No heat over 45MPH (1984 300D) - PeachParts Mercedes-Benz Forum

ChrisBowen · #1 02-17-2004, 03:17 PM

I have a 1984 300D with no heat when driving over 45 MPH, and any advice on how to fix this would be appreciated. (it's cold in CT). At speeds below 45 MPH, the heat is fine, and the climate control thermostat settings are fine.

When I say "no" heat, that's not entirely true - the temp at the vent drops to about outside temp + ~10°F. So, if it's 25°F, the air coming out of the vent is between 35°F and 45°F.

It's not a problem with the cooling system. The temp gauge is working fine, and even at speed in 15°F weather the engine temp never drops below 80°C.

I have replaced the climate control unit, the mono valve, the engine thermostat (just in case)... No joy. (except the buttons on CC head are nice and new)

My working theory is that at speeds in excess of 45 MPH, cold air is entering the system and mixing with the warm. Unfortunately, I can't figure out where that much air would be coming in from, or if it isn't a completely different problem.

Any clues?

Thanks,
Chris

amg280 · #2 02-17-2004, 05:41 PM

Did you check the cimate control servo? Im not sure if the 84's still did use that expensive unit. But if they do, it sounds like that could be the culprit. The unit is usually planted by the firewall on the pass. side. It has four water hoses going to, and a host of vacuum lines going to it on the top half. It is usually mounted near the aux. water pump.

good luck

ChrisBowen · #3 02-19-2004, 01:10 PM

Hey, thanks for the response, I'll check it out.

I was also wandering around the Diesel forum, and there are a number of references to the vacuum actuated center vent door. Any ideas? I haven't a clue where to start...

psfred · #4 02-19-2004, 02:38 PM

No servo on the 84, you have a monovalve. Should be easy to locate, has a two wire connector.

Unplug the connector -- you should be roasted out of the car -- full heat. IF not, you have either a plugged heater core or stuck monovalve (four screws in plastic -- be carefull going back in) -- if the bellows is wrinkled and bulged, replace valve. If the armature is stuck in the solenoid part, you will have to replace that, too.

Also check the aux. heater pump. If it isn't running, it will cause heat problems. Sadly, when it burns out it usually takes the pushbutton unit with it, you may need to replace both.

Peter

ChrisBowen · #5 02-19-2004, 05:20 PM

Hey, thanks. I'm getting a lot of good advice, and I really appreciate it.

OK, I've replaced the monovalve already - that was one of the first things I did (without bothering to test it, naturally...). I'll try the test you recommended.

Now, what's up with the aux water pump? What should I be looking for, and if it's operating, how can I tell?

My biggest problem is that *none* of the things in the system seem as though they could be effected by speed.

When I'm idling, or driving below 45 (seriously - that's the magic speed), *everything* functions to spec. Set the heat to 22°C, put the fan on auto, and it'll keep the heat right there (verified with my digital thermometer). Want it warmer? No problem. Cooler? No trouble at all.

Above 45 MPH (I swear I'm not making this up), the heat coming out of the vent drops to ambient +10°F. (again, verified with my handy RadioShack indoor/outdoor digital thermometer)

So, I'm confused and frustrated at this point. I doubt there are many components left to replace, and the problem persists...

amg280 · #6 02-19-2004, 05:38 PM

Unplug your speedometer to trick it. he he he. Perhaps a possible vacuum leak?

ChrisBowen · #7 02-19-2004, 05:59 PM

I like that! I'll give it a try!

Wow, vacuum leak... I've pondered that a bit after learning about all the zillion and one things that operate on vacuum in the car. I have a mighty-mite vacuum pump with gauge, so I could do it. I just don't know where to start. Also, I've been all over the forums and have seen many scary posts about taking the dashboard out to get at things...

Still, my car doesn't exhibit any of the low vacuum symptoms (e.g., won't shut off). How would vacuum be related to speed? Is the pump mechanically linked to the engine? (or does it have a little camera pointed at the speedometer

I've seen things about actuators having gone bad, and doors getting stuck... I went to the library to see if there were any books on Mercedes repair, but non of them had any useful diagrams. Any advice on what book(s) to get?

psfred · #8 02-19-2004, 06:44 PM

Look for a soft heater hose -- suction (return) side is most likely. Mushy hoses can collapse, preventing coolant circulation through the heater core -- this is most likely to be the problem (poor circulation).

You should be able to feel the aux pump running at idle by the internal vibration. It its not running, you can get exactly the symptoms you are describing -- nice heat at idle, but none at speed.

Peter

ChrisBowen · #9 02-19-2004, 09:30 PM

Ah! That's easy enough to check! Hose and aux pump... Thanks! I'll let you know what I discover.

BTW, this site is much better than trying to get those Car Talk knuckleheads on the phone!

psfred · #10 02-19-2004, 10:42 PM

Tom and Ray are a blast, but they don't do Benz or diesel. Nobody's perfect!

Peter

ChrisBowen · #11 02-20-2004, 01:45 AM

Tom an Ray aren't perfect?!?

Oh NO! I've staked my reputation (and the health and well being of my car) on those BUMS!

Diesel is way too cool! It's sad they don't touch it.... As a diesel owner I know where every diesel vendor (and mechanic) is, much to the amazement of my friends... (really, if more people drove diesels...)

So, what you're saying is, Tom and Rat (yes, it was a typo, but I'll let it stick just in case they're watching) are never going to let me on the air... Those BUMS! Well, so be it. I knew they were bums from the moment I got hooked on their ridiculous waste of air time. No big deal. (So you and I are their two listeners... Their loss)

Anyway, diesel isn't the issue... Nor is Mercedes. If they had *half* a clue (and, again, just in case they're watching, THEY DON'T!), they'd go beyond the rusty Duster and the Honda...

Unfortunately I had a date this evening and didn't have a chance to perform diagnostics on the car... (BTW, don't let the car know that I have a life! She'd go all Christine on me. German style)

Ah, well... Tom, RAT, I hope you're listening - CURE MY CAR!!! Rotten Little Monkeys... I've seen the photos, and you surely don't have a clue. I'm amazed you can find the car, much less your keys!

http://cartalk.com/content/about/history/

OK, I hope they were watching, or at least having ther lackeys watching...

(Thanks for the humor, Peter! It made a good night better.)