REST (residual heating) pump not working
12-30-2007, 04:12 PM
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W203 C220 CDI
REST (residual heating) pump not working
Hi, I have a W203 C220 CDI (European model diesel) which has a problem with the residual heating not working. When I activate the REST function I can hear the air vents change position and the fan goes to a low speed, but after a few minutes all I get is condensation building up on the windows. Closer investigation revealed that the auxillary coolant pump was not working.
I checked the voltage across the pump (with REST on) and it was 0v. I have checked ground continuity to one side of the pump (ok) and measure about 13kOhm resistance between the 12v end and ground (with the pump disconnected). So far I have been unable to trace the wiring back, but believe it goes to the SAM module on connector M15.
The next thing I tried was powering the pump independantly, just to see if it worked. I powered it from a separate battery and I can feel coolant circulating in the system. Also, when I activate REST the condensation doesn't build up and it works as I suspect it should.
Therefore, I conclude it must be a problem with the pump control circuitry. The pump is obviously working, but is not being activated for some reason. I do not know why.
I have checked battery voltage and this is normal, so the REST function should be working.
Does anybody know if there a fuse or a relay that drives this pump?
On WIS the wiring diagram doesn't show any and the 13kOhm resistance suggests it could be transistor switched, but I have not been able to confirm this.
BTW - the mercedes dealer thought it was because the coolant preheater wasn't working. Apparently they checked the pump on STAR and it worked fine, but quoted me about £300 to check the preheater (located behind the starter) and another £300 if I need to replace it and wanted the car for 2 days. However, I am not convinced this is the problem or is necessary as the pump never works, even when the coolant temp is normal. Also I thought surely it should be possible to check the heater is working by measuring the current it is consuming (or not).
Does anybody have any thoughts or guiding light on this as it is one of those niggling faults that is getting at me - particularily at this time of year when a bit of extra heat on idle is welcome.
Many thanks in anticipation
Mark
W203 C220 CDI (2002).
I checked the voltage across the pump (with REST on) and it was 0v. I have checked ground continuity to one side of the pump (ok) and measure about 13kOhm resistance between the 12v end and ground (with the pump disconnected). So far I have been unable to trace the wiring back, but believe it goes to the SAM module on connector M15.
The next thing I tried was powering the pump independantly, just to see if it worked. I powered it from a separate battery and I can feel coolant circulating in the system. Also, when I activate REST the condensation doesn't build up and it works as I suspect it should.
Therefore, I conclude it must be a problem with the pump control circuitry. The pump is obviously working, but is not being activated for some reason. I do not know why.
I have checked battery voltage and this is normal, so the REST function should be working.
Does anybody know if there a fuse or a relay that drives this pump?
On WIS the wiring diagram doesn't show any and the 13kOhm resistance suggests it could be transistor switched, but I have not been able to confirm this.
BTW - the mercedes dealer thought it was because the coolant preheater wasn't working. Apparently they checked the pump on STAR and it worked fine, but quoted me about £300 to check the preheater (located behind the starter) and another £300 if I need to replace it and wanted the car for 2 days. However, I am not convinced this is the problem or is necessary as the pump never works, even when the coolant temp is normal. Also I thought surely it should be possible to check the heater is working by measuring the current it is consuming (or not).
Does anybody have any thoughts or guiding light on this as it is one of those niggling faults that is getting at me - particularily at this time of year when a bit of extra heat on idle is welcome.
Many thanks in anticipation
Mark
W203 C220 CDI (2002).
12-31-2007, 11:06 AM
#2
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Excellent legwork and diagnosis for helping to rectify your situation.
The residual engine heat system circuit is protected with three separate fuses. Two are behind a plastic cover on the side of the dash, adjacent to the front door jamb. #30 is a 40 amp (amber) while #41 is a 15 amp (blue) fuse. Check those first. Maybe you will find your fuse chart there, as that’s where MB put mine. Finally, check the fuse panel in the motor compartment, adjacent to the firewall, for #51. It’s a 7.5 amp (brown) fuse. Hope that it is simply a bum circuit breaker that has interrupted your REST system’s proper function. Why it blew in the first place is the next hurdle. Perhaps some of the more electronically savvy members here can chime in…
Best wishes with your C220 CDI. We yanks apparently cannot be trusted with such a desirable model.
01-03-2008, 05:14 PM
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W203 C220 CDI
Rest
Thanks for the reply, I have noticed a distinct lack of CDI's in the US (or any other diesel car for that matter). I do have my eye on an E270 CDI though :-)
Anyway, I have located, removed and tested each of those fuses and they were all ok. I retested the pump and it is still not coming on.
The MB main dealer had a brief look and thought it was working, but I think they could hear the vacuum pump right next to it rather than the coolant pump and thought it was running (it does not). At least they didn't charge me for it, it is going back for 'another look' tomorrow.
I have toyed with the idea of just adding a separate timer to keep the pump running for about 30mins every time I turn the ignition off - with a battery voltage detect circuit for override. That would solve the problem, but is obviously going to take a while to design and build. I would prefer it is just fixed but am bracing myself for a quote that is simply beyond economic repair.
Thanks for your help. If anybody else has any ideas I would appreciate any further comments.
Regards
Mark
Anyway, I have located, removed and tested each of those fuses and they were all ok. I retested the pump and it is still not coming on.
The MB main dealer had a brief look and thought it was working, but I think they could hear the vacuum pump right next to it rather than the coolant pump and thought it was running (it does not). At least they didn't charge me for it, it is going back for 'another look' tomorrow.
I have toyed with the idea of just adding a separate timer to keep the pump running for about 30mins every time I turn the ignition off - with a battery voltage detect circuit for override. That would solve the problem, but is obviously going to take a while to design and build. I would prefer it is just fixed but am bracing myself for a quote that is simply beyond economic repair.
Thanks for your help. If anybody else has any ideas I would appreciate any further comments.
Regards
Mark
