Hi all.

Recently the duo valve on my '97 S420 has been acting flaky. When I first start the car, everything works great. After I have driven for a while and then park briefly (think stopping at a store to get some milk), I get back in the car and start it and I get heat coming out of both sides regardless of the temp or AC setting.

When I turn on the AC controller diagnostic mode I can see that both the heater cores are at 150+ deg F. (values 3 and 4), while the evaporator is at a cool 40 deg F (value 5).

If I open the hood and gently tap on the duo valve with a screwdriver handle the heater core temps immediately start to drop and in a couple of minutes they are within a few degrees of the evaporator. That step tells me that the plungers inside of the duo valve are probably sticking, but what has me puzzled is that both sides are doing the same thing at the same time, which doesn't sound quite right.

So my question to you is should I pull the duo valve apart and give it a good cleaning or is there something else going on I should check.

Thanks!

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles

The "duo" valve on that model isn't serviceable, no parts available.

Most likely you will need to purchase a new one $$$.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Thanks, M.B.DOc. That's not the answer I was looking for.

So it seems that I have my project for this weekend. I'll try and take it apart and see what I can do. If I mess it up it's no loss because I was going to wind up getting one anyway.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles