W124 no cabin heat

[mej]

Having just stripped down the duo valve and back flushed the heater matrix (from duo valve (stbd) side to the inlet (port) side I still have no cabin heat (E300DT S124.191). There is 12v to the duo valve plug on cold, and 0v on max hot, as there should be; the duo valves can be heard and felt to jump when 12v is applied to the terminals from an external source. My arms aren't long enough to check this at the moment the heater wheels are set to hot or cold. When intermediate temperatures are set, there is no 'pulsing' of the valves. As for the matrix, I obtained 5.5l per minute on the back flush with a tiny head of water (about 6' max so little pressure). This suggests to me the matrix is OK. I have the WIS sheets on checking the system, and the last ditch stand is to replace the matrix, but I can't think this is necessary. Has anyone been here before?

 

[Bolide]

AFAIK the diesels run the electric circulation pump all the time. If the pump has failed this may explain the lack of heating.

Can't remember where I read this but recall the petrol cars run the pump when needed but the diesel run all the time

Makes sense - there is less wasted heat in the diesel engine and the heater does take a while to warm up

Nick Froome

 

[mej]

Omitted to mention that the aux pump seems to be running as it should. It can be felt vibrating. Will try disconnecting the DV plug to see if the system defaults to max heat. Also failed to check the coil resistances - all new info I've just now picked up. The confusing thing is the 'click' response of the solenoids to 12v and the good flow through the matrix. Am looking for a wiring diagram to see if there's an easy way to check the 2 interior temp sensors' resistances.
Wil be some time before I get back to this though.

 

[television]

I have the diagrams and will put up in the morning

 

[wireman]

There are many tubes in the matrix and with only a few of them clear a good flow will be quite possible but little heat would be available, there are two matrices on the 124 LH/RH it is unlikely that both would block up at once and so I would guess that blockage is not your problem, unless it failed on on side and then the other. Does the pipe from DV to aux pump get hot? if it does and there is no heat it is the matrix at fault.
The coolant flow is from cyl head to the matrices and then back via the DV and pump.

My 124s aux pump runs all the time as Bolide says, without it (its brushes went wonky) the heater works but in town at low engine speeds it is poor and quite useless in very cold weather, I don't think that's your problem either, but the motor brushes are metal loaded 4mm square 8mm long with a wire pigtail if you need to find some.

The DV pulses only at cabin temperatures near the set point on the controls, a few degrees colder and they are fully open, any warmer they are fully closed.

Pull the DV plug and see if it gets hot, if it does this leaves the controller and sensors (one in the interior light assy and two in the heater behind the dash) as the prime suspects. The sensors in the air pipes behind the dash are easy to find behind the heater control unit.

Beware that the DV signals are switched by transistors which will escape to the happy hunting ground if any shorts or heavy loads are applied to the DV wiring.

 

[mej]

Thanks for that Wireman; I now know a bit more than I did before on this vexed subject. The pipes from the matrix to the DV do not get hot at all, this is why I suspected the matrix. The fact that the pulsing isn't happening you have explained I think, but I'm now a long way from the car and it will be some weeks before I can check any further. I hadn't checked the coil resistance or tried the system with the DV plug off. The temp sensors I'm familiar with from other W124s so will move on to those lastly. Pix are of my flushing set up with the 3 way splitter into the 2 hoses taking water from the matrix (but I was back flushing of course). This car (no aircon) has a by pass pipe which forks off the matrix inlet pipe shown disconnected and with the back flush out pipe in place on the matrix inlet pipe. There doesn't seem to be a tap to close this off, and my others (aircon) don't have this. Wondered if you'd any thoughts on its purpose (other than as a by pass!!) but why necessary.

 

[television]

There are many tubes in the matrix and with only a few of them clear a good flow will be quite possible but little heat would be available, there are two matrices on the 124 LH/RH it is unlikely that both would block up at once and so I would guess that blockage is not your problem, unless it failed on on side and then the other. Does the pipe from DV to aux pump get hot? if it does and there is no heat it is the matrix at fault.
The coolant flow is from cyl head to the matrices and then back via the DV and pump.

My 124s aux pump runs all the time as Bolide says, without it (its brushes went wonky) the heater works but in town at low engine speeds it is poor and quite useless in very cold weather, I don't think that's your problem either, but the motor brushes are metal loaded 4mm square 8mm long with a wire pigtail if you need to find some.

The DV pulses only at cabin temperatures near the set point on the controls, a few degrees colder and they are fully open, any warmer they are fully closed.

Pull the DV plug and see if it gets hot, if it does this leaves the controller and sensors (one in the interior light assy and two in the heater behind the dash) as the prime suspects. The sensors in the air pipes behind the dash are easy to find behind the heater control unit.

Beware that the DV signals are switched by transistors which will escape to the happy hunting ground if any shorts or heavy loads are applied to the DV wiring.

This information is not correct, the output from the control units are short circuit proof and and faults in the duo valve coil or recirculation pump can be read out on a pulse counter. Never yet in all my years have a sen a control unit damaged from a Duo valve.

The valves in their normal state are open in the valve, so with the plug out, they should be on max heat.

What happens is the rubber seals can fail and swell up preventing much movement of the solenoid core.

I do not advise anyone to take one down as far as this one in the picture for you will never get it back together again as the rubbers have swollen up preventing much movement of the valve.

You can see the valves are spring loaded to keep the valve open, the spring can be seen it the left hand picture.
The swollen rubber can be seen in the second picture and this one had virtually no movement at all.

A simple test is to ground the center pin and apply 12volt to either outside pin, and the clonk can be heard,, if one side makes a dull clonks, then it is hardly working at all and only has a tiny movement.When working well you can feel the pulses about 3 per second on both output pipes.

 

[mej]

That's very relevant TV, because my DVs didn't have a spring that I could see. Like a pirrock I didn't take pix of the disassembled valve, but it was like Parrot of Doom's (on the other side), with the tubes which descend from the upper half holding the shafts of the sealing caps which had a 'Chinese hat', circular, but V cross section shape, unlike yours. The cylindrical pistons which fit in the solenoid cavity were immovable in the bottom part of the valve, but I didn't fully understand how they should operate. The shafts with the Chinese hat brass head on did shoot out of the tubes when 12v was applied to the appropriate pins. I have a homemade pulse counter. Do you have the diagnostic plug pin and LED flash info by any chance?

 

[television]

The valves that are loose in the ends never give any trouble, The loose valves go in the top of the right hand picture.

The valve in my picture has been dismantled completely, unlike parrots where only the end that never give any problems is shown. the valve in my picture is the same as yours throughout in the way that it works.

Yes I have the paper printed off and how to check it all.

If you heater matrix is blocked, the water cannot flow anyway so you would feel no heat on the valve pipes

 

[television]

I have sent it

 

[mej]

If you heater matrix is blocked, the water cannot flow anyway so you would feel no heat on the valve pipes

Just my fear, and yet the flow was significant - 5.5l/min, as stated above. If you would post the work sheet number on the WIS I'll download it. Just got in after 1500km drive, so a little foggy, and why I can't pop out to check the car for a few weeks.

 

[wireman]

In which part is it incorrect Malcolm?
The only apparent omission is that the solenoid drivers may have current limiters in their design, but I have spent enough time blasting the grey smoke out of transistors to know that in spite of any protection they can still be prone to failure if inadvertent overloads are applied, especially if the overload is sustained.

If you heater matrix is blocked, the water cannot flow anyway so you would feel no heat on the valve pipes
Completely blocked yes, but it is not uncommon for most of the tubes to become blocked and the remaining few allow sufficient coolant flow that the pipes do get hot but no significant heat is transferred to the air.

It does sound like the duo valve is going to be the culprit.

 

[television]

In which part is it incorrect Malcolm?
The only apparent omission is that the solenoid drivers may have current limiters in their design, but I have spent enough time blasting the grey smoke out of transistors to know that in spite of any protection they can still be prone to failure if inadvertent overloads are applied, especially if the overload is sustained.

If you heater matrix is blocked, the water cannot flow anyway so you would feel no heat on the valve pipes
Completely blocked yes, but it is not uncommon for most of the tubes to become blocked and the remaining few allow sufficient coolant flow that the pipes do get hot but no significant heat is transferred to the air.

It does sound like the duo valve is going to be the culprit.

It is a pulsed short proof circuit, we have still in the 8 years that I have been on here and elsewhere never had a controller damaged through a short circuit duo valve. If I can fine the paper again from Mercedes I can send it to you.

 

[Dec]

AFAIK the diesels run the electric circulation pump all the time. If the pump has failed this may explain the lack of heating.

Can't remember where I read this but recall the petrol cars run the pump when needed but the diesel run all the time

Makes sense - there is less wasted heat in the diesel engine and the heater does take a while to warm up

Nick Froome

Omitted to mention that the aux pump seems to be running as it should. It can be felt vibrating. Will try disconnecting the DV plug to see if the system defaults to max heat. Also failed to check the coil resistances - all new info I've just now picked up. The confusing thing is the 'click' response of the solenoids to 12v and the good flow through the matrix. Am looking for a wiring diagram to see if there's an easy way to check the 2 interior temp sensors' resistances.
Wil be some time before I get back to this though.

As has already been suggested, check out the circulation pump and its brushes to confirm that it is indeed pumping properly.
If the vibrations were felt with the engine running? then the engine water pump could have been where the vibrations were coming from.

Below, similar problem, different car, the upshot was that replacing the circulation pump solved the fault.
See… http://forums.mercedesclub.org.uk/showthread.php?t=108985

Dec

 

[mej]

Good thinking Dec, but I have run the car on extended journeys at a reasonable speed, and still no heat, so it's something other than the aux pump which I understand is mainly for maintaining flow at low revs. The favourite seems to be the duo valve, and as my 'pistons' were not moving, but seem to require to slide into and out of the solenoid cylinder, I hope that's the case. Doesn't mean the aux pump IS working of course, but it does vibrate with ignition only on, so less likely.

 

[television]

Just turn on the ignition and place the blade of a large screwdriver on the pump , then put your ear to the handle and you can hear it running

 

[mej]

Interesting link in Dec's post, esp the solution of it being the aux pump in the end. Have done as TV just suggested, so happy with the running of the aux pump, but I wonder if the driver (an impeller) can be worn, as I hear can happen with some water pump impellers? By the way, there seem to be many different types of DV. On the cabin side mine has 2 returns from the matrix plus a 3rd pipe forked off from from the matrix inlet (port side) which must act as a bypass, and an outlet at the engine side of the DV back to the cooling system.

I included a pic of these connections at post no. 6.

 

[wireman]

Pics of pump from 1991 124.

A disk on the coupling can be seen rotating through the drain holes at the front of the motor, centre of pic.

The impeller is plastic on a metal shaft ~4mm dia.

 

[mej]

Will be one of my checks when next with the car. Thx.

 

[wireman]

Aux pump pics

2 pics of 124 aux pump

The disk visible through the drain holes on the very back of the pump can be seen rotating, or gently felt with a wire, centre of pic 1.

The impeller is afixed to a metal shaft about 4mm dia.

I have not attempted to open the gland on this pump not wishing to damage it.