Hi guys and gals!

I was driving home tonight, and had the CCU set in auto, with the temp between 18 and 22 C. I was alomost home, and the heat came on full heat. I moved the wheel selctor to the Min detent. The blower ramped up , and the temp stayed hot. I opened the window and shut the CCU off. About 5 minutes later, I tried the CCU again, and left it at Min. It blew nice and cold. When I set the temp back to between 18 and 22, the fan slowed, and then the heat came back. Moving the temp wheel back to the Min detent did nothing, until I let the CCU rest again.

Any ideas out there on what to look for? I did replace the monovalve diaphragm assembly two years ago.

Thanks!

__________________
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

yuppppp this is what mine does. i got some answers in a thread i made but haven't begun troubleshooting yet. search for "what might cause this climate control problem"

Yeah, it's crazy, and hardly any info on that. I have not really seen a way to test the signal going to the monovalve, since it's a pulsed signal, and it need voltage to close. I'm wondering, if it's "gummed up", and there for not responding to the pulse in a timely fashion.

__________________
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

System worked fine at lunch today...

__________________
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

Yes, it needs voltage (well, current, actually) but what it really needs is ground. The PBU and the CCU (two different boxes) send a ground path to the monovalve to make it close.

If you want a "real time" monovalve voltage differential indicator, you can wire an LED across the terminals and display it in the cabin.

A better course of action may be to check the solder joints on the PBU.

Here is more craziness...it's still working...

I'll try the LED trick. It would be good to see if voltage is there, when the air temp is hot. If it is, can we assume that the monovalve is indeed faulty?

__________________
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

If it's working, (i.e. not getting heat when none is called for) then the monovalve is probably okay. The easiest way to verify the physical condition is to just unscrew the four screws and inspect it.

The difficult part for troubleshooting happens when it's physically okay, but still acts up. That's when the electrics kick in. Your description sounds more like solder joints in the PBU. Possibly the temp wheel. Outside chance at temp sensors. How's your foam tube?