Hi, How hot is the air commig out from CCU in a 85 300sd ? Mine only reaches 80-100 deg. or so ! mono valve has no holes and the coil is good ! What next ? Thank you. J

Check the auxiliary water pump, low down on the right front inner fender. If it doesn't work, water flow through the heater core will be very slow and could produce your symptom. With the wires to the pump disconnected and the engine off, apply 12 volts to the pump connections (polarity doesn't matter for this test only). Listen for the pump running. If you cannot hear it, the pump has failed.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Are you sure ? I cant find one, On my car,both lines from the water pump go right to the firewall bulkhead. No Pump! thanks J

on the 126 it is not on the fenderwell. the aux water pump is behind the false firewall. follow the 3 hoses leading into the cabin and they lead to the aux water pump. the hose(s) from the monovalve lead to the pump to. if im not mistaken

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1981 300 SD 213k miles "Stock for now"
1999 Super Duty 7.3L 113k miles
1981 300 SD 180k miles "Heavily modified" SOLD

How is your thermostat? I think mine is stuck open or missing, as the tem guage does not make it to 80c when the temps a below 50 degrees F.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

The aux. water pump in on the inside of the main firewall. Look for the metal bracket on the firewall with a bolt in it. Take the partiton out of the inner firewall then you can access it better. The plug for the pump is also near the firewall to the left of the monovalve.

Hope this helps

My vent temps reach well over 130F, i think it was 138 F or something. Maybe a W126 needs the water pump more. I think the biggest purpose of the water pump is to prime the coolant lines in the event that they get drained. However this is easy to do with a 7.00 hand pump. If there is air in the circuit, the water just wont flow down that circuit. I get good coolant flow all the way back to my trunk to heat the veggie tank without a pump. I question the necessity of one of these 150.00 pumps and I have never had one that worked on 3 cars now

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

When mine didn't work (due to a flaky CCU) I would notice a lack of heat at idle only, but the heat would be OK as soon as the engine speed increased.

The temperature of the outlet air is dependent on several factors.

The most important is the difference between the ambient temperature and the cabin temperature. If this difference is only 20F. or so, you won't get anywhere near 130 degrees from the outlets.

The SD hasn't provided anything above lukewarm since last winter. It hasn't been cold enough.