1988 300TE AC Problem
 

I have a 300TE. You start the car and the AC Clutch engages, the suction line gets cold and blows cold air. Ambient pressure on the suction side 88#. As soon as the system runs I get 55# Low side and 225# High side at 90F ambient temp. The AC runs a couple minutes. Then the front electric fan kicks on. Runs another minute or so and the AC clutch kicks out and the electric fan starts. Will not cycle on again. Stop engine, restart and the whole thing repeats.

So I read and got the suggestion it was the serpentine belt. It was indeed in piss poor shape. It's a good thing i looked at it before it left the car stranded. Replaced it. No change in the behavior.

Where are the usual Gremlins at that cause this?

Thanx for any help! It's hot down here in S. Florida!

Les

You said "As soon as the system runs I get 55# Low side and 225# High side at 90F ambient temp. The AC runs a couple minutes. Then the front electric fan kicks on."

What do the pressures do after the fan comes on? Does it drop below 30 or so?

Does the '88 use a compressor speed sensor? If so, a low speed reading from the compressor will cause these symptoms.

The speed could be low because of a dirty sensor, or a slipping belt or clutch.

Yes, when the Fan Kicked on the Pressure went from about 250 down to 225 pretty quick.

There was a suggestion elsewhere to a few folks that they had loose belts, that's why I just changed mine and it's pretty tight. The tensioner scale I never figured out how to read it, so i just tightened it up till it had maybe 1/4" deflection when pushed on.

I'm more interested in the low-side pressures once the fan kicks in. More than likely, there's a low-pressure switch that can be tripped if the pressure drops low enough (which will happen if you're low on charge and the fan comes on).

What does the low-side pressure have to do with anything?

Low side cut-out; an 'absense of refrigerant sensor' if you will. Like I said, I'm not an MB tech, but I would assume the A/C system would be equipped with one. I know my '85 grey market 380SEL has one.

There is no low-side cut-out switch. M-B's have a low pressure cut-out switch on the high-side line (as do all other TXV systems).

I hope you're referring to "all other TXV systems" in MB's ONLY. Most other manufacturers have their low pressure switch in the low pressure line, becuase it makes more sense to do so. A high side pressure of 30 psi in the high side is a VERY low pressure, and likely to not be circulating compressor oil.

I'll be the first to admit, I know little about MB's, but I've spent 15 years wrenching on every other make, along with handling stationary, residential, commercial & "other" systems.

You could absolutely be correct - but having a low pressure switch on the high side seems silly, especially since a leak on the suction side could easily not trip a low-side switch placed on the high side because the compressor could still pump atmospheric air above 30 psi.

Hi,
The Low side was 55 and the high side was 250.

The Fan came on and the Low side stayed 55 and the High side went to 225.

I know that I am correct. Most other manufacturers use CCOT systems, and these do use a low-side cut-out switch. However, most (but not all) European cars use TXV systems which do not cycle during normal operation. The only thing that cycles a TXV system is the evaporator temperature sensor.

Do you know if you have a compressor speed sensor? If you don't, the easiest way to find out is to find the cable going to the compressor. One wire means no, three wires means yes.

If you do, look for anything that may cause slipping. Oil on the belt or clutch would do it, and a worn clutch might. Water on the belt too, but that would only occur when it rains.

There is an Electromagnetic RPM speed sensor on the compressor. The belt was just put on yesterday?? But you guys are sure trying to latch onto belt slippage. Is there a possibility the sensor is bad? Also the clutch engages 100% on a restart.

Yes, your car indeed has the speed sensor. You did not say how many miles are on the car, and you choose not to fill out your profile so I don't know if you are in a climate that heavily uses your a/c system.

IF you have lots of miles on the car in a hot climate, and the belt tensioner is holding the belt snug, then you are looking at one of two or a combination of the two issues:

Either the clutch on the compressor is worn with too much air gap and/or the compressor is worn allowing excessive thrust which causes the speed sensor not to read as it should at all times.

If the belt tensioner is in good shape and properly adjusted, then a replacement compressor is probably the best cure.

With the engine off clean the compressor clutch with carb cleaner. Allow to dry. Test A/C system. My 91 300TE had a slight leak at the front compressor seal which deposited oil on the clutch and caused the the clutch to slip just enough for the rpm sensor to detect and disable the system until the next start cycle. I had to repeat the cleaning every year until I ultimately replaced the compressor before selling the car to a neighbor. Mark

Hi,
I'm in South FL. The car has 280K. I don't know if the compressor is original or not. I know the evaporator has been replaced. Possibly the compressor. I can't figure out how to tension the belt. There is supposed to be some type of pointer, but it's not apparant to me. I tightened the new belt till it had about 1/4" deflection. Seems pretty tight to me. The new belt didn't change anything. Is there a way to bypass the speed sensor to see if that's it? I haven't timed the cycle, but it seems regular as clockwork from start to stop. That seems like it would be a good clue for anyone who understands these systems. I think if I could time it with a watch and it would be very close each time. I'd obviously like for it not to be the compressor. I certainly would hate to replace it and it not be it!

You really need the FSM wiring diagram to understand the system, but I'll do my best, leaving out much of the description since the system IS turning on the compressor to start with.

The Climate Control Unit (CCU) monitors about a half dozen temperature sensors and sends a signal THROUGH the low pressure switch to the Klima (Deutsche for climate) relay behind the battery. The Klima relay monitors the speed sensor and the signal that comes through the low pressure switch and decides when to energize the contacts which engage the compressor clutch.

The monitoring of the RPM sensor is done as magic inside the Klima relay. One can bypass the speed sensoring function by using a standard ice cube relay in place of the Klima relay, but it takes some innovative electric fabrication work due to it's location. I have never done it, but you could gut the Klima relay and use it as a mount for the ice cube relay. That way you could, on the bench, do the soldering in place of the relay or a relay socket onto the Klima relay base, then plug it into its original location.

Before doing all this as a shot in the dark, make sure that you are getting a signal from the low pressure switch to the Klima relay and still have that signal after the clutch has kicked out. This is where you need the FSM wiring diagram to know the pinout for such testing, and of course how to wire up the ice cube.

Hope this mess helps.

Actually it does help, some. I have Alldata and the Factory manuals I believe. The main thing is that they are for mechanics, real mechanics, so they assume you know a lot. AC stuff is new to me because you just don't fool with it a lot. I'm thinking that a lot of the wiring harness troubleshooting is already done because the Compressor starts everytime you start the engine.

There is an ohms spec for that RPM speed sensor, but I guess what you're saying is it's not the sensor because it starts up each time?

I just checked the compressor price and it's nowhere near as expensive as i thought it would be. I've been fooling with Porsches too mIt sounds like uch!

Listen to Larry - he has the right advice here. Before messing with the expensive, complicated stuff, check the simple.

First of all, you root cause very much appears to be that you are triggering the compressor clutch cutout. The speed sensor on the compressor is used to compare the compressor RPM to the engine RPM. Too much difference for too long, and the compressor is disabled until the car is restarted. If you can get the compressor going immediately by shutting off and restarting the engine, this is almost certainly your root cause.

Next, of course, is the question of why the cutout is tripping.

Start with the easy stuff. Clean the clutch surfaces. A little bit of slipping when the clutch engages is sufficient to trigger the cutout. Try giving the clutch a little more bite by cleaning off any residual oil/grease/gunk. Just spray your favorite degreaser into the air gap (engine off, please). Simple green, brake cleaner, Mr. Bubble - whatever. Wait five minutes, rinse a little bit with water, give it a try.

Second, check the air gap between the front clutch plate and the driven pulley. You're an old Porsche guy? Good - you must have a set of feeler guages laying around. As the clutch wears, the air gap increases, and the opportunity for slippage increases. The gap is specified at ~0.5mm. My anecdotal observation is that once it increases to ~0.8mm, clutch engagement will become iffy. If the gap is too large, you can purchase a kit to reset it from a good auto a/c specialty supplier.

Let us know if the clutch gap is too large and we can advise on procedure to reset. It's $5, 1 hour, and no need to disassemble & discharge/recharge the system.

- JimY

Thanx! I degunked this pig yesterday. I'll concentrate on the clutch and measure the gap. This thing cuts out regular as clockwork. I never runs more than a few minutes. But conversely, it always runs for a while. I can always drive about 1/2 mile before it stops. It does it at idle or driving in about the same time interval. What if the cutout time was the exact same number of seconds each time? because that's what it seems like to me. How would that factor in?

There's nothing related to clutch cycling in the climate control system that is time based. Generally what happens is that clutch cyling is triggered normally and the compressor triggers the cutout function when it is switched on again. There is more resistance to starting the compressor when the high side pressure is up than when the car has been sitting.

Does the interior fan slow down as the cabin approaches the set temperature, then it cuts out? Or does it cutout while blowing gale force? The former would be expected and point toward the clutch slipping. The latter case is more inidicative of a control problem where the compressor is intentionally turned off due to a fault elsewhere.

In the last year or two I recall a discussion about a very difficult to solve cutout on an older 124. It was similar to your situation in that it was a very consistent time from when the car was started until the a/c shut off. None of the usual suspects were to blame. If memory serves (iffy proposition with me...) it ended up being the evaporator temperature sensor. After a few minute of cooling the sensor went non-linear, causing the CC head unit to think the evaporator was freezing up. The a/c would not come on until the car had sat for a good long time - then it would do the exact same thing.

My problem is: start the car and it runs the same amount of time each start. I can drive 200 miles and it will not come back on till the car is restarted and then it will run the same few minutes and stop. Very repeatable pattern. That's why I was wondering about the speed sensor. Why is it the same time length each start to stop cycle. It also restarts every time. Everything is like clockwork.

I expect that the time to trip is related to the time to build high side pressure.

Hmm, ok, thanx!

Les,

I went back and reread Jim's last post where he mentioned the sensor. It's not too difficult to pull the panel on the CCU, remove it and then check the temp sensors with an ohmmeter. If you have Alldata, I would expect that you could get a pinout and resistance values from there without too much trouble.

Several years back, I made a test box for the X/11 diagnostic connector near the battery. It is SUPPOSED to trap any errors with the temp sensors and blink a code. The test box consists of a switch and an LED. Somewhere deep in the anals of this site is a schematic for building it. You don't really need to build it if you have a jumper wire and an LED with some leads you can accomplish the same thing. As I recall the poster that posted the schematic was named sort of a common name like Jim Smith or something.

Do a search in the Tech Help Forum for maybe test box, or X/11.

All that said, I am suispecting a bad temp sensor in my 300E right now in spite of the fact that the diagnostics show no codes set. I'm looking for a little bit of time to check the sensors with an ohmmeter.

Good luck,

FYI, here is the thread I referred to: http://www.peachparts.com/shopforum/tech-help/190653-long-covoluted-intermittant-c-problems-88-300te-post1531158.html

Unfortunately, there is no follow up with the final resolution. However, there is lots of good discussion on diagnostics and possible root causes.

Thanx for the link! I'll pour over that one. I couldn't find the one about the LED and jumper wire to get trouble codes. For the long haul with the car, that would be interesting I'm sure. I'm still looking for it.

Search for a post entitled "Getting Troublecodes on 300E." In the second post of that thread there is a link labeled "this page" This shows how to build the test set and may have more information.

I didn't take enough time to see if all the codes are in the write up. If not and you do get a trouble code, post it here. I have the CD and will try to find time and remember to look up the code for you.

Try this link http://home.comcast.net/~chwilka/Error_Code%20_reader_reset_tool.htm

I went back and read that article a little. It focuses on the CIS trouble codes, but use the same procedure, except CCU codes are pin 7.

Also, that shows using only a diode and doesn't show the cathode and anode. When I did it, I made a box that had the switch and the diode and it also has a lead that picks up B+ from the battery. I expect you should be able to make the one in the article work once you determine the polarity for the LED.

Hope this helps,
Larry

Thanx Larry, I've got some LEDs laying round.

I read that thread, Holy Smokes!

Just to make sure we're using the same info, I just did some measurements

I did 5 Start - Restart Cycles and timed everything. Times are + or - maybe 5-10 secs. I first started the timer when I started the engine and then figured out that the compressor starts 5 secs later. But you'll get the general drift.

Ambient temp 88F
Resting Pressures Hi and Low 101#
1st start Engine Cold for 24 hours, all restarts are immediate

1st Start Low= 50 High=225 Compressor Cutoff=110secs No FAN
2nd Start Low=58 High=220 Compressor Cutoff=115secs Fan on
3rd Start Low=55 High=225 Compressor Cutoff=115secs Fan on
4th Start Low=58 High=225 Compressor Cutoff=110secs Fan on
5th Start Low=52 High=210 Compressor Cutoff=115secs Fan 0n

The aux fan usually triggered on at 45-60 seconds.

The compressor sounds a little noisy for the first 45 seconds after it starts then quiets down.

I can measure the air gap, just need to figure out where to take the measurement. I had cleaned the hell out of the motor to get it better to work on so the compressor is pretty clean. I can clean it some more though. The belt tensioner I can't really figure out how to get tensioned properly since I don't really see a pointer. I also don't notice the marks moving when you tighten the tensioner, even though the belt gets tighter.

If the belt is loose, it will feel loose. If it doesn't feel loose, meaning there is no movement of the tensioner arm when you pull on the belt, then consider the belt tight enough and move on.

Your pressure readings are virtually the same every time it trips out.

It would be interesting to have a voltmeter on the low pressure cutout switch and see if the voltage there trips when the clutch quits or if the voltage remains when the clutch trips. This would tell you if the CCU is giving up its signal or if the Klima relay is tripping. If the voltage remains after clutch cutout, then the CCU is still asking for compressor and the Klima relay or circuit is the culprit. If that is the case, an ice cube relay in place of the Klima should fix you up.

If OTOH the voltage at the low pressure switch goes away at clutch cutout, then the problem is in the CCU system, very possibly a temp sensor or temp sensor circuit.

Hmmm, I can pull the belt up and the tensioner moves some.

This is one of those things that's difficult to describe. If the tensioner is in good shape and properly adjusted you might get a very slight amount of movement. The key is feeling tension on the belt such that the belt is wrapped with full contact around all pulleys. If there is not enough tension such that the belt is not pulled down in full contact with the pulleys it is not correct.

Hope this helps

Ice Cube Relay
 

Hello,
I've looked and there's a lot of references to the Ice Cube relay to bypass the klima, but I can't find the specs for how to do it. Anybody have a PDF or anything that describes what it is and how to do it? I did a search on Ebay, and there's a ton of things called Ice Cube relays!

Thanx!

p.s. still workin' the problem here. It ain't grease though.