'78 w123 (280ce) Climate Control turns off under load - PeachParts Mercedes-Benz Forum

Ten13 · #1 07-27-2019, 02:05 PM

Hey All, I hope this finds you well.

I've had a long-standing problem with the climate control system in my 1978 USA-spec 280ce. A previous owner upgraded the compressor and it's charged with 134a, and has the 1st gen servo upgrade kit from Unwired Tools. Everything actually works, including the A/C, however inadequate when it's super hot out.

The problem is this: the blower fan turns off (regardless of which mode, EXCEPT for defrost) when the engine is under load. Accelerating from stop (hot or cold engine), or climbing a grade (minor or not) causes the fan to just stop, until the load is removed/greatly reduced. The A/C compressor cycles even in this state. All of the flaps within the system function as-normal. It's just the blower motor!

The vacuum source for the Unwired servo module is off the intake manifold, so this to me seems like a vacuum issue, ultimately.

Does anyone have any sage advice on where I can next analyze? The vacuum lines for the servo are fine, and I have even isolated them to rule out a leak in the door lock circuit and front seat lockout mechanisms.

Thanks in advance, I welcome any thoughts!

Greg

BWhitmore · #2 07-27-2019, 02:29 PM

Is your engine producing adequate vacuum? 16-21 inches of vacuum is normal for a healthy engine.

Ten13 · #3 07-27-2019, 03:21 PM

Quote:

Originally Posted by BWhitmore

Is your engine producing adequate vacuum? 16-21 inches of vacuum is normal for a healthy engine.

hey there, thanks for the reply.

I just took at reading from the intake manifold (top, closest to firewall) where the vacuum lines attach for locks/seats/climate-control.

At idle: 10-11
At 3000 RPM: 17-18

Drawdown when throttle is quickly applied (not under load) is 5-7, but quickly goes to 17-18.

I'm sure, under load, it's in the 5 inches range. Need to get a longer tube to bring vacuum gauge into cabin during a test drive. Is this too low for under-load? Seems a possibility is that the drawdown is below the threshold for the CC system to function properly.

BWhitmore · #4 07-27-2019, 03:32 PM

The vacuum should be the highest (16-21 inches) at idle. As load is applied the vacuum level will go down. You may want to check for a vacuum leak. A good test is to spray WD40 around your vacuum fittings and the intake manifold. If the engine speeds up while spraying the WD40 at a particular location you have found your vacuum leak. There may be more than one leak.

Mike D · #5 07-27-2019, 05:52 PM

A Mityvac is your friend and best ally when it comes to trying to troubleshoot the ACC system.

You can use the tool to supplant the supplied vacuum to determine if it is adequate to activate the system.

10"HG is low at idle. The M110's are known for low vacuum at idle, a product of the combination of the hemispherical combustion chamber and the DOHC, but you should still be at a minimum of 13"-15" HG at idle.

Idle · #6 08-05-2019, 02:45 PM

As weird as this sounds some of the A/C systems from that time were designed to shut off during a load. The theory was if you needed power the compressor would be using about eight to ten HP, so it cut out to let you use the power.

This would kill your entire system which may be why your fan is cutting out.

This usually took the form of a 'magic box' that would be wired into your AC and your ignition. It acted as a tach that senses the RPM jumping up quickly and would switch off your AC.

I know this sounds far out, but I have seen such systems and wondered if there were a way around them. They can usually be cut out.

These were popular on small engines from Japan, but if the system was built that way and the installer was just following the instructions it could have been hooked up.

So just check your wiring from your compressor to it's power source. If a small box shows up in the middle and the function of this box is unknown perhaps you have one.

Also... Many of these had a small adjustment on them that would allow you to set at what point the ac cut out. Perhaps this could be dialed to zero?

97 SL320 · #7 08-06-2019, 06:09 AM

An AC compressor cut out at full throttle is possible however, I think you have a vacuum leak in the system and the air doors default to defrost as do most vacuum operated systems on all brands.

There should be a check valve between the engine and HVAC system and many times a vacuum reserve chamber. The other issue could be a leak in the HVAC system that allows vac to bleed off. ( Not the freon system )

Idle · #8 08-06-2019, 01:44 PM

Quote:

Originally Posted by 97 SL320

An AC compressor cut out at full throttle is possible however, I think you have a vacuum leak in the system and the air doors default to defrost as do most vacuum operated systems on all brands.

There should be a check valve between the engine and HVAC system and many times a vacuum reserve chamber. The other issue could be a leak in the HVAC system that allows vac to bleed off. ( Not the freon system )

If you have ever had to replace a fender you would find under it a vac chamber that is made up of what looks like a bunch of balls glued together. These can crack and drive you nuts when trying to find a vac leak.

Look for vac lines running from the engine area to the inside of a front fender to test the chamber for a leak.

Ten13 · #9 08-24-2019, 05:43 PM

Hey there, and thanks to everyone who offered suggestions on getting my blower fan to work while the engine was under load.

It turned out to be a combination of things, including an extra check-valve under the dash which was creating a false reading from within the engine compartment when testing for leaks. Turns out to be something related to the doorjamb vacuum switch on the driver's side door that was causing a loss of vacuum, bringing the whole system down with it.

I bought an electric aux vacuum pump, so I could diagnose the problem by ear.

As it's a coupe, and it's not a big deal to me that the locks don't work, I just disconnected that circuit, and everything works well.

I still need to find the lower-than-ideal vacuum condition at the manifold, but that's another issue altogether. For now, my AC works in the hot California summer.

All the best! You guys pointed me in the right direction.

Mike D · #10 08-24-2019, 08:15 PM

On the coupe there is a vacuum line which runs underneath the seats. This activates diaphragms which lock the seats from folding forward when the engine is running. Another "genius" idea from the Daimler-Benz engineering team. A simple mechanical spring lock would have served the same purpose but nooo, "'Let's make it needlessly complicated" seems to have been their thought process.

On the '78 W123 this is a blue tubing which runs through a grommet in the firewall, down each of the side panels and connected beneath the seats.

Quite often this has been knocked loose by someone cramming something under the front seats or an over vigorous application of a vacuum cleaner wand.

Easy enough to check. Disconnect the line at the check valve "Y". Use the Mityvac to check for leaks and holding of vacuum.

97 SL320 · #11 08-24-2019, 08:58 PM

Quote:

Originally Posted by Mike D

On the coupe there is a vacuum line which runs underneath the seats. This activates diaphragms which lock the seats from folding forward when the engine is running. Another "genius" idea from the Daimler-Benz engineering team. A simple mechanical spring lock would have served the same purpose but nooo, "'Let's make it needlessly complicated" seems to have been their thought process.

But then owners would have to move a lever to release the seat. Remember, this was in an era where seat backs on older cars didn't latch. Owners would have complained that their older car didn't have a latch and why should a " better car " have one?.