FSM description indicates that aux pump provides coolant flow at slow speeds.

I think that would be more of an explanation of purpose than it is a description of operation.

Yes. They mean slow engine speeds, not vehicle speeds. I just got a 1984 300TDt and it's got heater system issues in spades. Haven't gotten to probe what works and what doesn't. It's got a manual switch hooked up and a ball valve under hood but I didn't feel a lot of anything.

Anyway back to your issue. Can you sit in neutral at highway engine speed and get heat? Or better yet get hauling down a big hill on the highway and drop it in neutral and see if heat returns while coasting at speed(DONT HIT NEUTRAL BY MISTAKE!!!), rev match or wait till your slowed down alot to put it back in gear. Maybe a heater hose or component is internally failing and blocking flow under higher pressure when the water pump is running at highway rpm, the way a brake line can fail and become a check valve not allowing the brakes to release.

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Thanks. Will try engine-speed dependent operation as soon as possible. Will get back to you-all.

I think a hydro-locked solenoid is unlikely since I just installed an OE hi-$ one.

The operation of the aux pump is not speed-dependent. It isn't rpm dependent. It's function is to improve coolant flow at low rpm, but that doesn't mean that it operates only at low speed or rpm.

Don't jump to that conclusion too readily.

I meant (DONT HIT REVERSE) by accident when testing.

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Tried engine speed dependency and it is inconclusive. Cant coast long enough.

I tested my aux pump today and it is inop. I think I will bypass it to see if I get heat that way. If so, I need a new pump and I don't understand the system.

I replaced literally the entire HVAC system on my car, including several different monovalve housings and cores (OE and aftermarket) and never got it to work properly. Same problem, no heat at highway speeds. Do yourself a favor and search for "mercedes w123 monovalve eliminator" - There's a kit available that replaces the unit with a vacuum actuator, assuming your vacuum system is in working condition, the kit will dramatically improve the system. It's an easy one one night installation job. The kit also includes plumbing to bypass the auxiliary water pump, though that's not strictly necessary, I used it and haven't had any problems since.

Monovalve & Aux. Coolant Pump Eliminator
 

THANK YOU ! .

I found it easily, seems $pendy $125.00 but looks to be a good solution :

https://klimakit.com/product/monovalve-and-coolant-circulator-pump-eliminator-upgrade-kit/

Klima II Heat
 

As it turns out, it's rainy & cool in So. Cal. so I fired up my Coupe, rolled the temp. wheel to heat (not the MAX setting, just before the click) and headed out ~ in seven blocks the dash vents were softly blowing hot air, I use the 'EC' button and the low fan setting 99 % of the time as the fan noise bothers my wrecked hearing .

This upgrade _may_ be the co$t effective way to go.....

When the auxiliary water pump is dead, it drastically slows down the hot water flow to the heater core so by passing it might just be the solution if everything else is O.K. .

For a simple , NO CO$T test : block the radiator with cardboard and duplicate your highway speed no heat drive ~ if you get more heat, the thermostat isn't working well .

If you still get the same cool air there's a hot water flow to the heater core problem, monovalve or aux. pump .

I could be wrong and it may be different on the diesel, but I remember looking at my 450slc with a m117 v8 heating system and thinking that it was taking the hot coolant and returning it to a place where there was no positive pressure to move the coolant without help from the aux pump. Meaning that there wasn't any (or very little) change in the pressure from the main water pump in the in and out flow from the heater core. So the aux pump may be the only thing really moving a good amount of coolant through it. It's possible at higher rpm the heater core(and valves and bad pump)is just a more restrictive path and the hot coolant is just not going that way because it is taking the easier route. I have all of the heater control valves removed in my car and only have a blower and aux pump switch. It had the dreaded Chrysler garbage and it's a drift car so I only need defrost occasionally and to help cool it down. I can still feel a little heat at speed though. I've considered a ball valve to kill it in summer but haven't had to yet.

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In a previous post I noted that my aux pump is not working. My theory is that a BD pump blocks flow to the heater core.

Today, I bypassed the pump and the road test showed plenty of heat at both high and low vehicle speeds. It also seemed to regulate. I will replace the pump in the Spring but for now I am satisfied.

Auxiliary Water Pump
 

There are now inexpen$ive Chinese ones, I've not been able to find the original BOSCH or HELLA ones in a few years .

If you like to tinker, take the oil one off the car, invert it and ever so carefully pry off the delicate and crispy white cap n the motor end, it's a lubrication point and once you turn the impeller a little bit with a few drops of light oil in the now open bearing, it'll run when you power it up ~ the Dealer Mechanics were supposed to dribble in a drop or two of oil every so often, I doubt any ever did .

I'll try that nate. Thanks.

Bosch universal water pump E-bay $ 60.00 . Same pump?