85 300D AC and Coolant Question - PeachParts Mercedes-Benz Forum

tobybul · #1 05-22-2006, 04:35 PM

I just got an 85 300D with about 100K+- miles on it (odometer is broke). I noticed the drive belt to the ac compressor is missing. According to prev owner, they took it out since ac is not working. Not quite sure about that so I am wondering whats gonna happen if I slap a new belt in there. Say the AC may not be working, whats gonna happen? Will I be able to trace any leaks with a dye assuming the comp is working? How would I know if the comp is kaput?

Coolant, also noticed is just plain water. Will any Anti-Freeze be ok to put with the right mix (50/50? in Michigan)

rhodes2010 · #2 05-22-2006, 04:50 PM

I'd put the belt on and see what happens (in the driveway).

The compressor pulley may have a bad bearing (make a lot of noise - shut off engine).
Compressor may be siezed, belt will make a lot of noise - shut off engine.

Compressor may not engage at all - different problem, leave belt on.

If you get this far, you have a belt on and a seemingly good bearing so you can move onto checking if the system is charged and deciding how to get the cold coming out.

If you are super lucky - the system is all charged up.
If you are just lucky, the system is only low and needs a charge.

Anything else and I would think flush, replace drier and maybe expansion valve, maybe even compressor, charge properly and enjoy.
This last part could cost up to $500 if you do most of it yourself.

Well worth it.

Pete Burton · #3 05-22-2006, 05:00 PM

before you put the belt on, try rotating the triangular plate on the front of compressor. It should move easily. A new comp. can be had for about $225, a good reman for $150. If you plan to use it a lot, the new's probably worth it.

t walgamuth · #4 05-22-2006, 05:56 PM

ten to one the compressor is locked up.

tom w

bkeese · #5 05-22-2006, 10:30 PM

I also just got an 85 300D with a missing AC compressor belt. I tried rotating the compressor (the triangle thing) and it wasn't frozen, so I put a belt on it yesterday. No noise, so the bearing is fine. The clutch never engages, so now I'm waiting for a gauge set and a couple other things to check the charge. If there is no charge, I will pull a vaccuum and see if anything is leaking. I'll fix any leaks I find, then replace the receiver/dryer and expansion valve, flush, and recharge. If there is already a charge, then maybe the low pressure switch is defective, if not I'll have to go looking for some other electrical issue, maybe the compressor clutch coil. I suppose what I can do before my new AC tools arrive is test the clutch to see if it is shorted or open.

Brian Keese
85 300D
81 Isuzu Pup running WVO

#6 05-22-2006, 10:37 PM

There's not a whole lot you can do until you verify if it's charged. You don't want to force it to engage unless you are sure it's charged. Sounds like you're on the right track.

tobybul · #7 05-23-2006, 01:46 PM

I also checked the compressor pulley and and it turns freely. I didn't see the triangle thing being mentioned. Can I check for leaks without putting a belt in?

By the way, I haven't gone this far yet but is there a belt tensioner on the 85 300D to loosen the main belt so I can slip the AC belt on? Sorry for the question but I had not checked. I would expect see a tensioner otherwise how can you replace belt.

Thanks

rg2098 · #8 05-23-2006, 01:51 PM

You can jump the contacts on the pressure switch (At the dryer behind the passanger headlight with the 2 connections pointing up) for a second to verify the compressor is in fact good. You won't harm anything letting it run for a few seconds but not much longer.

The last car I saw with a belt off the compressor had a good clutch like yours and a fully charged system but the compressor siezed so you can imagine what happened every time the A/C kicked on.

#9 05-23-2006, 07:39 PM

Quote:

Originally Posted by tobybul

I also checked the compressor pulley and and it turns freely. I didn't see the triangle thing being mentioned. Can I check for leaks without putting a belt in?

The pulley will turn free anytime the clutch isn't engaged. You need to try to turn the piece in front of the pulley to actually make sure the compressor turns. It should turn harder than the pulley, but it should be smooth. If you can't get it to turn by hand the compressor may be locked up.

Quote:

Originally Posted by tobybul

By the way, I haven't gone this far yet but is there a belt tensioner on the 85 300D to loosen the main belt so I can slip the AC belt on? Sorry for the question but I had not checked. I would expect see a tensioner otherwise how can you replace belt.

There is no "main belt," there are two belts that drive the water pump and alternator (closest to the engine), then the AC belt, then the power steering belt (closest to the fan). At a minimum you need to remove the power steering belt to install the AC belt. You may just want to replace them all while you're at it. The power steering pump, AC compressor, and alternator each have an adjuster bolt to loosen and tighten each belt (after the mounting bolts have been loosened). The procedure is to loosen the mounting bolts, turn the adjusting bolt CCW to loosen the belt, remove old belt, install new belt, tighten adjusting bolt, tighten mounting bolts. It's not to bad, but it can be a PITA to get a wrench on some of the mounting bolts if you haven't done it before. You do not need to remove the fan, the belts can be put over the fan blades.

leathermang · #10 05-23-2006, 10:38 PM

"Can I check for leaks without putting a belt in?"--Tobybul

Yes, you can install nitrogen on both the high and low side and watch for any changes in the pressure. This is not an uncommon procedure after a flush or replacement of parts and seals.