No heat during medium fan and high fan

For some reason, I have no heat in my 300 SEL when I put the blower on medium fan or high fan. I do get some kind of heat when the blower is on low fan. The tech replaced the “heat pump” and I told that there is now warm heat, but still not like it is supposed to. Any ideas?

Cold in NC,
mattsuzie

[psfred]

Most likely poor flow through the heater core, but the cause must be determined on site.

Check the coolant level -- low coolant will airlock the heater core. Also check to make sure the aux coolant pump is working to move coolant through the heater core -- if it's burned out, flow will be slow.

A stuck monovalve will do the same thing, although they usually stick open.

A bad CC unit may do this too, getting "stuck" on full AC.

Peter

Thanks. Coolant level good. A new heater pump was installed.

According to tech, pretty good heat now comes out of the "defrosters", but no heat still when touching the button for the other controls. The tech ordered another part that is to arrive today, do not know what part it is.

I am not totally sure about the monovalve and the CC unit. Will update. Thanks again.

The tech determined that it was the "heater core". Even after the new pump, the downstream does not heat up as much, only in the defrost mode.

So now I only have acceptable heat in the defrost mode.

Heard it costs over a $1000 to put in a new heater core.

[cgoodwin]

$40 part, cheapest diagnosis there is...Not to mention that what you describe really sounds like a monovalve!

he put in a new monovalve

the core is more like $120, but he said it is a day worth of labor because you have to take out the dashboard.

[psfred]

Yup, if the heater core is plugged, it isn't going to give you good heat at high fan speed.

However, if it gets hot in defrost and not other modes, it's probably the PBCU, not the heater core. Hot water has to go through the core to get heat in defrost, too!

That doesn't mean the core is good, though -- if the outlet hose is still only warm in defrost, it's plugged, and it's a real pain to replace.


Peter

You are correct about the defrost mode, but the tech said that in defrost mode a valve opens a full 100% and inside air is recirculated.

With the control unit, when choosing the other options, a valve does not open the full 100% , but some value less, and air from the outside is utilized. Something like that.

Therefore, heat in the defrost mode is very warm, but heat in the other modes are just ok warm. It apprears that the control is working, just not flow in the core.


Thanks.