Any AC Experts out there? - PeachParts Mercedes-Benz Forum

spinedoc2304 · #1 05-11-2009, 10:03 AM

Good Morning to all;

After posting this in the regular tech help area, I was advised to post it into the diesel area because there are alot of AC guys here who like to talk...so here it goes...

Yesterday, after leaving my 1991 300E parked all day, the AC never came on upon starting. The fan didn't work, the compressor never came on, and the air recirculating switch with the light didn't work. It had worked fine up until yesterday morning and blowing cold air, so the system is charged. This happened intermittently last year and just as I was at the end of my rope, it started working again and has been for the last 7 months-until yesterday.
Last year, if I would hit the AC off button and then turn it on, I would sometimes hear the vents operate (sounds like it opens or shuts) and the AC would work along with the air recirculating switch. Sometimes back then, as it is doing now, the AC switches don't do anything, the air recirculating switch is doing nothing, and hot air blows out the windshield defroster vents and I can't turn it off.

Help!

leathermang · #2 05-11-2009, 11:23 AM

We just like to talk... we know nothing about AC stuff..

Your car is newer than I have real factory manuals for but I will venture a couple of ideas...

You first need to check out regular stuff like fuses before assuming AC problem.

You can not assume the system is charged because it was working fine yesterday... or even this morning... while slow decline and leaks are the norm.... you can blow the seal on the front of the compressor or at some other place... and thus have nothing in the system the same day it was working fine....

It sounds like you have a system with some of the controlling done by vacuum... and have had small problems with it for a long time.. you need to check out that system before you assume AC problems... Controls for the AC, if not working look JUST LIKE the ac not working...

ah-kay · #3 05-11-2009, 12:21 PM

I am not an AC expert by trade but I have done a few A/C jobs with newer cars as well as old MBZ. Like changing out evap, compressor, condenser and converting R12 to R134a.

I have not worked on a 1991 300E but I have a few pointers that you can check. You need a multimeter to do the simple checks.

1) Check the fuse to the A/C unit
2) Measure the pressure switch across receiver/driver. It should not be opened. It is used to detect freon. Your car is probably using R12 for 1991. If it is opened then you can jump with a wire for a short while and see whether the compressor kicks in. If it does then freon is low. If it does not then compressor is questionable.
3) 1991 has a ACC mono-valve. Remove the connector and measure the voltage across the connector of the mono-valve. When the climate control calls for heat, the voltage should be 0v. When the climate control calls for A/C, the voltage should be 12v. This is primitive check for the unit, not exhaustive.
4) If you have access to the gauges then you can measure the pressure on the high and low side with the engine not running and check it against spec.

Good luck.

Hit Man X · #4 05-11-2009, 08:18 PM

Go back to the gasser forum, http://www.peachparts.com/shopforum/showthread.php?t=252111

I posted there, you have to TEST MORE before you can get further with it.

Matt L · #5 05-11-2009, 09:05 PM

Quote:

Originally Posted by ah-kay

4) If you have access to the gauges then you can measure the pressure on the high and low side with the engine not running and check it against spec.

There's no spec for the pressures without the engine running.

Hit Man X · #6 05-11-2009, 09:06 PM

You can tell if you have a block as I understand w/ gauges and vehicle off. They should be balanced.

tangofox007 · #7 05-11-2009, 09:22 PM

Quote:

Originally Posted by ah-kay

If you have access to the gauges then you can measure the pressure on the high and low side with the engine not running and check it against spec.

That's a meaningless exercise. If there is any liquid refrigerant in the system, the pressure will be constant for a given temperature regardless of how much refrigerant is present. It's not like adding air to a tire; more refrigerant does not create more pressure.

Quote:

Originally Posted by Hit Man X

You can tell if you have a block as I understand w/ gauges and vehicle off. They should be balanced.

It would take more than one obstruction to prevent the pressures from equalizing with the compressor not operating.

ah-kay · #8 05-11-2009, 09:39 PM

Well, I am only go by the manual of the "A/C manifold gauge set" from Harbor Freight to conduct a STATIC pressure reading, see Page 5. You can measure the low and high ports pressure in 2 conditions, with engine running and not running. I do not want to endanger the life of the any members with the engine running because I am not an export'.

Also thank HOT_MAN_X to the rescue.

http://www.harborfreight.com/manuals/92000-92999/92649.pdf

Matt L · #9 05-11-2009, 10:23 PM

How about step 7, at the top of page 6?

Find me the spec in the service manual for that one. Whoever wrote that doesn't really know what they're doing.

And actually, Tango was more accurate than myself in his answer. The static pressure tells you the temperature. There's no reason to look that up, as it should be printed on the gauge face.