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Discussion Starter #1
(Regarding my '79 MB 300D...)

I know that a few weeks ago, and a couple of times prior to that, I used heat for a short while in my car and it blew very warm.

The temperature dropped tonight and there was NO heat to be found after trying it for 35 min. I tried my BI-LEVEL and A/C settings (LO & HI) with the temp gauge on the hottest temp setting (85). Again, not once did I feel any heat, only cold air blew the entire time.

What should I begin checking and should I wait a certain time after starting to try using heat?

Thanks.
 

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90 300 SE ,85 300 GD
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Temperature Gauge

What is your dash gauge reading? You may have a stuck thermostat. The climate control system can "go out" without warning on the 79 model.
 

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Hi there Ed!

Well, my temp gauge is at 175. This is what it's been sitting at lately a...


Hi there Ed!

Well, my temp gauge is at 175. This is what it's been sitting at lately after warming up. Do you think the thermostat is stuck?

I took my car to the mechanic today for a quick inspection about the heat issue.

He's "almost sure" the problem is the heater servo, and this part is about $600 new. He also mentioned that there could be other issues causing this heat problem that he couldn't tell by a quick check (vacuum lines, etc). However, he said he'd need to inspect the car for about an hour to know for sure.

Should I have him do further checking or check some things out myself?

Thanks!
 

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A quick update...

The heat worked just fine tonight, and it felt good!

Don't know wh...


A quick update...

The heat worked just fine tonight, and it felt good!

Don't know what happened. When I took it in to the mechanic yesterday, we both felt cool air blowing for the heat. Well, now that I think about it, the temp dial setting was at the lowest setting when we tried it.

Tonight though, I waited about 10 min after cranking and driving before turning on the heat. I didn't think it would work but wanted to test it again. (The temp gauge was at 175 before I tried.) I thought I did this same procedure though 3 days ago.

The inside got warm at about 70 degrees, then I raised the dial to the 80s and it was toasty, as it should be. I was only able to keep it on 80 for a few moments before turning it off because it was too warm. I used the Auto-Lo button, where the heat blew just fine from the side vents and footwell.

Wonder what caused the heat to work tonight and weeks ago, whereas it didn't 3 nights ago?

Any ideas? Let's hope this stays working for the winter!
 

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Servo is the problem most likely

That servo is getting ready to go out. I agree with your mechanic. The price is about $600. The real issue is that it is located deep into the dash and requires removing the control box. Very labor intensive. If it is working, great! I hope it will continue to keep the cab warm!
 

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The default setting to get heat should work on
"defrost" The servo needs to be exercised ...


The default setting to get heat should work on
"defrost" The servo needs to be exercised reguarly to keep working. But can go bad anyway. Mine quit on the 78 300D and was a vacuum problem
John Gregg
 

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Discussion Starter #8
Ed and dfw, thanks for the info.

Tried the heat again last night and it blew cool again...


Ed and dfw, thanks for the info.

Tried the heat again last night and it blew cool again, so it's an intermittent problem. Heat works one day and doesn't the next (or a few days later).

Would you still say this is a sign of the servo going bad Ed?

Also, dfw, did your vacuum issue require replacing the servo, vacuum lines or something else?

I've been searching online for a lower price on the servo. The lowest I found was on eBay, $425, but it's an "aluminum" servo unit I've never heard of. If this is something to stay away from, please let me know. It's here: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=46094&item=2435063834

I emailed the seller about it and they just told me they've had good success with it and that it's made by a company whose name begins with an "M." [?]

Thanks.
 

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Check the re-Built price too

E-mail this source with your VIN number and year. Let him quote you a price on a re-built servo unit. His e-mail is: [email protected] Fast. Reliable. Ed
 

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It was the vacuum pump that was going bad on my 78 300D. Replaced it and the power assist ...

It was the vacuum pump that was going bad on my 78 300D. Replaced it and the power assist came back to the brakes and the servo started working.

Oftentimes lack of power assist for the brakes is a clue for a large vacuum leak or bad vacuum pump.
A mityvac can be a big help for finding vacuum leaks and problems. Get the mityvac with a pressure gauge.

George Murphy who sells the servos gives talks on Mercedes restoration and repair questions. The talk goes all day and is worth the modest price. He is also the national MBCA technical director.
John Gregg
 

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Discussion Starter #11
I just emailed George Murphy tonight and awaiting his reply. Thanks a bunch for his email...

I just emailed George Murphy tonight and awaiting his reply. Thanks a bunch for his email address.

I've noticed a few types of MityVac kits when searching the web, like a "Testing and Brake Bleeding Kit," a "Universal Cooling System Test Kit," an "Automotive Test Kit," a "Compression Tester Kit" and a few others.

Any particular one I should go with?

Thanks again...you guys have been a great help.
 
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