MB airconditioning

Amboman

New member
The MB front air conditioner seems to work cool OK for 3 minutes then it goes
to just fresh air, no chill factor... any ideas would be welcomed :thinking:
 

Bob of QF

New member
Could be a variety of failures, as the MB has an actual thermostat, I'm led to believe-- behind the little grille on the dash?

Anyway, as above, low refrigerant, an overheating compressor clutch, a slipping compressor clutch or a failure of your thermostat.

The most likely? Low refrigerant. It's easy enough to check that-- locate the large pipe (versus the small one). Look for a service valve-- usually there's a plastic cover on it. You'll need a refrigerant guage, but there's quick-and-dirty kits which include exactly that.

Once you start it, the guage should show positive pressure when it's running (cooling)-- nevermind the value, here-- automobile charges are really based on weight, and there's a sticker underneath the hood telling the total charge.

But for a quick-and-dirty fix? You can use the "it gets cool" method... :)

However, if the low side pressure is less than zero (or even negative)? There is a switch which will defeat the clutch from running-- protecting it.

There are two ways to proceed, if that's the case (i.e. when running, after the cooling stops, and you note that the clutch is not engaged (spinning) even though you've set the temp as cold as it can be, and you've engaged the compressor switch on the dash).

You can do it the "offical" way, of course: remove and recover any remaining refrigerant. Pump a good vacuum for 1/2 an hour. Then weigh in the correct (sticker) charge into the system-- you can do this entire process without the engine running at all, except perhaps for the last few ounces of charge. To recover the refrigerant, you'll need a recovery tank and a recovery pump-- neither is cheap, but you might be able to rent the pump unit. Of course, you could discover that all the refrigerant had "accidentally" leaked out... but I cannot in good conscience suggest that officially. You can rent a vaccum pump--different from the recovery one. You'll also need a digital/accurate scale to weigh in the charge. And finally? You'll need to rent or borrow actual hoses and guages calibrated for automotive use. To use the scale, I find it's easier to zero it with the bottle on it first, then weigh in the "negative" value to the correct spec, that way you can ignore the weight of the bottle.

Or, there's the home-brew "good old boy" method, useful if you're trying to get by before a real repair, or trying to determine where the leak is-- you purchase a "recharge kit" at your local auto parts--- get one with some sort of guage on it. You'll need an approproate can, 134a of refregeriant. I suggest getting one with dye in it, to look for leaks.

Anyway, if your safety-switch is disengaging after 3 minutes? You can proceed one of two ways: try to get enough 134a from the little can, during those 3 minutes, or cheat the safety-switch with a jumper, fooling the system into thinking it's charged.

Either way is okay, just remember to remove your jumper and re-enable the safety switch--it is there to prevent you from destroying a compressor from low or no 134a.

How much to put in using this method? Easy: until it blows cold. I know, I know, that's hardly optimum, and certainly not official. But it works-- on your little guage, you need positive pressure-- i.e. above zero, but not too high in this case.

One good rule of thumb is this: the low pipe (large) will begin sweating in warm weather about the time your charge is good enough-- and your guage will show roughly 10psi to 20psi or so-- at idle. This obviously varies with engine RPM. But the real test? A thermometer-- a typical A/C unit ought to be able to go 10-20 degrees differential temperature at the grilles, on high, at idle. That is, from the intake temperature (outside, if you're not recurcurlating air) to the output grille on the dash, should show a drop of 10 to 20 degrees F. Again, this is just a quick-and-dirty measure.

Another "good old boy" rule of thumb, is the safety switch will close when there's enough 134a in there, and open when there isn't. So putting in just enough to engage the lock-out/safety switch -- and a wee bit more-- can also work good enough.

Once you get it to stay on? Run it for a few days of normal use, then .... look for your leak.

A leak is only going to get worse-- and if you blow a hose suddenly? You can take out the compressor... cha-ching! $$$

Good luck!

Edit: I see I did not mention to check which refrigerant your van uses-- most use 134a these days, but an older one might still need R12-- use an aftermarket replacement in that case. To convert from R12 to 134a? You must remove all the old R12, pull a vacuum, and weigh in the new 134a-- you cannot mix the two. But there are "R12 friendly" replacement gasses out there- "hotshot" is one such. I doubt you have the older stuff, though.

Edit: I see I did not address the electrical issues at all-- these are more complex, and since yours runs for 3 minutes then quits? It's really likely low refrig., as the system cools down, it lowers the overall pressure, which is dropping below the "off" threshold of your safety/limit switch. The only way to fix a slipping clutch, is to replace it-- you cannot rebuild these AFAIK. And I've had poor success at clutch replacement-- it's just as easy to put back a new/rebuilt compressor as it is your old one with a new clutch-- I always trade mine in for a rebuilt, in that case.

Edit: once you locate your leak? Replacement of the offending part is the only real fix, here-- hopefully it's just a fitting that needs new O-rings, and not the compressor-- or worse, not the condenser (in front of the radiator) or the evaporator (buried in the dash). Those require hours to disassemble enough other parts, just to get at them... not fun.

Replacing hoses is relatively simple, as is the compressor--but you'll need that vacuum pump, and a scale, and hoses.

Good luck!
 
Last edited:

hkpierce

'02 140 Hi BlueBlk Pass
Edit: I see I did not mention to check which refrigerant your van uses-- most use 134a these days, but an older one might still need R12-- use an aftermarket replacement in that case. To convert from R12 to 134a? You must remove all the old R12, pull a vacuum, and weigh in the new 134a-- you cannot mix the two. But there are "R12 friendly" replacement gasses out there- "hotshot" is one such. I doubt you have the older stuff, though.
134a for 2002+ Sprinters.
 

Top Bottom