Evaporator Temperature Resistor - PeachParts Mercedes-Benz Forum

dannym · #1 10-27-2004, 12:56 PM

Hey,
What exactly does this part do?

From information I got from a search this may be causing AC problems for me.

Thanks

Danny

MB STARS · #2 10-27-2004, 02:02 PM

Danny
maybe you can tell us more about your problem with a\c system first we need to know years & model of the vehicle.

dannym · #3 10-27-2004, 03:07 PM

Sorry, forgot.

I have a 1984 300SD.

I have been having problems with the AC since I bought the car. I put a can of leakstop in and it seamed to work but the pressures are way off.

I get over 300lbs on the high side with 45lbs on the low side. That is the only way I can get it to cool.
If I drop the low side down to 30lbs it will take the high side down to about 270, but it won't cool hardly at all at that pressure.

Also, the compressor runs continuously, it never stops.

I did notice some oil on the TEV valve if I can't tighten it I will replace it but I think the ETR valve may be causing problems also.
I guess I should just replace that too at the same time. They are both fairly cheap to purchase on line.

Thanks

Danny

dmorrison · #4 10-27-2004, 03:20 PM

Sounds like you have a blockage in the expansion valve. Your eveporator is not getting cool so the AC control is telling the compressor to stay on. This in turn is causing the high pressures and the low pressures are being caused by the lack of freon getting by or to the evaporator. What is causing the blockage we will need more info. Leak stop? Why?. Was the compressor Ok or has it trashed itself and the debris is blocking the expansion valve?
We will need more info. What have you done and what have you added to the system.

Dave

dannym · #5 10-27-2004, 03:49 PM

Hey,
I agree with your blockage assesment. That's exactly what I was thinking.

I put the leakstop in because there was a leak. I didn't notice it was the expansion valve until yesterday when I was removing the instrument cluster.

The compressor has always worked fine it just doesn't shut off. The previous owner had a service ticket concerning the AC. All they did was pull a vacuum and recharge though. The ticket doesn't say they did anything else.
Don't worry I won't be going to them for repairs.
Anyway, these AC problems aren't something new, i'm just more active in getting to the problem and making a final fix.

Once in a while I can hear the TEV valve spit out some refrigerant into the evaporator. It's just a short hssing sound.

I was wondering if the ETR valve would cause problems like this also but i guess if I'm replacing the TEV then I'll just replace the ETR valve also.

Thanks

Danny

MrCjames · #6 10-27-2004, 05:08 PM

Perhaps you have a blockage but I would think that your low side would go into a vacuum state if the expansion were truly blocked. High side would go up but if the refrigerant was blocked then there is little, or not getting past the blockage. Maybe some other times need to be examined prior to jumping into an expansion valve replacement.

Have you checked?
Mono-valve (split rubber seal on insert)
Mon-valve (electrical control, grounded through pushbuttons)
A/C comp (Always on, regardless OR with A/C demand)
ETR (quite possible IF A/C demand-comp runs continuous)

Let's explore some of these other contributing factors to determine if there is pressure issue to address.

BTW: Was the A/C system converted to R-134?

psfred · #7 10-27-2004, 07:41 PM

Lots of air in the system, very greatly reduced cooling capacity, and very high pressures.

A blockage at the expansion valve will give high head pressure (as will a blocked condensor, by the way --is the aux fan running, and is the condesor clear of crud?) and VERY low (as in vacuum) pressure on the low side.

If it was leaking freon out, it was leaking air in.

You need to do the following:

Check the condesor for buildup of crud between condesor and radiator, or just plain accumulation of slop preventing air flow.

Verify that the aux fan comes on (it must, at that pressure) -- check fuse if it doesn't, it's very likely cracked as mine was.

check to make sure the high pressure side is hot and the low pressure side is cold -- suction hose fitting should be condensing water out of the air, but no frost.

Chances are the how side isn't very cold, in which case I'd say you have way too much air in there. You need to vent it, fix the leaks (almost surely bad orings, replace them ALL, including the ones on the pipe manifold on the compressor, renew the oil, and replace the reciever/dryer. I'd do that only after you KNOW it will hold vac for at least an hour (you will need to pump overnight if it's been leaking a while).

Lots of work, but leakstop won't fix eroded o-rings. If you want to stay with R12, get it recovered if you can, otherwise you need to flush and replace the oil with PAG 46 oil and charge with R134a.

Been there, done that -- save yourself repeated work and do it right the first time.

PAG oil picks up water terribly, and withR134a water makes hydrofluoric acid -- you must change the reciever dryer EVERY time you open the system for any length of time.

Peter

dannym · #8 10-28-2004, 09:03 AM

Hey,
Ok, the condenser is clear no crud at all and good air flow. The auxilliary fan does not come on, I will have to check that this weekend. Is that on a fuse or a relay? I think it's a relay but I don't know which one or how to test them. I did try jumping the pressure switch with no joy.

The system is still R-12 and I would like to keep it that way. I was thinking about air in the condenser but I don't know how it would get in there. The compressor should not pump into a vacuum, it should not run at all below around 30 psi. Though I do realize that it is possible.

Quote:

If it was leaking freon out, it was leaking air in.

The low side hose does get condensate on it, It just doesn't cool very good unless I overcharge it. I know that's not good to do.

Actually yesterday was the first time I heard of a monovalve though I do remember reading in the Hayne's manual about a valve in the engine compartment behind the firewall.
How do you troubleshoot this valve?

Yesterday was definetely the first time I heard of a ETR valve. After searching through posts I was thinking this may be at least part of the problem.

Your right about fixing it right the first time. If I open the system up I want to do it only once. Though I do have some R-12 laying around

I was wondering if the hissing from the TEV was normal. I would assume that would mean that it is working. I was only going to replace it if I couldn't stop it from leaking.
I used a very sensitive electronic leak detector on the whole system and the only leak I found was on the TEV.
But I guess if I'm opening the system I should replace the o-rings anyway??

Oh yeah I almost forgot, It takes about 10 minutes for the pressures to equalize after shutting the AC off. It shouldn't take that long should it?

Thanks for all the feedback I appreciate it.

Danny

jim16671836 · #9 10-28-2004, 11:47 AM

You might want to test your aux. fan by chasing the wiring from the fan to the connector which is under or near the dryer and disconnect and jump the fan from a battery. I think the temperature switch on the dryer operates the fan and the pressure switch operates the clutch on the compressor...Jim

dannym · #10 10-28-2004, 01:18 PM

I may have jumped the wrong switch I'll test both of them again when I get a chance. When I jumped the switch I did test for power to the fan and I didn't read anything. Would you know if the fan is on a fuse or relay or both and how would I test the relay??

As an update I just went for a drive.
Outside temp: 81 deg Humidity: 58%
The AC was just barely making the cabin cool though I did feel cool air out of the vents.
SO there is definitely not enough liquid refrigerant in the evaporator though with those head pressures I'm sure there is enough available.
Or maybe it's the monovalve? Could the monovalve be leaking hot water into the heater core?? I think I'll pinch the lines off and test that. Or is there a more definitive way?

I still don't have a good idea what the ETR valve does which is kind of funny because that was my original question

.
Not that you guys aren't a great help though. I'm really learning a lot.

Danny

jim16671836 · #11 10-28-2004, 08:20 PM

I don't know where the ETR valve is let alone what is does....I am suggesting that you hook your aux fan directly to a battery and see if it will run first...I checked 8 fans that I have and ony 2 were working...if your fan works then it could be your relay and I can send you a picture of the relay...the relay gets 12 volts from the fuse box and I don't know if it is feeding the other relay like a 123 turbo does...the 123 Aux.fan relay also feeds another relay right next to it on the driver's fenderwell...
I think the monovalve is designed that when it does not work it will stick in the open position and let you have heat all the time...when you are running the AC check and see if the heater hoses going and coming thru the firewall are hot..Jim

psfred · #12 10-29-2004, 08:20 PM

If the heater hoses are warm to hot, the valve has failed open (no power) or the pushbutton unit is bad. If there is voltage on the monovalve when you want AC, but the hoses are hot, it's the valve. If no voltage, it's the pushbutton unit.

Either failure is pretty common. Monovalve on the W123/W126 can be fixed with a replacement valve, very easy (4 screws in a plastic housing, pull the valve out, put a new on in, put the xcrews bach in).

Pushbutton unit isn't hard either, just more expensive.

If hte suction hose is cold, the AC is working properly. Aux fan is a fuse for the current, probably over a relay. there is a pressure or temp switch on the receiver/dryer to switch the fan on, also a resistor in the fan circuit to run it a low speed, check the connections, they tend to corrode and cause a no fan condition.

If you run high side pressures that high, eventually the compressor will detonate or you will blow a hose, venting all the freon.

You must get the aux fan running, it will overpressure without it.

Peter

dannym · #13 10-30-2004, 04:38 PM

Ok,
The valve definitely has power, just a tad over 12v. And the hoses are warm to the touch. So from Pete's post it's the monovalve.
The expansion valve is ice cold with condensate on the line going to the evaporator coil.
There are 3 hoses going into the firewall 2 from the valve and one from the engine block/pump. The 2 from the valve are warm. The one from the block/pump is HOT!!. Is it normal fror it to be that hot?
There are also 2 more hoses going to some kind of defrost unit for the wipers??. Both are HOT. Do I need to worry about them. One of these lines is going into the bottom of the valve.
I wish I had a diagram of the water flow circuit.

Also i put power to the auxiliary fan directly from the battery and it didn't work. So I need to replace the motor.

Danny

psfred · #14 10-31-2004, 12:29 PM

If I remember correctly, the supply side comes from the block, and if it's hot, asi should be (engine runs around 90C there), coolant is running through the heater core, reheating the air so it appears that the AC isn't working.

Replace the monovalve insert, it's about $35 I think. Easy job, no coolant drain or anything.

Replace the fan, too -- head pressure will drop quite a bit.

Peter