116 (450SEL) HVAC Amp, Reg-Valve, or.....

[JRBrown]

Folks,

HVAC won't maintain a set temperature.

Ran through job 601 (Quick Test without tester) and zero faults. Works like a champ. BUT, it doesn't evaluate "quality"

Moved on to job 609 (Testing ambient, in-car and potentiometer). They all check within limits.

Checked In-car sensor for air flow pulling through. It checks okay.

I'm left with job 611 (Balancing of sensor chain). I suspect it will check okay since it simply measures the three resistors in series.

So, assuming I'm stumped to here, is it:
Amplifier (any quick checks)
Regulating Valve (only 3 yrs old) (any quick checks)
Or, what else?

Obviously I don't have the tester. I was thinking of locating the pin that sends control voltage from the amplifier to regulating valve and then run a "quality type check to see what voltages are being sent. I am aware that the amp can send out of spec voltages that can damage the reg valve.

Thoughts and help very much appreciated.

J.R.
80 450 SEL
86 280 GE
81 Vovlo