450 SEL Issues - Mercedes-Benz Forum

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post #1 of 6 (permalink) Old 05-09-2004, 04:19 PM Thread Starter
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450 SEL Issues


I have a 78 450 sel - a nice machine. However, the heat is stuck on full blast. The temp wheel won't do anything. Everything was working fine and then this happened. Also, the battery is running down now when I park the car for a few days. These may be related, may not. Any ideas?
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post #2 of 6 (permalink) Old 05-12-2004, 09:49 AM
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RE: 450 SEL Issues

langpfeife - 5/12/2004 3:07 AM


The biggest weakness of these cars (later US 116s) is the climate control servo. The system itself is fairly
easy to troubleshoot by an experienced Benz technician, but the servos crack and develop
leaks. The part is expensive, and quality of rebuilt servos varies quite a bit. It's nice when it works,
though. ;)
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post #3 of 6 (permalink) Old 05-13-2004, 03:58 AM
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RE: 450 SEL Issues


Unfortunately it does sound like an ACC servo. Often they will "stick" in one position or
another (heat or cooling), usually the one you don't want. ;) The battery drainage is caused by the
stuck servo motor, indefinately attempting to return to it's "park" position after you
turn off the key.

[:(] Expect to pay between $4-600 USD (rebuilt) and $1500 USD (new MB) for the part, and
about 1-2 hrs.

Another consideration - the "amplifier", which among other things controls movement of the servo motor, will get extremely hot whilst continually attempting to relocate the stuck servo...it *may* need replacement as well. (About $150 USD, IIRC.)

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post #4 of 6 (permalink) Old 05-13-2004, 08:03 AM
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RE: 450SEL Issues

If the problem is the servo, we keep them in stock for a lot less than the price mentioned above... Let's hope that isn't the problem.

"If spending money you don't have is the height of stupidity, borrowing money to give it away is the height of insanity." -- anon
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post #5 of 6 (permalink) Old 05-17-2004, 05:39 PM Thread Starter
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RE: 450 SEL Issues

Hey guys. Thanks for taking the time to help me out. I appreciate it!
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post #6 of 6 (permalink) Old 05-25-2004, 03:04 AM
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RE: 450 SEL Issues


I had a 450SEL for about 11 years, and had the same problem as you, but although mine had aircon this was not the automatic type, so my solution may not work, but it's worth a look.

On mine, when the heater switch for either passenger or drivers side is moved up, this moves a vaccumm switch which goes through the firewall and directly to another vacumm switch next to the heater pipe. This switch literally lets the flow of hot water from the engine into the heater matrix. This switch can corrode over time, but can be freed with a little WD40. Look for a heater pipe going from the engine to inside the vehicle. From memory, this pipe on my car was located at the top of the engine bay around about where the the wiper motor is situated. Look for a pipe with a vacumm unit connected. The vacumm unit will have an small arm on it which fits inline to the heater pipe.

I found that the heater switch inside the car was working (you can hear the vacuum hissing if this is working), but as the arm was corroded there was not enough air pressure to move the switch. Mark the position of the arm, start the car. move the temperature completely in the opposite postion and see if the arm moves.

This is at your own risk, but I did find I could move the arm "manually" until I freed the vacumm unit up. This should block the flow of heated water into your matrix.

As stated my 450 was slightly older and UK spec, but it did have aircon, so I am presuming the heater setup was similair.

Good Luck

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