1969 Mark III ATC Questions

Hi Bill,

The ATC works as it should on my ’69 Mark III with the exception that when selected to HIGH, if I accelerate hard, the fan drops off and only returns once cruising speed is reached.
This only occurs on HIGH.
Set on LOW, on hard acceleration, the low fan speed remains constant.
In have looked for vacuum leaks and found none.
Hope you may be able to assist. much appreciated if so.

Kind regards,

Deyan

Hello Deyan –

What you describe sounds like a classic vacuum loss issue. On HIGH position the system uses more vacuum as the system may select to recirculate the air from within the car to provide maximum cooling for the interior. I suggest that you “T” in a vacuum gauge into the A/C feed control vacuum in order to observe the readings in  the LOW and HIGH positions under acceleration. This will prove or disprove loss of vacuum for you. The factory service manual with complete a/c  vacuum circuits shown is very necessary to properly understand, diagnose and repair these units. Please let us know what you find.

Sincerely,

Bill

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Bill –

Thank you for your response.
I tee-ed a vacuum gauge into the main vacuum feed line near the reservoir and at idle the reading was 15 in Hg on both Low & High settings and bringing the revs up made no appreciable difference…of course there was no load as would be the case when driving.

It seems to me that there may be a vacuum leak either at the firewall plug or beyond, inside the car.

The vac reservoir and the check valve all check out ok.

Next, I will use a smoke machine to trace the leak.

Any other suggestions greatly appreciated.

Sincere thanks,

D

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Deyan –

When you tested the vacuum using the gauge without driving the car did the problem occur? The vehicle should be driven exactly as you indicated in your opening question and the gauge observed while driving at this time when and if the problem occurs. The gauge must of course be teed into the vacuum line at the controlled side of the vacuum valve for this test. When the engine vacuum drops as it normally does during heavy acceleration, the valve will close and not allow the saved vacuum to return to the engine. If the vacuum at the gauge does drop at this time and the faulty issue occurs , the valve could be faulty or the reservoir or some other component in the vacuum circuit must be leaking vacuum to cause this.

Sincerely,

Bill