Intermittent air conditioning

[e500.org]

Advice needed please - 1994 E500, 114,000km.

I thought my evaporator was on the way out but recently I’ve had ice cold AC and the system has now been charged for many months so I’m quite baffled…

The AC works intermittently.

Some days it’s ICE COLD, others, warm air ONLY through the vents. No obvious patterns or start/restart makes a difference. Some days it’s ice cold from start, others it goes from warm to cold after 20-30 mins of driving. Others it just stays warm.

Generally there is better performance in the evenings when it’s cooler and no direct sunlight. Evening still has an ambient temperature of 35C (95°F).

The only attention I’ve given the AC system is a full recharge of the system a good few months ago.

Where would a good place to start be? I have read the belt tensioner may have slack which doesn’t activate the AC compressor? How do I tell the AC compressor is functioning at any one time?

Basically I want to avoid the dash pull job for the evaporator so wanted to try diagnose the issue before getting into a job major like that - particularly given my AC is intermittent rather than completely dead.

Given I’ve never replaced the belt tensioner nor the AC compressor - these are obvious areas to begin. Anything else reasonably readily accessible worth looking at?

Many thanks

 

[gsxr]

Belt tension is probably not the issue. The BM/GM compares compressor speed with engine speed, if there is a discrepancy, the compressor is shut off until the engine is restarted. This would result in intermittent no A/C until the engine is turned off / turned on. Doesn't sound like that's what you are dealing with. And, fault code 6 or 7 would be stored on the BM/GM.

If the evaporator was faulty (leaking), you'd start with ice cold AC after recharging, and performance would gradually diminish over time as the system lost refrigerant. Your description doesn't match this either.

My guess is either there's a control issue (HVAC control unit, or faulty temp sensors), or something affecting refrigerant flow (TX valve?). Less likely is something up with the vacuum pods that control the main / fresh air flap (assuming you have ACC, not manual). When the main air flap (recirculation) pods stop working, they won't block outside hot air as they are supposed to. I don't know how the manual systems work, or if they have the same control for the main air flap.

 

[gerryvz]

You can visually check if the compressor is running by observing whether the clutch on the front of the compressor is engaged or not. You can see this looking down onto the driver's side of the engine. If the clutch is not engaging when the A/C pushbutton is activated at the dashboard, then obviously the compressor will not be turning.

Connect a set of gauges to the system (high side and low side) to see what the system pressure is. That will tell you whether you need refrigerant or not. If your pressure is low, then a recharge should "reactivate" the compressor....and you will confirm that you have a leak somewhere in the system.

Check the three-prong electrical connector on the top of the compressor to make sure you have power to the compressor. You can even wiggle the wires with your fingers to see if moving the wires creates an electrical connection that engages the clutch and starts the compressor.

There is some information here:

HOW-TO: Replacing 500E/E500 A/C Compressor | "HOW-TO" Tutorial Articles

This HOW-TO is focused on replacing the A/C compressor in the V-8-based W124 Mercedes-Benz models. Since the compressor and key components of the A/C system are very similar on other W124 models, most of this HOW-TO should also apply to the inline-six models. The Nippon Denso compressor as...

www.500eboard.co

 

[emerydc8]

This would be further down the suspect list of what the @gsxr and the @gerryvz have already mentioned, but if you have an extra base module, you can try swapping it out. A bad base module has been known to cause intermittent compressor engagement.

Also, you can check the strength of the low-speed fans by grabbing the hub with leather gloves to see if either is weak. Compare the left with the right. I had a bad fan on the drivers side once and that was causing the pressure to increase to the point of electronic cutout. Too bad I went through two tensioners, a belt, new expansion valve and PBCU before I figured it out.

 

[e500.org]

Picking back up on this.

My AC compressor works intermittently, comes on when it feels like it. When it’s on, AC is ice cold.

This is even after a regas (car had not lost much gas over time however it’s a concern overall for any loss in a short time as the system should be sealed, perhaps the evaporator may be leaking).

So I need to deal with the triggers related to the AC compressor not coming on when it should.

Today I had a look at the drier, sensors and connectors. Everything visually appears ok, after reseating the wires from the sensors and spinning the AC pulley by hand, switching on the car and the compressor ‘woke up’ and I had AC again.

Parked for the day, switch the car on and the compressor isn’t running.

I think the first fix should probably be the sensors and the drier given their age. As there is no OEM drier available so I may try the sensors first and clean up the contacts and go from there.

It appears I need pressure switch A1248208310 (GBP 118, eBay) and climate pressure switch A0048206810 (GBP 85, eBay).

DENSO AC compressor appears available at GBP 400 also on eBay, I’ll try and find one locally in Dubai.

I do not have a spare BM otherwise I would have swapped that out to test if it makes any difference to activating the compressor when I need it to.

Any leads on alternatives for the drier? Aftermarket from BEHR/etc appear to be for non V8 124s.

 

[gsxr]

If you have not yet swapped out the BM/GM... do that first, before any other part replacement! Good used ones are inexpensive and easy to locate.

Do not replace the drier unless there's a specific reason to believe the old one is contaminated and cannot be left in service. There are no good aftermarket replacements, all you can get are the 6-cyl units which don't fit properly (search the forum for details & photos). The aftermarket units are of questionable quality as well now that both OE/Genuine and OEM Hansa are NLA.

Wouldn't hurt to trace the wiring and make sure all connections at the switches and connectors are clean, tight, and not contaminated with oil.

As an absolute last resort I'd try replacing the compressor (Denso only) and the 2 switches at the drier (OE only).

I'm suspicious something is flaky with the BM/GM, which is the "brain" which controls the compressor engagement.

 

[gerryvz]

Check the condition of the three-prong plug that plugs into the top of the compressor, to make sure those wires have continuity.

Did you check refrigerant pressure with gauges, to make sure there is enough refrigerant to trigger the compressor?

 

[e500.org]

Update on this - I do not wish to jinx it but I removed the BM and reinstalled it - since then, the AC compressor has activated without fault every single time! So thank you for pointing me to the BM!

I’ve not driven the car much and the weather is now getting warmer so the car will really be tested over the next month or so, however the problem appears to be resolved (with the jury still out!!)

 

[nocfn]

Often that is as easy as removing the fuses and reinserting them randomly