Wondering where to start trouble shooting my non fuctional heater in my 1992 400E 160K miles. The blower motor works, and warm air comes out at first , but then turns to cold within about 20 seconds. Another symtom is that my windows seem to fog up slightly after trying to engage the heater. The problem started right after having a valve job. Heator Core? control valve? perhaps some hose was left dissconncted? Don't know where to start and all I have is the very usless mitchell CD-ROM, if someone can reccomend a good CD-ROm or shop manual for this car please let me know.

Any info would greatly be appreciated.

Please elaborate on what you mean by "valve job". Heater valve, or engine valves?

It sounds like you're not getting coolant circulating through the core. I believe (not sure) that there's an aux coolant pump for the heater core, which could be dead. Or - there could be air in the core.

The fogging of the window could be a bad sign. is there any sweet smell that accompanies it - like coolant? If not - then it's likely condensate, and the problem becomes less troubling.

Troy
1995 E420 76k

The diagram might help you with understanding of operation. I am no expert but I would check flow from the auxiliary water pump (140) as well as flow through the valve (56) make sure connection were not left off or the piping changed after the valve job. The pump (140) should be running at idle if the heat is on. If you put the heat to max and the fan to max (not auto) there is still no heat?

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2010 ML350 Bluetec
2012 Mustang Convertible

Thanks for the reply,

Sorry I wasn't more specific,. I recently had an complete engine valve job, resurfaced heads, the whole deal.

I haven't noticed a sweet collant smell, but that might just be my bad sence of smell. I'll ask someone to ride in the car with me as see if they smell anyhting. I've checked my coolant levels and I don't seem to be loosing any.


If its the aux coolant pump for the heater core, is that part easily accesible? Is there a remedy to air in the core? without removing the the the whole unit?


I guess what I'm trying to determin is if there is anything I can check, pre heater core that might be the problem. The labor involved in removing the dash to replace the heater core might be quite expensive? Trying to get the shop that did my valve job to fix it but they are not willing to do so.

Thanks

It would be worthwhile to try and find the aux coolant pump and make sure that the garage didn't disconnect it when they removed your head. This would be the easiest, and most likely fix.

The design of the heater core, as shown in the diagram, appears unlikely to trap air - as the influent and effluent are both connected to the top. If both were at the bottom, it would be possible.

I just thought of something - is your defrost button on, or is it set for regular heat?

Getting cold within 20 seconds almost sounds like the A/C is activating - which occurs when the defrost button is pressed.

Does the airflow change when you select different buttons? If not - your A/C control panel may be shot, in which case it defaults to defrost mode for safety reasons - and explains why your air is cold.

Let us know...

Troy
1995 E420 76k

I'm still trying to repair my non functional heater. I've been having trouble locating that aux coolant pump, anyone happen to know its location?

Thanks

Thanks again to everyone for all the usefull insight. I couldn't have done it without your help. It was the mono valve (heater valve), replaced it and the heater worked like a dream. Took less than an hour, hardest part was getting to it. The Bosch part cost me $103, and that was it. The dealship wanted over $800, they said I needed a new climate control unit, sure glad I didn't go that route.