Losing heat at higher speeds in '80 500SE

I would think the thermostat is the most likely candidate. At high speeds, the mass of air might actually be cooling the system to below where it should be?

Does the temperature gauge indicate a normal temp at high speed (ie 80c)?

Just a thought. Marty

And lemme guess, the air that comes out isn't that hot, either?

That's usually a sign it's the aux. heater pump. It's right below the monovale, pictured here:

When they fail the water going into the heater core isn't as hot as it should be and at highway speed the flow thorugh ventilation cools the car down more quickly and without the heater working at full temperature it seems like it's not working.

My old pump spun when 12V was applied, but a new one fixed this problem.

That's the thing, at slow speeds it can be brutally hot, typical older MB sort of heat.

This seems to be a common culprit but why would it only work at slow speeds?

John '80 500se '80 300sd

Sounds like the monovalve would be a good place to start. Thanks.

John '80 500se '80 300sd

If your engine temperature gauge stay steady on highway as well as local driving, then I suspect the heater monovalve problem. However, I believe your problem may be the thermostat... but this would be easily diagnosed by the fluctuating engine temperature gauge... highway speed would be much colder.... and in city driving... wildly swinging... going up when at the light... go down when moving.

Had the same problem on my 300D. Turned out to be the climate control module. Apparently it turned off the auxillary pump that puts hot water through the heater core. Never did figure out why but you know solid state stuff...it doesn't need a reason.

Just for giggles, also consider the thermostat - they can stick in the open position as well as the closed. At high speeds, you are shoving lots of air through the radiator, and a thermostat that is stuck in the open position will fail to close up as the temp of the coolant falls below ideal operating temperatures. When that happens, you get much cooler coolant going through your heater core, regardless of how well any aux pump may be working. Thermostats are also way cheaper than climate control modules.

I caught this post after the original message had expired, so if you've already tried this, I apologize.

Conrad

Typically, the coolant will cool off gradually with a thermostat that sticks in the open position. If I understand the original post the air went from warm (read hot) to cold almost instantly. This is exactly what happened to me. If I shut the engine off and immediately started it again the air was warm for a minute or so and then suddenly turned cold. Any suggestions.......?

Sigh - I've got the best of both worlds right now - a climate control module that works fine for about 20 minutes, then decides not to blow warm air anymore AND a stuck open (actually missing - it's a long story) thermostat.

My assumption about the "stuck" open thermostat was based on the fact that I thought that the original post said something about losing the warmth at high speed. When I get on the highway at high speeds, I definitely don't get all the warmth I'd like due to the fact that I've been running sans thermostat for a month or two, but you're right - it's not a sudden chill. My optimistic debugging technique is usually to replace all parts in a system starting with the cheapest possible part. One of these days, I'm going to have to actually learn what I'm doing ;-)

(Bizarrely enough, my cruise-control works for about the same length of time, then gives up the ghost)

Conrad

Have you heard a clicking in your dash (you have to listen close). My

MONO VALVE

the case, minus a few cans!

OOPS if it has one. if tour temp gage is reading 80C and your heat output drops off fast that is i bet it. mine would do that. i turned the car off restarted had heat and it cooled once again.

pulled the valve apart and it had a tear in the gasket. the coolant gets in the magnet and makes it stick.

easy fix

the case, minus a few cans!

Exactly where is the mono valve and how will I recognize it?

on my 82 300SD it is on the pass fire wall. you should see the input of the heater hose going to the valve. on mine there are 4 small screws that hold the top on. park the car facing up hill remove the 4 screws