost 300k miles (298k) on the car right now. Bought it a month ago to get me through the winter. Don't wanna be driving a M3 in the winter. I'd guess the rears were replaced at this point.