Hello-

Done some researching on failed auxiliary water pumps, but most are regarding car models newer than mine - so I'm not sure how relevant they are to my situation.

Symptoms are the heater works fine when the car is moving but when stopped in traffic I get lukewarm air out the vents. When I'm moving, the heater seems to work correctly; ie it seems to hit the setpoints and delivers warm air out the side vents and cool out the middle. If I change a temperature setpoint there's a short lag but the ACC seems to respond.

ACC is a rebuilt unit that I purchased and installed last year. I didn't test the auxiliary water pump at this time.

With the water pump installed, I disconnected the 2 prong connection and connected directly to the battery. Nothing. Resistance across the water pump prongs was infinite.

Removed the water pump and connected it directly to the battery. Again nothing.

Opened the cover of the motor. I don't know DC motors too well, so forgive my terminology but there were blobs of dark grey over the windings and on the magnets. Smelled like some overheating was going on at some point. The little brushes seem to still have plenty of meat left on them. I chipped off the carbon, reassembled, then took it back to the car to see if it would work.

Connected it to the battery directly, and it ran. But when I hooked in my VOM I found I was getting over 5A running through the motor...

Ran it (again connected directly to the battery) for a minute. Disconnected and the housing of the motor was hot.

I'm not putting the pump back in, I've read that over 1.2A is dangerous to the ACC. So I connected the hose that fits the bottom of the pump directly to the ACC housing.

I measured the potential across the plug that mates with the auxiliary water pump with the motor running, both with defrost on (max heat) and with the temperature setpoint below ambient (ie cool air blowing) and am getting voltages on order of hundreds of millivolts.

A couple of questions:

1. Is the aux water pump toast? I assume yes, and not reparable.
2. Is the ACC toast? Seems to be functioning but I was expecting higher voltages across the plug that mates with the auxiliary water pump.

Thanks for the advice.

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Rob
1979 300SD W116

Checking up on this, could someone confirm I'm on the right track?

Thanks

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Rob
1979 300SD W116

Following up on this, I ended up replacing the aux water pump with a new unit. Before installing I checked the current draw on the new component when hooked directly to the battery, was about 0.5A.

It should be working now...haven't seen the water pump switch on because from what I understand there's a temperature switch that activates it and the weather's still pretty warm out. The location of this switch is a mystery to me. According to the manuals on w116.org it's supposed to sit on/near the manifold or something...only switch I see here is the overboost protection switch. The person who sold me the pump said it was the switch that sits on the theremostat housing, but that's water temperature not air temperature. Maybe it's under the dash, this makes a little more sense to me?

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Rob
1979 300SD W116

I'm familiar with 123's and 126's only, I'll be happy to send any info, pics via regular e-mail, PM me.

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83 SD

84 CD