Mercedes-Benz Forum banner

1 - 20 of 25 Posts

·
Registered
Joined
·
124 Posts
Discussion Starter #1
1984 300D. Picked up with just over 70K miles! All systems worked beautifully when I purchased it, however the AC/heater started experiencing a "delay" when turned on. You would select a mode & nothing would happen for a few minutes, then it will kick on.

This "delay" became longer & longer, and now it seems to no longer work. On occasion it will turn on after driving for a very long time (an hour or more!). Usually it never turns on.

I can hear baffles or something moving inside the dash when I select a mode, but the fan never kicks on. In those rare instances when it does, the AC is usually cold, heater is warm.

Any ideas what could cause this & how to fix?
 

·
Registered
1985 300CD Turbo coupe, 2006 E320 CDI sedan
Joined
·
573 Posts
Delay when starting a cold car is normal. That's so you don't get blasted with cold air from the vents while the engine is heating up.

But what you describe sounds like the power plug on the right side of the ACC controller (push button unit in the console) is loose. It provides power and speed control to the blower motor. It is located about where a passenger left knee would rest. Try banging on that console panel. If that works, remove the panel and reseat the connector.

//greg//
 

·
Premium Member
W-1-2-3 Go!
Joined
·
16,161 Posts
Good recommendation from Greg.

In addition, see if moving the temperature wheel up and down will trigger the blower to turn on. Sometimes they get dirty and need some cleaning.

If it's too cold outside (I forgot what the specified engine temp should be so the AC blower could turn on) then yes the blower won't turn on.

Check the temp switch on the engine.

Also make sure the blower carbon brushes are good and the assembly is clean. If it doesn't come on for an hour something else might be preventing it from working properly. At 70k miles the blower should not be a problem, but worth to check nonetheless.
 

·
Registered
'89 300TE, '79 450SL, '01 ML320
Joined
·
1,051 Posts
Not a bad idea to give the fuses a spin also, lets face it these things are old, electrical connections get crusty.
While you're under the hood, check connections at blower resistor, mono valve, and auxiliary pump. The latter two wont effect the blower but hey... tis the season :)


Fish
 

·
Registered
Joined
·
124 Posts
Discussion Starter #5
Delay when starting a cold car is normal. That's so you don't get blasted with cold air from the vents while the engine is heating up.

But what you describe sounds like the power plug on the right side of the ACC controller (push button unit in the console) is loose. It provides power and speed control to the blower motor. It is located about where a passenger left knee would rest. Try banging on that console panel. If that works, remove the panel and reseat the connector.

//greg//
Pulled the pannel off ... I think I found what you're referring to:



Connections seemed very solid, but I unscrewed the connector on the left side of passenger compartment and pulled off both plugs, cleaned, & reinstalled. Also saw the plug for the fan itself (pictured) - it was seated, but I cleaned it as well:



Are these photos the things parts you were referring to?

Will do a test drive shortly, but everything was well seated so not holding my breath...
 

·
Registered
Joined
·
124 Posts
Discussion Starter #6
MBeige said:
...check the temp switch on the engine.
...check connections at blower resistor, mono valve, and auxiliary pump.
Not to advertise my ignorance, but how would I locate these things?

Gave the fuses a spin & hit them all with DeOxit spray. Some were pretty seized up, so that was a definitely a good call.
 

·
Registered
1985 300CD Turbo coupe, 2006 E320 CDI sedan
Joined
·
573 Posts
First photo doesn't look right, appears to an under the dash shot - rather than the passenger side of the ACC box (the one in the console with the buttons). Some owners refer to it as the CCU. Note the rectangular black connector plug on the blower motor. There should be a similar one - 8 pins perhaps - plugged into the passenger side of the ACC.

Cleaning those plugs is only half the job. The "lid" of the connector body pops off so you can see each individual conductor inside. That permits you to inspect for loose or disconnected wires too.

//greg//
 

·
Registered
Joined
·
124 Posts
Discussion Starter #8
First photo doesn't look right, appears to an under the dash shot - rather than the passenger side of the ACC box (the one in the console with the buttons). Some owners refer to it as the CCU. Note the rectangular black connector plug on the blower motor. There should be a similar one - 8 pins perhaps - plugged into the passenger side of the ACC.

Cleaning those plugs is only half the job. The "lid" of the connector body pops off so you can see each individual conductor inside. That permits you to inspect for loose or disconnected wires too.

//greg//
Ah ... got it:







Everything looked really solid, but I popped all the wires out & hit them with DeOxit nonetheless. Did connectors on both sides - one on the driver's side next to temp control wheel looked like it had been worked on before. Tool marks on plastic to remove connector. Wires looked good in any case.
 

·
Registered
'89 300TE, '79 450SL, '01 ML320
Joined
·
1,051 Posts
You'll find the blower resistor on the passenger fender well, looks like a metal cube with
holes in it, resistor coils are visible thru holes. This is what gives variable speeds to the blower.

Below that and a bit forward on the frame rail you will see the auxiliary pump. It has two
heater hoses and an electrical connection. This insures flow to the heater core at idle and low RPMs.

The monovalve is found on the passenger side firewall, it controls flow to the heater core
to maintain desired cabin temp.



Fish
 

·
Registered
Joined
·
124 Posts
Discussion Starter #10
test drive results...

Promising perhaps...

After pulling & cleaning all the various connectors, I turned on the heater controls, then fired the car up. Nothing turned on right away. Went for about a 15 mile drive involving sidestreets + freeway and the heater didn't activate the entire time.

Parked for about 3 hours or so for a family dinner & went back to the car. It was very cold out, and as soon as I fired the car up, the heater blower came on!

Am wondering if perhaps the reason it didn't turn on then entire drive initially is because the car is always garaged and wasn't as cold, and it was after being parked outside for three hours or so.

Am going to check out those parts Fish mentioned in the previous post. Clean all those connectors & see what happens.

Seems like all the parts are working properly (switches, blower, etc...), just that there is perhaps a sensor someplace telling the system not to actviate.
 

·
Registered
'89 300TE, '79 450SL, '01 ML320
Joined
·
1,051 Posts
just that there is perhaps a sensor someplace telling the system not to actviate.
There is, as MBeige mentioned below, it's located on the top of the thermostat housing.
I seem to remember it as a single wire, supplies ground as the temp reaches 40 degrees or so.
Just disconnect and run to ground to check, if you replace it don't use any sealant for obvious reasons :)



Fish
 

·
Registered
2013 MINI Copper S Clubman, '84 300CD-weekend car
Joined
·
10,152 Posts
The ACC unit itself is know to deteriorate as soldered points inside, this will give you flaky results. Pull it out and check for cold solders, search around here, you'll find some info on it.

The pic shows one, see the crack surrounding a solder point? Sometimes you'll make contact, sometimes not.
 

Attachments

·
Registered
Joined
·
124 Posts
Discussion Starter #13
The ACC unit itself is know to deteriorate as soldered points inside, this will give you flaky results. Pull it out and check for cold solders, search around here, you'll find some info on it.

The pic shows one, see the crack surrounding a solder point? Sometimes you'll make contact, sometimes not.
That sounds really promising ... I recall when the symptom started if I turned switches on & off the blower would activate. Will check those solder joints asap...
 

·
Registered
Joined
·
124 Posts
Discussion Starter #14
There is, as MBeige mentioned below, it's located on the top of the thermostat housing.
I seem to remember it as a single wire, supplies ground as the temp reaches 40 degrees or so.
Just disconnect and run to ground to check, if you replace it don't use any sealant for obvious reasons :)

Fish
This the one?



If so, running it to ground should cause the AC/heater unit to fire up right away correct?
 

·
Registered
'89 300TE, '79 450SL, '01 ML320
Joined
·
1,051 Posts
Yep, that's it, if everything else is ok then yes, the blower should come on.


Fish
 

·
Registered
Joined
·
124 Posts
Discussion Starter #16
So sure enough, there were some severely cracked circuits on the ACC (pics below) ... hit them with a bit of solder, buttoned everything back up, fired the car up, grounded out the engine temp switch by simply running a wire from the switch connector to the negative battery terminal, and within a minute or so, the blower came on. Not sure if it should have come on immediately or not with the switch being grounded out, however having the blower come on that quickly was a welcome symptom.

New symptom cropped up ... and I think this means all the work is headed in the right direction:

When I just barely touch the fan switch, or even push on the faceplate, there is an electrical relay clicking/clacking sound coming from being the rear window defroster switch area, above the ACC unit.

Both high & low fan modes worked once the blower fired up, but the click/clack electrical sound coming from behind that upper faceplace happened very frequently - virtually every time you touched the switch, and this has never happened before.

What is up in that area that could cause a noise like that, connected to the fan switch?




 

·
Registered
Joined
·
124 Posts
Discussion Starter #18
Sounds like your GEBLASEREGLER :)

It's the blower speed regulator, located up behind the glove box.

Fish
One more thing to hunt down & clean :)

Occurs to me, if pushing on/around the switch makes the blower speed regulator click intermittently - issue would most likely lie with a loose/intermittent connection at the switch, yes?

Fan switch seems fairly simple, so not sure how I'd address it, but seems that's the first place to go poking around...
 

·
Registered
Joined
·
124 Posts
Discussion Starter #19
Sounds like your GEBLASEREGLER :)

It's the blower speed regulator, located up behind the glove box.

Fish
Odd ... heater worked great immediately after pulling everything out & cleaning, etc... now has gone back to previous state (?) Exception is the "clicking" from what is apparently the blower speed regulator - that still happens from time to time - a new thing.

What's the trick to accessing the blower speed regulator? Somehow removing the glove box perhaps or...?
 

·
Registered
'89 300TE, '79 450SL, '01 ML320
Joined
·
1,051 Posts
My guess would be that you've still got a bad connection in the fan switch. You just need
to touch your soldering iron to the connections long enough to re-melt the solder that's there.
I may have a couple spare switches if you still have problems.
Yes, the glove box comes out after pulling a few plastic pins, maybe the two latch screws too,
it's been a while.



Fish
 
1 - 20 of 25 Posts
Top