Flashing Recirc Light - PeachParts Mercedes-Benz Forum

sjh · #1 02-01-2011, 01:00 PM

1990 300D/w124 2.5L

I have the standard 5 button (horizontal) climate control unit (CCU). Just above the 5 button CCU are another set of switches including the 5 minute recirc button. Normally depressing the recirc button while the HVAC system is running causes the recirc pod to engage and closes the recirc vent by 80%. This action continues for about 5 minutes and then automatically turns off. While the recirc pod is engaged the red LED is illuminated on the recirc switch. Until last night this is the only time ai have seen the LED illuminated on the switch.

Last night I hopped in the cold car and turned on the ignition and started the car. The red LED on the recirc switch was flashing on and off about once a second or so. The HVAC system did not appear to work. I turned off the car and turned it back on. The flashing LED went away and the CCU worked normally.

Has anyone seen a flashing recirc light and/or does anyone know what it means? Thanks.

compu_85 · #2 02-01-2011, 01:36 PM

A flashing resirc light means the system detected a short and went into safe mode.

Good first place to start? The aux water pump.

-J

sjh · #3 02-01-2011, 03:06 PM

Thanks. Can you tell me where that is located?

Jeremy5848 · #4 02-01-2011, 04:41 PM

The "circulating pump" is located in the engine compartment, under the windshield washer bottle.

AFAIK, the flashing light warning is only in post-1989 models. My 1989 ACC service manual says only that a short circuit will cause the circuit with the short to be turned off (Kammerer-made PBUs) or all circuits except the blower (Bosch-made units). There is no mention of the recirc switch's light flashing.

There are apparently a number of differences in the PBUs during the years of the W124, more than I realized. Perhaps we could put together a list.

Jeremy

sjh · #5 02-01-2011, 05:04 PM

Thanks as well Jeremy. I assume PBU = "push button unit".

I only had the error occur once. I turned the car off and back on and the problem was gone.

When the recirc light was flashing NONE of the ventilation controls, including the fan, worked.

I'm not sure where to find the PBU identifier unless it is written on the unit itself.

The w124 diesels, which I think were a turbo and non-turbo 6 cyl (3.0L), a turbo 5 cyl (2.5L) and a non-turbo 3L with 4 valves had a shorter production run than the w123 but appear to have had more changes. It probably would be helpful to collect and organize the w124 info. I don't have the time but if someone else wishes to do so I will help as I can.

fullcircle · #6 02-01-2011, 05:53 PM

I've had the same issue for about two weeks nowin my '87: no climate control operation, preceded by intermittently flashing recirc light, occurring only when the car is cold. I assumed I had a short in the PBCU (which I have resoldered once). Two things seem to work for me, turn off the system ("O" button) until the engine temp is registering at least 40 degrees, then turn it on and it works normally. I also put a small electric heater in the car once for awhile, and the system worked normally on start-up. To my simple way of thinking this reinforced the cause as a short....when cold, the short is caused from contraction. A little heat causes enough expansion to close the gap and allow normal operation. I may be wrong. Hopefully, this will get me through until I have the opportunity to resolder the unit once again.

Jeremy5848 · #7 02-01-2011, 06:39 PM

The PBU has a maker's sticker; you have to remove the unit to see it. All of the PBUs I've ever seen have been Bosch. I wonder if Kammerer units are used in Euro versions only?

babymog · #8 02-01-2011, 08:41 PM

Actually very easy to tell: The Bosch unit has black cloth tape over the light-bulb holes, the Kammerer one has aluminum tape. If you remove the wood panel the tape will be evident.

Also, the Bosch units were recalled for bad solder in the late '80s on 124s.

I've seen several of the Kammerer units, but it seems that all of the ones I currently have, ... are Bosch.

Finally, my asessment of the works when it's warm? If the aux-pump were drawing too much current cold due to gunk built up in its bearings etc., warm water passing through it might soften it enough to run at normal speeds and not draw "locked-rotor" current. A sign that your aux pump might be on its way out. The easy test is to unplug the pump.

bustedbenz · #9 02-01-2011, 09:11 PM

Mine does this every single time it's colder than 30 degrees or so ambient. Start the car, it's silent for a few minutes and then the recirc light flashes on and off many times, you hear relays clicking in and out about every second or so, and no controls of any type will respond.

Let it warm up 10 minutes, and then cycle the ignition key, and usually it works normally afterwards.

Has never been explained to my satisfaction *AND* putting in a known good push button unit did NOT make the problem go away.

sjh · #10 02-01-2011, 11:30 PM

Quote:

Originally Posted by babymog

A sign that your aux pump might be on its way out. The easy test is to unplug the pump.

If I unplug it will the system still work?

I appreciate all of the comments.

jcyuhn · #11 02-02-2011, 09:28 AM

Yes, it will work fine. You just won't get quite as much warm air when the engine is idling.

Jeremy5848 · #12 02-02-2011, 11:05 AM

Quote:

Originally Posted by Dr_SJH

If I unplug it will the system still work?

I appreciate all of the comments.

The reason Mercedes added the pump into the system was that, in cold climates people complained that there wasn't enough heat at slow speeds (parked or driving in slow city traffic).

The "circulating pump . . . operates in heating mode, when the mono valve . . ." is open to allow hot coolant into the heater core. [Source: Service Manual, Automatic Climate Control, Model 124, 1989.]

People living in warmer climates could get along just fine without the aux pump. Some have even replaced it with a piece of pipe.