85 300D Heater problem - PeachParts Mercedes-Benz Forum

JMH · #1 11-03-2003, 06:51 AM

I'm helping a friend with his 300D that the heater will not work at all. We replaced the monovalve with the kit yesterday, but unfortunately, that is not the problem. He is not getting electrical power to the monovalve. I cannot find a schematic for the wiring on that but suspect that the ACC unit is the issue. We had already replaced his Relay on the left fenderwell (about $270!) so that his a/c would work....all of that is working fine..... just cannot get heat. The electric pump for the heater coolant to the heater core works fine....... it is pumping.

If that is the issue and we need to repair the ACC, is that something that can be done by looking for shorts/solder breaks and repair it without buying a rebuild/new unit?

Is the problem with this typically a problem with the temprature wheel or the pushbutton portion of the unit? The fan works fine, the a/c works fine, etc.

Need some direction! My 84 300D has not had that type of issue so I am a neophyte to this type of problem.

We're stumped so far,
JMH

engatwork · #2 11-03-2003, 07:12 AM

Is air blowing out of any of the vents? The monovalve closes with 12 volts to it and opens when voltage is 0 volts if I remember correctly. Is there anyway you can test the voltage at various settings of the temperature thumbwheel?

Ridge · #3 11-03-2003, 09:28 AM

JMH,

You don't say whether the fan is blowing but I'm going to assume that it isn't. I've had this problem on several of my cars and it's just that over the years the brushes in the blower motor have worn down.

If you dissconnect the blower fan connection up under the passenger side footwell, and check that electricty is getting there, I'll be it is and if the fan doesn't work, it's the brushes.

The later model fan motors have brushes that are easy to get at so they can be replaced. www.**************. com has both the brushes and the instruction manual on how to replace them.

I have rebuilt the sealed motors, too but that's another story, hope this helps.

Ben

JMH · #4 11-03-2003, 05:54 PM

Thanks for the replies.

I will restate the symptoms for clarity.

The temperature dial on the ACC moves smoothly and the Air conditioning works flawlessly... nice and cool. The blower motor works fine in all settings, it is just that the heater does not provide heat when the dial is moved to the red "heat" temperatures. When we tested the power getting to the monovalve, there was no electricity getting to the wire pigtail/plug that goes onto the top of the monovalve. When we traced the wiring back through the firewall, it merges with a giant wire harness that snakes underneath and through the the dash to the ACC unit. We did redo the monovalve with the kit. Tested it with a gator clip wire set... it pushed out with power, retracted without power.

We did notice a white plastic ball in the bottom of the monovalve that looks to act as a checkvalve of sorts. I am wondering if that is really supposed to be in there or if the previous owner put it in to stop the warm water and thus plug the heater off. Anyone know about that?

Still, there is no power to the monovalve. Gotta solve that.

Anyone have a wiring schematic of the 85 model 300D Federal?

Thanks,
JMH

engatwork · #5 11-03-2003, 08:28 PM

Can't say I have ever looked down in there for a ball

.

Sounds like you may have an issue with your pushbutton unit.

JMH · #6 11-03-2003, 09:10 PM

Engatwork,
Thanks so much for the willingness and ability to help!

The monovalve tube has a white plastic ball in the lower part of the unit. It looks like it is supposed to be in there (maybe) but this beloved ol' Benz has suffered a bit of abuse, although the engine runs good, transmission the same, etc.... the air conditioning suffered the indignity of having been "hotwired" to bypass a blown relay switch that sits on the firewall (about a $275 item to replace).... after purchasing this minimally rusted, clean engine classic, I noticed the clutch running all the time on the compressor and that could only mean one thing: bypass of the temp control panel..... sure enough, I found a splice; at least they had the good sense to put an inline fuse in there!!! The heater issue is the last of the PO (previous owner) foulups we have to correct. I don't mean to sound as if though I begrudge the PO; for all I know they might have been economically disadvantaged and were merely seeking relief from the oppressive heat of these Florida summers, although if they had just refrained from the habit progenerated by Sir Walter Raleigh, they might've been able to purchase that relay in a shorter period of time!!

Again, thanks for the help and if I could just question one part of your reply. When you said that it may be a problem with the pushbutton unit, was that to say the temperature/thermostat wheel or the actual pushbuttons vs. the unit in general??

I am unsure of how the power flows thru the unit so I want to have a really good idea of what to pursue here.

thanks again,
John

psfred · #7 11-03-2003, 09:53 PM

You have no flow to the heater core, most likely from the white ball in the monovalve (if I remember correctly, all that should be in there is the valve itself).

Check the monovalve for a deteriorated rubber bellows -- if it is badly wrinkled or torn, it will leak past and cause the heat to be on all the time.

If you have no voltage to the monovalve, the pushbutton control unit is calling for heat and opening the monovalve. It stays open unless closed by 12v from the PBCU.

Peter

JMH · #8 11-09-2003, 07:14 PM

Well today we solved the puzzle of the non-working heater. I took the hose off the bottom of my 84 300D monovalve. It too has the little white plastic ball in there as a sort of checkvalve. So we now know that it is supposed to be in the bottom portion of the monovalve assembly. So I took my ACC unit out and installed it in my friends 85 (the subject with the heater issue).. still wouldn't work although it was now getting power to the monovalve pigtail..... I was thoroughly confused. I had read here numerous times that if there was not juice, you got permanent heat. On this car, with power, without power, irregardless, no heat.

We checked the electric pump motor. It was spinning for all it was worth... monovalve was working, still no heat. We were about to delve into the ducting, vaccum elements, etc... when one of the gang of 3 that were attempting to tackle this most difficult problem started taking apart the hoses from the electric pump. Despite it working, nothing came out the end of the metal pipe!!! Checked antifreeze/coolant, enough in the system to get this heater core filled... took off more of the hoses....

the culprit was found at last: the Previous Owner (PO) had put a plastic nob in the hose to block the flow of antifreeze/coolant to the heater core. Problem now diagnosed. ACC unit is bad for the heater portion; so it will be replaced. But without removing that plastic plug/cap, we would still be without heat..... not now!!

Wanted to be sure I posted up our results in case the PO for anyone else had the same issues and solutions/jerry rigging!

JMH

psfred · #9 11-09-2003, 08:00 PM

Diagnosis:

Bad pushbutton control unit ($75 at the junkyard, usually) "fixed" by cutting off the coolant flow to the heater core.

Get the "new" PBCU and the heater will work properly. The "fix" was inserted because the heat stays on all the time with a bad PBCU and overwhelms the AC.

Peter