(Somewhat Urgent) 87 300TD: A/C dies @ 90c - PeachParts Mercedes-Benz Forum

Biodiesel87TD · #1 08-05-2011, 09:21 PM

My mechanic has been trying to figure out what is wrong with my 87's climate control and is nearly at his whit's end. Here's the scenario: the A/C engages when the engine is cool, however when the engine temp reaches 90c the A/C shuts down. All of the sensors and inputs for the CCU and Klima relay are correct except one. We replaced the CCU with a rebuilt unit and also tested the system with a CCU from another car but the same thing happens. We also tried a Klima from another 87 300TD (however this car has had non-op A/C for 10 years) and the same thing happens. We are pretty sure that the Klima is not at fault because it shuts the system down and attempts to restart it once, then shuts down again until the engine is cool. Also he tried the Klima out of a V8 model and he said that it worked once (stayed on past 90c) but when he tried it again it didn't. He said that the engine speed sensor on the flywheel is not to spec but that would mean that the system would not work period (but it does, at least until 90c). He has checked most of the wiring in the car as well. Anyone have an idea what's wrong here?

I am trying to get this car in shape over the next two weeks so I can drive it back to college on the east coast. Before my last trip back from college the A/C worked reasonably well (fan would shut down randomly and would only restart if you used one of the lights)... I made it through most of that trip back with no A/C but I'd rather not repeat that!

sixto · #2 08-05-2011, 09:29 PM

Seems to me you need one of these - http://www.peachparts.com/shopforum/showthread.php?t=302802

Have you cleaned the compressor clutch surfaces with brake parts cleaner and flossing with a thin cloth?

Disconnect the 3-pin temperature switch atop the upper radiator hose fitting to see if that's what's triggering the compressor to disengage.

Aside from figuring out what's wrong, you can replace the Klima relay with a basic relay that bypasses the rpm comparator, coolant temperature and kickdown interrupts.

Sixto
87 300D

ashedd · #3 08-05-2011, 09:33 PM

Sounds to me like it's all junk and you should drive the car to me and let me take it off your hands

I am going to work on my '87's a/c in the fall... hope I dont have the same problems.

The question I have is: where is the ccu getting the 90* signal from? can you just unplug/defeat that? I am trying to think in my mind, of all places, how you would build a simple circuit to control the compressors clutch manually. Just need a hi/lo switch, which it should have.. hmmm

ashedd · #4 08-05-2011, 09:36 PM

PS..

pull that Klima apart and run a heat gun over the solder side of the board, CAREFULLY. That will reheat all the solder joints and fix any cold ones.

Opps I just let secret out

..you can do that with cruise amp, PBU's, anything.

sixto · #5 08-05-2011, 09:38 PM

My Klima relay is sitting in the trunk since the replacement compressor doesn't have an rpm sensor. I replaced the Klima relay with a simple ice cube relay.

I assume the mechanic confirmed that the CCU sends a continuous signal to the Klima relay socket beyond 90*C.

BTW, where are you measuring 90*C? If at the engine temp gauge, why is your car running so hot? If my compressor disengaged at 90*C, I might never know it.

Sixto
87 300D

ashedd · #6 08-05-2011, 09:42 PM

That is a really nice looking wagon btw. Just find yourself a newer hood and headlights and your set!

Jeremy5848 · #7 08-05-2011, 10:59 PM

You say the a/c cuts out at 90ºC -- are you going by the car's coolant temperature gauge? Those are not always accurate. The switch mentioned by Sixto should not turn off the a/c until the coolant is at least 110ºC or higher (depending on which version you have). One possibility is a defective switch that is operating at too low a temperature. There's a thread on this forum somewhere in which all of the switches and their temperature limits are listed. The different switches have different colored plastic insulation on the top where the 3 pins are. BTW, is your aux fan coming on in either low or high speed?

Biodiesel87TD · #8 08-06-2011, 02:51 AM

Ok starting at the bottom and going to the top:

I'm getting 90*c from one of the mechanics (tends to overstate), I've also heard 80*c from the other. I would assume they mean operating temperature based on the temp gauge. My observations support that as well. Basically we're talking about the space smack-dab between the two middle marks (80* and the one before 120*c).

The coolant temp switch (top of the thermostat housing) is a new blue top one. I asked for a grey and got a blue but I figure that is the least of my worries... My mechanic said he'd make sure that the aux fan is working, I would assume that he found that it does... He hasn't replaced it, but there's no harm in asking...

My mechanic says that all the signals to the Klima except for the engine speed are matching the specs in the FSM. So I'd assume that the CCU signal is included in that statement. I'll have him check that again.

We've used the original Klima (Which has been opened and examined) as well as one from another 87 300TD. I am relatively sure they are good.

The so called "90*c" signal seems to be inconsequential to the problem; they have rigged it so that it doesn't register as having a temperature (disconnected it I think...) and the problem still occurs. Also that part is new... And the old one behaved in the exact same way.

That Klima monitor looks like a handy tool! I'll look into it.

The clutch seems like it is engaging just fine as the air gets quite cold and it is pumping fluid. But I'll ask him to clean it anyway...

I'd like to keep as many of those safety features as possible. Having the belt break would really ruin my trip...

So far my mechanic has followed the FSM word by word but nothing yet...

Thanks to everyone who replied!

sixto · #9 08-06-2011, 02:57 AM

How about more basic things like dirt and oil in the compressor pulley grooves and an incapable belt tensioner?

Does the tach work all the time?

Sixto
87 300D

ashedd · #10 08-06-2011, 03:05 AM

Quote:

Originally Posted by Biodiesel87TD

We've used the original Klima (Which has been opened and examined) as well as one from another 87 300TD. I am relatively sure they are good.

It's not uncommon to have cracks in the solder joints that you cannot see. The other guys can help you more than I can, since I have yet to mess with the a/c on a om603 engine.

I dunno how much inclination you or your mechanic has to do this, but I always study the wiring diagrams hardcore if there is an electrical fault. That way I totally understand whats going on. But, I do the same thing with helicopters.. so I understand it's not for everyone.

Goodluck, I hope you get it sorted out. A cross country trip in this weather without a/c would suck suck suck.

Biodiesel87TD · #11 08-06-2011, 03:21 AM

Sixto:
Belt doesn't slip and it is new. In really hot weather I swear that there is a squeaking noise but my mechanic said that the tensioner was ok. I'll ask him again. I think he said that he can see the compressor disengaging... Still I'll ask.

The only time the tach doesn't work is when all of the electrical stuff in there doesn't work (the gauge pod never stays in all the way). It used to sometimes not work when I'd start the car in cold weather but it hasn't done this in a while.

ashedd:
BOTH Klima relays for the 87 diesel have displayed the exact same issue. Also a Klima from a V8 model has displayed the same symptoms. Since the V8 worked the first time and not the second, perhaps something is frying them on contact. I don't know what though.

As for diagrams him and his co-worker have the paper and digital FSM all over the tables and screens in the shop. I think they might be getting a little loopy because of studying those diagrams so much!

Again, thanks for the quick responses and suggestions. I will look into them ASAP and report back.

sixto · #12 08-06-2011, 12:20 PM

I have a client whose mechanic tries to save money by not throwing parts at her car. She begged him to throw parts at it because she was spending more on rental cars than she would on parts.

If the coolant temp switch is new, idle rpm is stable and the kickdown switch on the throttle linkage isn't sticking, I'd try a new compressor rpm sensor. Maybe Jeremy, the only person I know who actually reads the FSM, I only look at the pictures, will discover that the 'trying again' condition means a fault with the compressor rpm signal. It wouldn't try again if the engine rpm signal were bad or if the coolant temperature were still too high.

Sixto
87 300D

Jeremy5848 · #13 08-06-2011, 09:04 PM

Diagnosis of a/c problems in 80s-vintage Mercedes cars is compounded by the fact that the Klima relay looks at so many different inputs. All must be "correct" or you will get no cold air. Since you are getting cold air for a little while, the compressor and its plumbing clearly work. The challenge is to figure out what is happening to cause the system to turn off at 90ºC. I will hazard two guesses:

1. Your engine is actually a lot hotter than 90ºC. With a blue-top switch, the compressor should remain on until the coolant gets up to 128ºC. Sixto and I both believe this is way too hot but the blue switch is what the cars got in the factory. See post #13 in this thread for a complete list of switches and their temperatures. I have a red-top switch in my 603 engine and Sixto may also; we're conservative.

2. The blue-top switch is defective or the wires are incorrectly connected to it such that it is switching off too soon. [This is hard to believe with a new switch.] Incidentally, if the engine cools off, the switch (Mercedes calls it S25/5 in the diagrams) will turn the compressor back on again. This off-on cycle can continue forever if the engine repeatedly heats up and cools off for any reason.

I'm a little concerned about this "90ºC signal" that you mentioned. AFAIK there is nothing in a 1987 300D Turbo (W124, OM603.960) that sends a signal -- climate control or anywhere else -- at 90ºC.

Also, I forgot to mention that Klima relays from different cars CANNOT be safely interchanged. They are internally different, ranging from "a little" to "very" different. To be sure of getting correct results, you must use the correct Mercedes part number. For a 1987 300D/TD with an OM603.960 engine, that means part number A 001 545 80 05 (original) or part number A 003 545 69 05 (later factory replacement) "Control unit, refrigerant compressor and kick-down cut-off." These numbers are from EPC.

This is not to say that it's impossible to adapt a different Klima but I wouldn't advise it. For those who like to gamble, the critical items are (a) gas/diesel and (b) number of cylinders. Match those to your car and you might have a chance. YMMV and don't blame me, I warned you. Unfortunately, Mercedes doesn't provide internal schematics of things like Klima relays and the electronic portions are shown as boxes with wires running and out. Reverse-engineering is possible but I haven't heard of anyone doing it and it's not something I'd want to tackle.

To answer Sixto's implied question, FSM says that Klima will attempt to start even with a frozen compressor but if it doesn't like the signal it gets (or doesn't get) from the compressor RPM sensor then it will shut off the compressor. Specifically, Klima compares the speed of the ring gear to that of the compressor and, if it sees more than a 30% difference, it watches for an additional 200 milliseconds (0.2 second). If the problem goes away, all well and good. If not, Klima turns the compressor off and it stays off until you turn off and restart the engine.

FSM ("Service Manual / Automatic Climate Control / Model 124") section 83.605 page 7 has a Note "After the refrigerant compressor has been switched off [due to slipping serpentine belt or frozen compressor], the refrigerant compressor will only be switched on again after the engine has been switched off and restarted." That does not sound like your symptoms: FSM says Klima will not attempt a "restart" as you describe. Something else is going on. Wish I could be of more help.

Jeremy

Biodiesel87TD · #14 08-07-2011, 01:18 AM

Jeremy:

Those two ideas were what my mechanic thought when he first encountered the problem. Despite replacing the blue top switch with a new one (I'll see how much a red one costs) and disconnecting the switch thus neutralizing that input into the Klima, the problem still persisted. The car would have a functioning compressor until operating temperature was reached, then it would disengage, engage for a split second, and refuse to engage again until the car cooled down and restarted. What I meant by "90*c signal" was whatever signal was coming from the blue top switch at operating temp. I assume that my assumption that it sends a signal constantly is wrong?

I am not planning on having another Klima relay put in place of the correct one; I want the system to function correctly! So far the mechanic has used the one that is original to the car (A 001 545 80 05), one that is original to another 300TD (A 001 545 80 05), and one for the V8 model. I'll ask him not to put the V8 one in there again! What type of damage could that do?

As for my problem, it seems like the shut down symptoms are similar to the ones for a RPM mismatch. However the lead up to the shut down doesn't make sense with that diagnosis (even though the ring gear speed sensor is giving some wacky readings). Maybe it is combination of the incorrect ring gear readings and something like the belt or clutch that is only slipping when it is warm (due to expansion, liquefaction of grease-dirt, faulty tensioner, etc). I also read here that the Klima in the V8 models doesn't have inputs for the ring gear sensor or compressor RPM. This would support the functioning of the system when the V8 Klima was used... But that might have been a fluke since it didn't work again. What do people think about that? Does it make sense?

Thanks everyone!

Jeremy5848 · #15 08-07-2011, 03:30 AM

The switch, S25/5, blue top or whatever, doesn't "send a signal." All it does is close one of two switches, one at a temperature around 105ºC, to turn on the aux fan in "high" speed, the other, at around 120ºC, to tell Klima to shut off the a/c because the engine is getting too hot. Both switches close when their "trip" temperature is reached.

The common terminal of S25/5 is ground. For the fan, the ground completes a circuit that energizes a relay, that turns on the aux fan. The two pins on the switch for fan and ground use a two-pin connector (the two pins are close together); the one pin for the Klima relay uses a single pin connector and the pin on the switch is off by itself. It's almost impossible to get the connectors on the wrong pins unless you take the connectors apart. If the coolant gets up to 120ºC (or whatever, depending on the color) the single pin is grounded; that ground tells Klima to shut off the compressor.

I think the most important thing for you and your wrench to keep in mind as you work through this problem is that Klima has many inputs and every single one of them must be "right" or Klima will shut off the compressor. Let me try to list them all.

1. Power from Fuse 7 to Klima pin 5.
2. Engine speed from the Electronic Diesel System (EDS) at Klima pin 2. The engine speed must be at least 600 RPM for at least 10 seconds before Klima will allow the compressor to engage. This keeps the load of the compressor off of the engine until the engine has run for a little bit. (Later versions left this feature out and allowed the compressor to start immediately.)
3. The over-temperature switch S25/5 has to be "open" (not grounded). When the engine gets too hot, the switch closes and Klima pin 12 is grounded; this shuts off the compressor. Once the temperature drops below the set point, S25/5 opens and the compressor is turned back on.
4. The pressure switch in the refrigerant line, S31/1, must be "closed" (so it completes a circuit between the push-button control unit (PBU) in the dash and Klima pin 10. If the refrigerant has either too little or too much pressure, it will open and Klima will shut the compressor off. If the pressure returns to "normal," the compressor will run again.
5. The PBU must be "calling" for a/c, that is, one of the three left-most buttons in the horizontal row of 5 must be pressed (Defrost, Bi-Level, Normal). This "call" goes through the pressure switch (#4, above) to Klima pin 10.
6. The microswitch in the throttle linkage near the IP must be "open;" at full throttle it is closed, grounding Klima pin 4, and again, shutting off the compressor. (The idea here is that if you are running from the bad guys, the a/c is shut off so the car can go a little faster. Once you take your foot off, the compressor comes back on.)
7. The compressor speed, as measured by the speed sensor on the back of the compressor, and connected to Klima pins 9 and 11, must be close to the engine speed (see item 2, above) or Klima shuts off the compressor and keeps it off until the engine is turned off and restarted. This is a more "severe" shut-down because the problem, a frozen compressor, is more serious.
8. Klima pin 1 must be grounded at all times. The ground terminal is W10, the same one used by the negative terminal of the battery.

Any of these eight things could have a problem that makes them sensitive to temperature. I admit to being puzzled by the single restart attempt and subsequent shutdown until the engine cools. It has been discussed on this forum and I believe it to be true, that the compressor does not cycle on and off to control temperature. The compressor runs all the time, making the air as cold as it can, and heat is added via the monovalve to bring the air up to the temperature set by the thermostat in the PBU.

I had a problem in my 1985 300D-T in which the monovalve failed to close; the air was coolish but never got cold even though the compressor was running "as fast as it could run." That is apparently not your problem if your mechanic says he can see the compressor stop (and you can look down at the front of the compressor and see it switch on and off if you know what to look for).

One experiment I have not had time for is to wire a small lamp across the compressor's clutch terminals and lead the wires into the passenger compartment so I can watch to see if the compressor ever turns off. Unless the engine overheats or something else happens, the compressor is supposed to stay on all the time whenever the climate control system is in any of its a/c modes.

Hope that helps.

Jeremy