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blowing hot from the side vents

Posted: Tue Jul 21, 2020 11:27 am
by grumpy old man
I gave my '94 SL 500 to my granddaughter as a throw away first car. Didn't pay enough attention to the hoses and she blew the heater hose hidden behind the engine. Needless too say she barbecued it. I found an acceptable used engine and installed it. It takes a lot of digging around to replace the heater hoses and I don't know what I might have done.

I started it up and everything seemed to be fine. I had it running in the side yard for about 20 minutes checking for water and oil leaks. The A/C was on and working. The only glitch I saw was that the low oil light was on. I knew that it was full off oil so temporarily dismissed that. As I was pulling it out of the side yard suddenly it started blowing hot out of the side vents and a combination of hot and cold out of the center vent.

On the right side where the electronic box is located there is an abundance of wiring and vacuum hoses I was moving around to replace the heater hose so I thought maybe I had disturbed something in there. I dug back down even further than I had originally and can find nothing.

Don't know if anyone has encountered anything similar on a Mercedes but if so and you found the cause, I would appreciate any help possible.

Re: blowing hot from the side vents

Posted: Sun Jul 26, 2020 10:09 am
by ice-n-tropics
Hi Grumpy,
I worked for BEHR (headquartered in Stuttgart-Feuerbach) for 14 years and they made the MB HVAC using an abundance of vacuum motors and hoses.If Antifreeze enters the vacuum system as could happen in your overheated engine, the elastomer vacuum diaphragms are damaged over time. If this is your problem, maybe there is a way to flush coolant out of the vacuum system and replace leaky vacuum motors. although access is difficult, vacuum motors can be checked to determine if they hold vacuum or if they leak. the vacuum storage container and check valves are a starting point.
Some vacuum motors are double acting or two stage using double diaphragms.

Re: blowing hot from the side vents

Posted: Sun Aug 09, 2020 10:50 am
by grumpy old man
Thought that I would post the findings. In the absence of any useful information on the system operation I wound up taking the whole thing apart to figure out how it worked. My ancestry comes out of southern Germany on both sides so I believe that gives me the right to cuss the Krauts. Essentially I had to do the dreaded heater core or evaporator replacement but needed to take it a step further and completely disassemble the HVAC housing to determine the air flow and why it was blowing triple digit air out of the side vents. Turns out that the air flow to the center vent goes through the evaporator core up and out between the evaporator and heater core. The side vents on the other hand go through the heater core on the way out. This little detour was necessary to be able to control all the vents separately. Once again my crazy countrymen overthinking things. Bottom line, the problem was the heater valve! an easily replaceable item if I had just been able to find any operational information. Instead I spent three weeks taking things apart a bit at a time trying to figure out the system.

The up side is that the heater core was beginning to show a slight leakage on the bottom left. Figured that as long as I had it in my hand it was time to replace it. Turns out it is apparently a discontinued item. After calling everyone on the google search that claims to sell them but was out of stock I finally found someone on the opposite coast in Daytona Beach on ebay and I purchased the last one he had. Hopefully I will never need to dig that deep into that SL again.

Bottom line is that in A/C mode their should be zero coolant flow through the heater core. If anyone reading this has an elevated temperature coming out of the side vents simply clamp off the RIGHT side hose at the rear of the engine and see if that cures the problem.