Re: 89 300E AC Problems

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-ning Tan wrote:
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.

All,

[quoted text clipped - 65 lines]
I'd suggest you change out that part first before fuel pressure regulator...
but both can be at fault.

--
Message posted via CarKB.com
http://www.carkb.com/Uwe/Forums.aspx/mercedes/200707/1

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