300 SDL A/C electronic controls
 

I've spent the day attempting to diagnose the following symptom:

The a/c compressor will engage for about four minutes and shut down. It won't come back unless the vehicle is shut down and restarted.

I have done the following:

1) Checked the low pressure switch. Found a ground on both terminals when connected to 12V.

2) Isolated the system from the Klima. Removed the Klima and installed an ice cube relay in it's place.

No change in symptoms. Four minutes of compressor engagement and then it drops off line.

I'm at a loss as to what other device would control the compressor engagement. AFAIK, there is no high pressure switch that shuts it off??? :confused:

Any additional thoughts would be greatly appreciated.

What's an 'ice cube' relay?

The pressure switch on the receiver/dryer is a dual switch. One switch will open if the pressure is too low, and the other switch opens if the pressure goes too high. The connections on this pressure switch come from the push-button control, and the ACC (klima?) controller. When the proper button is pushed, this line is grounded. It is the ground signal that tells the ACC to turn the compressor on. The ACC will turn the compressor off if the engine temperature gets too high, or if the compressor speed does not agree with the engine rpm. From you symptom, (compressor stops and will not restart until the key is cycled) I'd first want to look into the speed signals going to the ACC controller. Jumper power to the compressor clutch and measure the signal coming from the speed sensor. Mine was under specification at about .17 volts at idle, but works fine. If it changes after 4 minutes, find out why. A slipping belt or incorrect compressor clutch gap are possibilities, as is a failing compressor. If the signal is steady and the compressor continues to run with direct power to the clutch, then you'll need to look at the engine speed signal, the coolant temperature sensor, and the compressor request signal. If all those signals are correct, then it's the ACC controller at fault.

An "ice cube" relay is one of those cube relays that look like an ice cube. There are about six of them in the fuse box.

Please correct me if I am mistaken:

I have eliminated the following items because the Klima (ACC controller) is not in the system. I have removed the Klima and replaced it with an ice cube relay. The relay switches power from to the compressor based upon the ground signal from the CCU off pin #10.

---engine temperature
---compressor speed
---engine speed


So, the only control to the compressor is via the CCU. If pin #10 gets a ground signal, the compressor engages.

The question is what could possibly interrupt the ground signal from the CCU?

The only thing that I see is the pressure switch. I believe that this pressure switch has only a single low pressure function (1986 wiring). However, to rule out the pressure switch, I removed the two leads from the switch and jumped them. Same results.

Just to confirm, the switch is the lower of the two switches on the dryer?? In fact, the switch is actually in one of the lines from the dryer. There is a second pressure switch (high pressure for aux. fan??) on the very top of the dryer. :confused:

Pressure switch: Looks like the pressure cutoff switch is 'binary', will open on either hi or lo pressure .
http://catalog.peachparts.com/item.wws?sku=W0133-1628267

Temp switch:
http://catalog.peachparts.com/item.wws?sku=W0133-1626582

Sounds like jumping should make the compressor 'ignore' hi or lo pressure...

Best Regards,
Jim

Thanks. I've definitely got the right switch.

Is it proper to jump this switch to eliminate it?

I tried this and found no difference in the results, so, I'm assuming that it is not the problem.

Any other ideas where the CCU would get a signal to drop the compressor??

I do not have a schematic for this chassis , but you may want to consider power feed interuption instead of the switched ground fault.
It is possible that you have a poor connection at the fuse feed or relay connection .
Hook a test light to ice cube coil side and load side and see if power is dropping at the terminals of each.
If suspect , bring a bat+ jumper over to ice cube pos side and run the car for a test .. see if it drops then.
Same goes for ground side .. Run ice cube with its own ground wire and see if you get a drop after the time limit you are getting.

Arthur, wouldn't the fact that the exact same symptom occurs with the Klima and with the ice cube relay would seem to rule out a poor connection to both of them. :confused:

Additionally, a poor connection would give a different time interval than originally noted for the Klima. This symptom is exactly the same, with the Klima in place, or with the ice cube relay in place. Four minutes of compressor operation and then nothing.

I have a suspicion that the CCU might be the problem, unless there is a high pressure switch that I'm not aware of.

BTW, I did confirm unlimited operation of the compressor by jumping 12V directly to the compressor socket in the 12 pin connector. So, if I wasn't worried about freezing the evaporator, I could run the compressor indefinitely.
I'm not seriously going to consider this. ;)

Yes .. I tried to find that schematic, but I just can't find one for that year/model..
Last time I used one from a gasser and it turned out to be different.
So , basically what I am saying is that if you are getting an interuption, it has to be on the feed side or the ground side ,, so, I would not assume which, as an imcomplete circuit can come from either .

If you bring 12v to the IC relay direct from bat and still have the problem , you then know it is not a feed problem... just like if you have a seperate ground wire to complete the IC circuit and the problem still exist, you know it is not a ground problem .. this is just a way of narrowing down your fault.
[ I am talking coil circuit of relay on this, not load]
If I were to just look at the symptoms , I would guess, with that time frame, that the aux fan was not coming On and the high side pressure was getting to high pressure sw cut-out.

but no schematic , so i do not know that...
Do you have the schemmatic???

I was thinking the same thing regarding the high pressure switch cutout. I do have the schematic and it appears that the high pressure switch runs directly to the CCU and, therefore, can interrupt the ground signal from the CCU to the Klima (or ice cube).

But, I disconnected the leads from the high pressure switch and jumped them. Same results. Is it possible that I should not be jumping those leads to the high pressure switch? Could the switch provide a necessary ground for both those leads, a ground that is broken when the switch opens?
Maybe I need to ground both those leads, independent of the switch?? :confused:

BTW, the auxiliary fan does start up properly when the tests are done at idle.

can you jpg the schem to me???

I have it as a .pdf.

I'm sure you can open it.

Can you kindly send me your e-mail address. I'll attach the file and return it.

Brian,

I found a diagram on the W126 service info CD. If this is correct, the pressure switch only opens with low pressure. There is no high pressure switch. Test the switch for continuity. Should be closed with refrigerant in the system. The ground signal comes from the pushbutton controler, through the pressure switch, and arrives at pin 10 of the ACC socket. This pin should be the ground for the control circuit of your 'ice cube' relay. This ground is controlled by the pushbutton control. You will also need power at two pins of your relay. One feeds the control side, and one feeds the load. Since you say you were able to jump pins and get continuous compressor opperation, it indicates that the load power supply, the wiring from the relay to the compressor clutch, and the compressor clutch ground are all good.

You are either losing the power supply to the control side of the relay, or you are losing the ground. Test by grounding the control circuit at the pressure switch. If the compressor runs, look at the pushbutton control. The pushbutton control monitors the temperature of the evaporator and heater cores. This is how the temperature of the evaperator core is maintained at a point above freezing. A faulty evap temperature sensor would shut the compressor down.

I'm fairly sure that I'm losing the ground. I'll ground the control circuit at the pressure switch. I'm fairly sure the compressor will run.

Funny thing about the evap temp. sensor, you would think that it would fail completely and not allow the compressor to engage, or would fail at a certain temperature and then, after it warms up, would close again. But, that's not the case. It allows the compressor to run for four minutes and then will NOT allow it to run again, until the vehicle is restarted. So, would this make any sense as to an evap temp switch failure?? Doesn't seem possible.


What about a faulty CCU? The evap temp switch opens, the compressor drops off line and, when the evap temp switch closes again, the CCU, for whatever reason, won't reengage the compressor??

I can't seem to reconcile the behavior of never engaging the compressor, with the exception of the first four minutes.

Have you read the following article from Steve B. on this website?

http://www.peachparts.com/Wikka/AcDiags

I'm not famliar with the specific wiring scheme in the W126 but assuming the W126 is the same as a W124 I don't understand how you could install in "ice-cube" relay in place of the KLIMA realy. The KLIMA relay has 10-12 pins and the standard ice-cube relay has 4 pins.

Tim, the ice cube relay eliminates all of the superflous controls that are not required to run the a/c. You lose the speed monitoring capability for the serpentine belt and the coolant temperature cutout and the kickdown cutout, but the system will work just fine, if the CCU is providing the proper ground signal.

You can install the ice cube on the W124 as well. It's been done before and works fine.