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W638 112CDi intermittent heater problems

2K views 18 replies 2 participants last post by  Dave2302 
#1 ·
Hi all

A little over a month ago the heating stopped working on my van. It blew intermittently hot/cold for about half an hour and then only blew cold for the last month (although my van hasn't had much use in that time).

It was in the garage this week for some other things so I asked them to have a look at it but after driving it a bit they decided it was an intermittent electrical fault somewhere and probably not worth having them spend the time looking for it. So I'd like to try and investigate a bit myself before the weather gets too cold.

I drove it in to work this morning and once it started to get a bit of warmth in the engine the heater blew hot/cold in a pretty regular 1 min cycle for the first 20 mins, after that it only blew cold for the rest of the journey.

Does anyone have any suggestions of what might need looking at?
I did last weekend have a look at the heat control knob on the dash, this is cable operated and the cables seemed to be running freely. Does this connect directly to the heater bypass valve? If the valve is mechanically controlled by these cables it doesn't seem like that would cause the problem. The cables ran back under the dash but I couldn't see where they went to. Where is the valve located and how difficult is it to get to?
Is there anything else I should look at?

Thanks for your help
 
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#3 ·
so I asked them to have a look at it but after driving it a bit they decided it was an intermittent electrical fault somewhere and probably not worth having them spend the time looking for it.
Hi,

Tossers !!

I love garages like that, most of my work comes from this sort of stupidity........

What they really mean is they haven't got a clue :wink

There is no "bowden" cables going to the heater water feed valve on these vehicles, they have an electric solenoid water valve which usually will be sticking, or have a bad connector plug on it. This is fed from the heater control panel temperature sensor circuits.
Best cure is replace that valve and clean tighten the electrical plug as it's a bit of a pain to get at, on RH inner wing up near outer wing lip, you'll know it when you see it as it has heater hoses going to it :wink

HTH

Cheers Dave
 
#4 ·
Hi Dave

Thanks for the reply, the garage didn't seem to interested and I wasn't going to push them if they didn't want my business.

I probably won't get to have a proper look till the weekend but took a couple of pictures this morning so hopefully you can let m know I'm looking in the right place. (Hopefully I've attached them correctly and the show up)

The first is looking down behind the airbox in the engine bay. Is this the heater bypass valve I've circled with the pipe coming out of it?

Second is behind the heater control on the dash showing the control cables I mentioned. If the heater bypass valve is electrically controlled do these run to another switch somewhere?


Cheers
Andy
 

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#5 ·
Hi Andy,

Pic 2 of behind controls, these cables go to heater flaps behind dash, don't usually give trouble.

Pic 1 is nothing to do with heater, that looks like the Servo pipe on the Vac pump.......

If you take out the Air Box, the part you are looking for is up under the edge of inner wing, near where bonnet hing attaches to bodywork, above inner wheelarch, the 2 or 3 pipes attached to the valve are about 25 - 30 mm outside diameter, black rubber with jubilee clips, which attach to metal pipes.

HTH,

Cheers Dave
 
#8 ·
Hi,

Lift it about 30mm at the engine end to disengage the lower grommets, then when these are free pull towards engine to release the lugs in grommets in the inner wing :wink

HTH,

Cheers Dave
 
#11 ·
Hi,

You are looking way too far forward / downwards :laugh

Look to the left of strut top mount, 3 water pipes and electrical plug going to it, it is under the wing lip :wink

Cheers Dave
 
#12 ·
I'm sure I am missing something obvious but I did look back there.

The airbox is back on and it's dark now so not a great photo but this is looking back over the airbox towards the top mount. All I can see back there are some electric cables coming through the firewall and the wiper motor.

The ribbed hose bottom left is the drain from the windscreen which I pulled out the way for the photo.
 

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#13 · (Edited)
Hi,

Well I'm starting to get puzzled now........

I've completely broke up 4 638's all 2001 - 2003 CDI's and worked on several others, including my own 52 plate Ambiente.
Both A/C and non A/C models, pre heater or no pre heater all have the electric valve with 3 hoses going to it !!
Our early model Sprinter Minibuses have it too :confused:

It should be right about where your ribbed hose is disappearing, it is in that wing cavity, and right awkward to get at :confused:

Yours isn't left hand drive or something is it ?? Perhaps located same place other side ??

The only thing I can suggest if it really isn't there is to try following the 2 rubber heater hoses, (around 20mm O/D), where they come out of the bulkhead :wink

HTH

Cheers Dave
 
#14 ·
I'm even more lost then.

I've followed the two hoses as I can. Looking from the front the one on the left goes down the back of the engine but couldn't see where it attached to. The right hand goes to the part in the picture. It only has one entry and exit and looks more like a pump, I think I've read that there was an electric pump for when the engine was off? Saying that I couldn't get it to turn on so not sure when it should run. The other hose from this part again disappears behind the engine.
 

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#15 ·
Hi again,

Difficult to tell in that pic, I can't get the orientation and I'm 700 miles from home on holiday, so I can't compare it to shelved parts :wink

Yes all the ones I have seen have a pump, but from memory pump hoses go in one end and out the other, and are mounted a little higher up and more behind the intake plenum, hoses are not at right angles like yours.

Pump is only used for the residual heat function which some vehicles don't have, and even if it doesn't work the heater will still work when engine is running.

It is possible that if yours does not have a residual heat button, then what you have photo'd maybe the valve, and again maybe if the vehicle doesn't have residual heat, then Mercedes have mounted the valve in a different place !!

God I hate possibles & maybes :laugh

Only thing I can suggest is that you take that part off and see if it is a pump, (will have a impeller inside), or if it has a valve inside.

Keep me posted, I'll try and get a look at some parts drawings later, unless someone else can chime in.......

Post up your vin, and I'll look at EPC for you :wink


Cheers Dave
 
#16 ·
The part in the pic is pretty much in the middle, just behind where the intake pipe comes across the top of the engine.

Vin number is WDF63809423505295.

I will pull it out next time I get a chance and see if I can work it out.

I appreciate the help, especially as you're on holiday.
 
#17 ·
Hi,

Well, I have to admit your vehicle has got me :confused:

I ran your VIN through EPC and it appears that yours does not have the electric water valve, but it does have the pump, which as you rightly say is located roughly in the centre of the bulkhead, behind the intake.

Only thing I can think of now is that you have a different heater in your model, so I suggest that we go back to the beginning, follow the cable that comes off the back of the heater controls, and find the other end, hopefully it will have come off a heater flap or something simple.

I'm sorry if I have misled you on this, but I've never seen a 638 with this system.

Hopefully you won't have to pull the dash out to sort this :wink

Cheers for now, and keep us posted,

Dave
 
#18 ·
I didn't have much time this weekend but have had another dig around under the dash.

I think I've found the valve, shown in pic 1. This is looking down behind the steering column under the dash, the bulkhead is to the right of the pic. The silver pipes look to go up to the heater matrix and the control cable goes up to the heater knob and I can feel the valve actuator moving when I turn the knob. I presume this connects to the two pipes going through the bulkhead in the middle of the engine bay.

Pic 2 is looking up the silver pipes which I think connect to the heater matrix.

Given that this is a cable operated mechanical valve is this likely to be the cause of my problems? The intermittent heat before it failed completely seemed to suggest an electrical issue.

And do you know when the inline pump in the engine bay is supposed to operate? My van doesn't have the rest function (or at least not the button).

I've seen the valve quite cheap on eBay but it looks really difficult to access so don't really want to go down that route unless I'm sure that's the problem.

Thanks again.
 

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#19 ·
Hi,

I have never had any dealings with this type of Heater, but all the ones with the rest function, only run the electric pump when the engine is off and rest function is on.

When your engine is up to temperature, are the pipes into and out of the matrix hot ??
Can you get at the matrix through one of the vent holes to feel if it is getting hot ??
Can you disconnect the heater hoses in the engine bay and get water to circulate from a hose pipe ??
If you do this, does your "Hot / Cold" knob stop / allow the flow of water ??

They would be my next checks.

HTH,

Cheers Dave
 
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