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  #1  
Old 10-07-2009, 04:31 PM
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1988 300TE AC Problem

I have a 300TE. You start the car and the AC Clutch engages, the suction line gets cold and blows cold air. Ambient pressure on the suction side 88#. As soon as the system runs I get 55# Low side and 225# High side at 90F ambient temp. The AC runs a couple minutes. Then the front electric fan kicks on. Runs another minute or so and the AC clutch kicks out and the electric fan starts. Will not cycle on again. Stop engine, restart and the whole thing repeats.

So I read and got the suggestion it was the serpentine belt. It was indeed in piss poor shape. It's a good thing i looked at it before it left the car stranded. Replaced it. No change in the behavior.

Where are the usual Gremlins at that cause this?

Thanx for any help! It's hot down here in S. Florida!

Les

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  #2  
Old 10-07-2009, 08:52 PM
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Quote:
Originally Posted by les_garten View Post
I have a 300TE. You start the car and the AC Clutch engages, the suction line gets cold and blows cold air. Ambient pressure on the suction side 88#. As soon as the system runs I get 55# Low side and 225# High side at 90F ambient temp. The AC runs a couple minutes. Then the front electric fan kicks on. Runs another minute or so and the AC clutch kicks out and the electric fan starts. Will not cycle on again. Stop engine, restart and the whole thing repeats.

So I read and got the suggestion it was the serpentine belt. It was indeed in piss poor shape. It's a good thing i looked at it before it left the car stranded. Replaced it. No change in the behavior.

Where are the usual Gremlins at that cause this?

Thanx for any help! It's hot down here in S. Florida!

Les
You said "As soon as the system runs I get 55# Low side and 225# High side at 90F ambient temp. The AC runs a couple minutes. Then the front electric fan kicks on."

What do the pressures do after the fan comes on? Does it drop below 30 or so?
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  #3  
Old 10-07-2009, 10:20 PM
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Does the '88 use a compressor speed sensor? If so, a low speed reading from the compressor will cause these symptoms.

The speed could be low because of a dirty sensor, or a slipping belt or clutch.
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  #4  
Old 10-07-2009, 11:11 PM
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Originally Posted by dhjenkins View Post
You said "As soon as the system runs I get 55# Low side and 225# High side at 90F ambient temp. The AC runs a couple minutes. Then the front electric fan kicks on."

What do the pressures do after the fan comes on? Does it drop below 30 or so?
Yes, when the Fan Kicked on the Pressure went from about 250 down to 225 pretty quick.
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  #5  
Old 10-07-2009, 11:14 PM
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Originally Posted by Matt L View Post
Does the '88 use a compressor speed sensor? If so, a low speed reading from the compressor will cause these symptoms.

The speed could be low because of a dirty sensor, or a slipping belt or clutch.
There was a suggestion elsewhere to a few folks that they had loose belts, that's why I just changed mine and it's pretty tight. The tensioner scale I never figured out how to read it, so i just tightened it up till it had maybe 1/4" deflection when pushed on.
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  #6  
Old 10-07-2009, 11:24 PM
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Originally Posted by les_garten View Post
Yes, when the Fan Kicked on the Pressure went from about 250 down to 225 pretty quick.
I'm more interested in the low-side pressures once the fan kicks in. More than likely, there's a low-pressure switch that can be tripped if the pressure drops low enough (which will happen if you're low on charge and the fan comes on).
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  #7  
Old 10-07-2009, 11:26 PM
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What does the low-side pressure have to do with anything?
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  #8  
Old 10-07-2009, 11:28 PM
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Originally Posted by Matt L View Post
What does the low-side pressure have to do with anything?
Low side cut-out; an 'absense of refrigerant sensor' if you will. Like I said, I'm not an MB tech, but I would assume the A/C system would be equipped with one. I know my '85 grey market 380SEL has one.
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  #9  
Old 10-07-2009, 11:34 PM
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Originally Posted by dhjenkins View Post
Low side cut-out; an 'absense of refrigerant sensor' if you will. Like I said, I'm not an MB tech, but I would assume the A/C system would be equipped with one. I know my '85 grey market 380SEL has one.
There is no low-side cut-out switch. M-B's have a low pressure cut-out switch on the high-side line (as do all other TXV systems).
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  #10  
Old 10-07-2009, 11:42 PM
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Originally Posted by Matt L View Post
There is no low-side cut-out switch. M-B's have a low pressure cut-out switch on the high-side line (as do all other TXV systems).
I hope you're referring to "all other TXV systems" in MB's ONLY. Most other manufacturers have their low pressure switch in the low pressure line, becuase it makes more sense to do so. A high side pressure of 30 psi in the high side is a VERY low pressure, and likely to not be circulating compressor oil.

I'll be the first to admit, I know little about MB's, but I've spent 15 years wrenching on every other make, along with handling stationary, residential, commercial & "other" systems.

You could absolutely be correct - but having a low pressure switch on the high side seems silly, especially since a leak on the suction side could easily not trip a low-side switch placed on the high side because the compressor could still pump atmospheric air above 30 psi.
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  #11  
Old 10-07-2009, 11:47 PM
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Hi,
The Low side was 55 and the high side was 250.

The Fan came on and the Low side stayed 55 and the High side went to 225.
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  #12  
Old 10-07-2009, 11:53 PM
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Originally Posted by dhjenkins View Post
I hope you're referring to "all other TXV systems" in MB's ONLY. Most other manufacturers have their low pressure switch in the low pressure line, becuase it makes more sense to do so. A high side pressure of 30 psi in the high side is a VERY low pressure, and likely to not be circulating compressor oil.

I'll be the first to admit, I know little about MB's, but I've spent 15 years wrenching on every other make, along with handling stationary, residential, commercial & "other" systems.

You could absolutely be correct - but having a low pressure switch on the high side seems silly, especially since a leak on the suction side could easily not trip a low-side switch placed on the high side because the compressor could still pump atmospheric air above 30 psi.
I know that I am correct. Most other manufacturers use CCOT systems, and these do use a low-side cut-out switch. However, most (but not all) European cars use TXV systems which do not cycle during normal operation. The only thing that cycles a TXV system is the evaporator temperature sensor.
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  #13  
Old 10-07-2009, 11:55 PM
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Originally Posted by les_garten View Post
Hi,
The Low side was 55 and the high side was 250.

The Fan came on and the Low side stayed 55 and the High side went to 225.
Do you know if you have a compressor speed sensor? If you don't, the easiest way to find out is to find the cable going to the compressor. One wire means no, three wires means yes.

If you do, look for anything that may cause slipping. Oil on the belt or clutch would do it, and a worn clutch might. Water on the belt too, but that would only occur when it rains.
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  #14  
Old 10-08-2009, 12:53 AM
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Originally Posted by Matt L View Post
Do you know if you have a compressor speed sensor? If you don't, the easiest way to find out is to find the cable going to the compressor. One wire means no, three wires means yes.

If you do, look for anything that may cause slipping. Oil on the belt or clutch would do it, and a worn clutch might. Water on the belt too, but that would only occur when it rains.
There is an Electromagnetic RPM speed sensor on the compressor. The belt was just put on yesterday?? But you guys are sure trying to latch onto belt slippage. Is there a possibility the sensor is bad? Also the clutch engages 100% on a restart.
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  #15  
Old 10-08-2009, 07:53 AM
LarryBible
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Yes, your car indeed has the speed sensor. You did not say how many miles are on the car, and you choose not to fill out your profile so I don't know if you are in a climate that heavily uses your a/c system.

IF you have lots of miles on the car in a hot climate, and the belt tensioner is holding the belt snug, then you are looking at one of two or a combination of the two issues:

Either the clutch on the compressor is worn with too much air gap and/or the compressor is worn allowing excessive thrust which causes the speed sensor not to read as it should at all times.

If the belt tensioner is in good shape and properly adjusted, then a replacement compressor is probably the best cure.

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