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1970 Mark III AC Issues

Hi Bill –
I have been working on this 70 MK III doing some minor restoration work for my customer so he can drive it this season. Everything has gone well except for when it comes to the HVAC blower motor operation. The issue is it does not work no matter what position I have the controls in. I have the factory manual but am having some difficulty making heads or tails of the power flow in the system. I can tell you my 30A breaker is powered and intact, the blower motor runs smoothly if 12 volts is jumped to the org/blk wire at the resistor. I pulled the resistor and checked it with ohmmeter and visually. OK there. The only power I have at the resistor is on the violet-white and red-white wires when the system is on but I believe that circuit relates to AC operation. I am concerned I may have a problem with the power servo switch assembly as it does not hold vacuum, but the ACC control box never applies vacuum to it anyway regardless of con trol head position. I do have vacuum at the top vacuum line on the ACC box. Anyway, any feedback you may have regarding blower operation would be greatly appreciated. Hope the info I gave helps.
Hi Chris –
These ATC controls are complicated and difficult even if you read and understand your correct FoMoCo shop manual. If you cannot use the manual you are only guessing.
The manual shows a flow chart from the dash switch. The blower circuit can be traced to a relay, the servo and the resistor etc. Your description of the vacuum issues lead me to strongly suspect some problems with the servo and the ATC box. We overhaul many of these two units as a package here at Lincoln Land. If the system has been hacked by a previous owner or mechanic the diagnosis and subsequent repair becomes more difficult. Let us know if what you find and if we can help further.

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