I have a 1989 Lincoln Town car that blows cool air when it wants to. I read the post from Justin in VA ( Permalink) which is pretty close to my situation except my car’s AC would turn on randomly, but most likely at start up. Here’s the basis for all the following scenario: it is 85 degrees outside (and probably hotter in):
If I cold start the car, leave on Park, turn the AC on (setting on Panel) with the temp set to lowest, the compressor will very likely cycle once, maybe twice, (cold air through the vents) then stops cycling (then 85 degree air vent plus some humidity). Switching back to Vent then back to AC/Panel, the compressor won’t cycle.
If I redo the scenario 1 by turning the engine off then on, the compressor won’t even cycle once.
If the car is running and I am at constant speed, and turn on the AC, I get the same scenario as above. The compressor just won’t cycle more than twice.
Please note that once, while I was testing scenario 1, going from AC/Panel to Vent on the climate control dash, the car stalled.
I think I must have a vacuum leak, but then, if I did have a leak, wouldn’t the compressor keep cycling at least when I am on idle (scenario 1), like Justin in VA?
I have tested the following for leaks:
– Vacuum canister (aka Coffee Can): perfect vacuum seal; holds 25 inHg for minutes…
– Cruise Control servo: bad leak there; have isolated by sealing the hose going from vacuum manifold on the firewall to the latter, re-tested scenario 1 with same results. I will replace the servo anyway but looks like it is not the issue there.
– Hose going to coffee can from firewall: odd seal here. Anything above 5 inHg is immediately lost, then vacuum is held at 5 inHg pretty well: should it be tight at higher vacuum?
– The actual AC freon pressure (134a) is super tight with the right pressure on both High and Low at the end of a compressor cycle
Of course, documentation is hard to come by so anything you can send my way would be greatly appreciated. Here are some questions you may be able to answer:
– Define holding a vacuum: is loosing 1 inHg in 1 minute qualifying as tight?
– I have located one check valve on the AC on the accumulator. Is there another one?
– What actually triggers the compressor to cycle? is there a vacuum servo in charge of that?
Thanks in advance.
Do you have a set of the correct Factory Shop Manuals, a set of AC gauges and prior refrigeration experience?
Thanks for responding so fast. To answer your questions:
Q1: no unfortunately I don’t have the manuals. I had the car for less than a year now. The previous owner did update from r12 to r134a most of the ac high and low pressure parts. I still have his phone number. I will give him a call. Maybe he has an idea.
Q2: I would say I am intermediate level. I know more than the basics and the equation of perfect gas PV=nRT.
Q3: yes I do have AC gauges with the proper r134a adapters. I can monitor both high and low pressure when that silly compressor is kind enough to run.
Let me know what you have in mind. I can take pictures and/or shout a small movie for better diagnostics. Let me know what works.
Again, thanks so much.
Hello Nic –
Thanks for the additional information regarding the a/c on your 1989 Lincoln Town Car. I will advise you that unless you have plenty of time on your hands, you will not get too far without a proper Shop Manual. These manuals contain wiring diagrams and vacuum schematics for the Automatic Climate Control as well as how most of it operates etc. There are two basic sections for the HVAC system. They are a control system and a refrigeration system.
I will answer your questions from your first email to us. Loosing 1in of vacuum 1 minute after engine shut off is not excessive. Loosing 1in per minute however will soon deplete the vacuum which may indicate enough of a leak to cause operating problems at certain acceleration levels. The accumulator tank does not have a vacuum valve on it. The vacuum check valve for the controls is located on the engine side of the firewall at the rear of the engine. The a/c compressor clutch normally cycles on suction pressure according to system refrigerant pressures. This cycling switch is located on or close to the accumulator tank. This sw. also serves as a low pressure cut off sw. and an ambient cut off sw. during cold weather. There is also a Wide Open Throttle relay that will disengage the compressor for certain acceleration demands. I have no idea if your refrigeration system has the correct charge of refrigerant nor do I know the system running pressures. We hope that this information helps you decide to get a hold of a maintenance manual.