Air Conditioning Diagnosis Question - PeachParts Mercedes-Benz Forum

Ace Taxi · #1 07-13-2009, 05:42 PM

Identical problem on '97 E300D & '92 300D: Initially, compressor engages fine; after a few minutes, compressor disengages. Can't get the a/c to kick in thereafter.

If the car is allowed to sit for a few minutes, turn everything off, then the compressor will again engage upon restart, and pattern repeats.

'92 was converted to r134a by me. PO hadn't used a/c in five or six years. '97 had a bad blower regulator when I bought it so PO hadn't used a/c in a year or two. When I replaced the regulator, a/c blew nice and cold. Both were working fine earlier in the summer, I can't pinpoint anything specific that's changed. To charge the '92 and top off '97 I used the ultra synthetic r134a w/lubricant & leak sealer that you see at the auto parts store.

Other data (maybe helpful for diagnosis, maybe not): auxilliary fans work, but kick in almost immediately after a/c starts. On cold summer mornings (exterior temps ~ 55 F) and a cold motor, it takes a little longer for a/c to disengage.

Any ideas on where to start looking?

I was thinking about evacuating the systems, re-lubing and recharging, but I was hoping for some guru-onic input first.

Thanks in advance.

compu_85 · #2 07-13-2009, 05:45 PM

What are the pressures?

-Jason

#3 07-13-2009, 05:48 PM

Either your a/c clutch or the belt is slipping. When this happen it is sensed via the rotational sensor on the back of the compressor. This shuts down the compressor and to reset it, the ignition must be turned off and the engine restarted.

Occasionally the cause will be a slipping belt or slipping clutch as I said, but it also gets worse with high mileage compressors. As the compressors wear the amount slack in the shaft front to rear increases causing the sensor to trip more easily.

Check the condition of the drive belt and check the gap in the a/c clutch. If this all looks good you might want to consider replacing the compressor. I saw one of these compressors on ebay for about $170 including the seals and a filter/drier.

Good luck,

Ace Taxi · #4 07-13-2009, 06:20 PM

Now that you mention it, the tensioner dampeners on both cars are on their way out...

nhdoc · #5 07-13-2009, 09:48 PM

Without reading pressures you cannot know what is going on in there. Get a set of gauges and read both the high and low sides from when the compressor starts to when it stops. It sounds to me like you might have overcharged both systems and they are cutting off due to the high pressure cutout. Either that or you might have a restriction causing the pressure to spike. That is why the fan kicks on immediately too.

#6 07-14-2009, 05:55 AM

He said that the systems ran fine for awhile after the conversion. Had they been overcharged, this would have shown up right away.

Now, to your point, if they were overcharged, the effect of the added strain MIGHT have led to the current slippage problems. Also to your point, it is VERY common for converted systems to get overcharged.

I used to subscribe to the 85% theory, that is, use 85% of the R12 specified weight in R134a. Since doing more work and now using a charging scale, I have learned that 70% is more often correct. I start with 70% and then go up from there in 1 ounce increments while monitoring high side pressure ensuring that there is no runaway.

All that said, BY FAR, the VERY BEST approach is to stay with R12.

nhdoc · #7 07-14-2009, 06:34 AM

Perhaps the OP can clarify the sequence of events. It sounded to me like he charged/topped off the systems and then started having trouble. What he describes is classic overcharging symptoms - right down to the fact that it runs longer when cold out- but without gauges he can't know this for sure. I suppose he could try reducing the amount of refrigerant in the system and see if that cures it.

I agree with Larry than R12 is the best way to go with the systems that were designed for it. I have never been happy with any converted system, period.

nhdoc · #8 07-14-2009, 07:01 AM

I just realized that the OP's '97 has the diagnostic capability to read the high side pressure without gauges. Start the car with the compressor disengaged (EC light on) and then hold down the "rest" button for several seconds until the display changes, then toggle the left and right end buttons until the left display reads item 07 - that is the high side pressure. It should read about 60-70 PSI or 4 bar static pressure with the EC light on. Now engage the compressor. Monitor item #7 once you turn the compressor on and see if it spikes to above 300 PSI or 20 Bar which is the high pressure cutoff on that system and report back. I'm still betting it is overcharged. The system capacity on the '97 should be three 12 ounce cans or right about 2.2 lbs.

Matt L · #9 07-14-2009, 10:51 AM

Be sure to confirm the reading at #7 with a gauge. If they differ, not only do you not know for sure what head pressure you have, but your compressor may not shut off due to high pressure and can vent.

jcyuhn · #10 07-14-2009, 08:07 PM

The systems in the 124 and 210 cars are quite different. The 210 car does not have the RPM sensor setup which compares to engine speed and disengages the compressor clutch - it uses a different (and permanent when it goes) mechanism.

Check the standard things on the 124 -clutch gap, clean the clutch, tension the belt, etc.

Not sure what to check on the 210. Does the EC light come on when the compressor fails to engage? That is commonly the evaporator temp sensor.

#11 07-15-2009, 05:39 AM

Jim,

As always, good information from you. I did not know whether the 210 had the slip sensor or not.

Matt L · #12 07-15-2009, 10:03 AM

Quote:

Originally Posted by LarryBible

Jim,

As always, good information from you. I did not know whether the 210 had the slip sensor or not.

It uses a thermal fuse in the clutch coil. The heat of the belt slipping permanently blows the fuse.

nhdoc · #13 07-15-2009, 12:41 PM

Quote:

Originally Posted by Matt L

It uses a thermal fuse in the clutch coil. The heat of the belt slipping permanently blows the fuse.

Yes, and I can personally attest to the fact that it works exactly as it is designed to!