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89 300E AC Problems

I think there are two possibilities about the A/C problem. I don't believe the air mass sensor has any relation to A/C.

Possibility #1, low on freon. This should be easily identified by checking pressure on both high and low sides when the system runs.

Possibility #2, loose serpentine belt (see description below). Check belt condition, tensioner, and oil at the compressor clutch (also gap).

The A/C compressor has a speed sensor at its rear end. The Klima relay compares the compressor speed and the engine speed (sensor usually at ring gear around starter). When the speed deviation is too much (I saw it somewhere in the MB manual saying like 20% - 30% but can't find it anymore), it assumes the compressor is seized so it cuts off the clutch to save the belt. Therefore, the belt tension and groove condition should be checked. Also, if oil is on the clutch plate, this may cause it slip so much that the compressor speed isn't up to spec. If you see oil on the plate, degrease/wash it.

The easiest way to check if it is the speed cut-off, turn off the engine and then restart. The speed comparison logic is reset by engine off. If the A/C runs again after restarting engine, it is definitely the speed cut-off.

This speed comparison appears in all MB I have seen in the 80s, including W201, W126 and W124. I am not sure about earlier or later MBs though MB tends to use similar design repeatly.

The air mass sensor may affect the engine speed but I would assume it also indirectly affects the compressor speed for the same degree. Therefore I do not believe there is any relation in these two problems.

I am surprised to know M103 engine still uses the potentionmeter. I thought it has been replaced by the more modern thin-film type by mid-80s. The potentionmeter is a variable resistor (like fuel level sender) and the contact surface always wears out after many years of usage.

GeneKelly via CarKB.com wrote:

Reply to
Wan-ning Tan
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I checked the belt for wear and slack and it appeared fine. It probably had a little more deflection then I'd like (1") but it didn't seem to be slipping while I placed a load on it.

I also didn't notice any excessive oil on the compressor or clutch. There was some minor leaking coming from the power steering that was dripping onto the compressor that I cleaned. Again, it didn't appear to be causing a problem during operation.

I checked the Klima relay for scoring, none noted. I did turn off the car after the AC went warm and turned it back on and it still blew warm air. Probably not the cutoff?

It's most likely low freon, wouldn't you say? I've made an appointment with an AC shop and will let you know the result of pressure and coolant level. Based on Tiger's comments, I'll make sure system is vacuumed for at least 1/2 hour.

On the potentiometer issue: I pulled the air cleaner off and check all hoses. I also sprayed a can of mass air flow sensor cleaner on everything and cleaned contacts. I didn't notice a change of engine speed while spraying or any loose connections. I did adjust the mixture setting on the mass air flow sensor and it did seem to improve idle. I messed with it for about an hour, ensuring I marked the original spot and number of turns before adjusting. If I went too far one way, it ran sluggish. The other way, and the engine ran rough. So by ear and a test drive, I was able to improve idle slightly. However, I think it can be better. Is ther some way to test mixture with a gauge or device while adjusting the mixture screw? Does this adjustment affect the potentiometer? I believe this adjustment is out of whack and is part of the problem. Last December, I had real stumbling problems that turned out to be a worn out O-2 sensor. With the new sensor, maybe the mixture needs to be adjusted to specs?

Standing by,

Thanks again,

Gene

Wan-n>I think there are two possibilities about the A/C problem. I don't

Reply to
GeneKelly via CarKB.com

You got serpentine belt... There should be ZERO flex/play. Your tensioner is shot. Change the tensioner and the shock.

Reply to
Tiger

Tiger,

I agree with you 100% that the belt tension may be causing the AC problem. I replaced the tensioner and shock in 2004 through a Mercedes shop (routine maint.). Before I replace again, I'd like to try and adjust if possible. I checked the tension scale on the tensioner and the arrow is pointing to the left of a line with a big notch in it. Can I adjust by loosening the bolt (17mm) on the top part of the bracket (between water/power steering pumps), then adjusting clockwise on the six-sided adjustment that sticks out? Or is it a lost cause? No noise, oil, greese, or other abnormality is apparent on the belt. Belt has five grooves in it. My owner's manual calls it a poly-v- belt and has a belt routing diagram on page 97. Is this the same as serpentine belt?

The only other thing that's happening along with this problem is that the SRS like comes on randomly. I know the belt runs alot of things, could this be causing the SRS light to come on intermitently?

I still have the surging idle problem. Any tips for adjusting the idle mixture or should I just leave this problem to the real mechanics?

Gene

Tiger wrote:

Reply to
GeneKelly via CarKB.com

Looking at the part supplier... yes, you got an adjustable version of belt tensioner... Your tension should be like 1/2" play...

Loosen the nut on the tensioner where your wheel pulley is... then tighten via your tensioner adjuster on the top... brass thingy... then lock it all down.

It is probably all related. Loose belt cause all these problems.

Reply to
Tiger

Tiger,

I tried to tighten the belt per your instructions and tensioner adjustment would not snug up. I replaced the belt thinking that would tighten things up a bit, it didn't. I think the adjuster is maxed out. I've got everything apart in my garage, any suggestions on what to do next? I took the car to an AC shop today where they topped off the refirgerant with 1 1/4 lbs, verified pressure was good, and observed good flow through the "eye" on AC valve. After a short while, the compressor shut down. They felt that the auxillary fan should be on whenever the AC is on? They also suspected that the serpentine belt was too loose and was causing all of the problems with AC compressor shutting off and fan not coming on. So, I attemped the job myself and now I'm stuck.

Please help!

Gene

:
Reply to
GeneKelly via CarKB.com

If you can't tighten it up, then you routed the belt wrong.

Reply to
Tiger

Tiger,

Turns out the adjustor bolt and nut were stripped. It probably has been that way for some time. I took out the tensioner, shock, bracket, and adjustor so I can get a better look. I had to remove fan shroud, fan blade (not clutch), power steering pump and pulley, and water pump pulley to get to them. It was real knuckle buster without the right tools. I didn't break anything though. I've got new Febi tensioner, adjustor, shock, and mounting bracket coming on Monday. The shock was replaced three years ago by my Mercedes mechanic, but nothing else. I think the shock went bad because the tensioner was wearing out? The tensioner and shock both looked good when I took them out, but since I was in there anyway I just decided to replace everything. The belt is new and was installed exactly as I took it off and matches the diagram in my owners manual.

Do you have any images of a pr>If you can't tighten it up, then you routed the belt wrong.

Reply to
GeneKelly via CarKB.com

Shoot me an email.

Reply to
Tiger

Tiger,

Not sure how to e-mail you. My e-mail is snipped-for-privacy@bresnan.net

Thanks

Tiger wrote:

Reply to
GeneKelly via CarKB.com

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