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88 Grand Marquis AC problems when in drive
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Vvandenn
New User
Jul 26, 2009, 1:53 PM
Post #1 of 4
(5172 views)
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88 Grand Marquis AC problems when in drive
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Hello, I have a 1988 Mercury Grand Marquis LS with around 150,000 miles on it. Many years ago we replaced the AC compressor and the AC had been working fine up until a few months ago. Currently, the AC only blows cold air when the car is idling, in park, or otherwise not accelerating. Once I accelerate, it blows hot. We suspect it may be the vacuum lines but there are so many lines that snake through the car that it's a bit difficult to diagnose. Could it be anything else besides the vacuum lines? Thanks for any help.
- Sean
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Hammer Time
Ultimate Carjunky
/ Moderator
Jul 26, 2009, 8:55 PM
Post #2 of 4
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Re: 88 Grand Marquis AC problems when in drive
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I would say it could be anything BUT the vacuum lines. They only control the mode doors and where the air comes out, not the temp. The temp control uses a cable but varies depending if you have auto temp control or not.
The first thing you want to try is to pinch off one of the heater hoses and see if that changes the air temp or not. That will tell you if you are dealing with a gas system problem or and air control problem.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jul 29, 2009, 3:42 AM
Post #3 of 4
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Re: 88 Grand Marquis AC problems when in drive
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Hammer is correct about usually not an obvious vacuum leak but could be! Some of those came thru or folks added a compressor cut out when under load as those 5.0s are gutless with A/C off never mind on. Make sure the reservoir can isn't leaking - looks like a juice can or food can on driver's inner fender shield, underhood. Take the hose off it a few minutes after shut off and it should release vacuum. If not they rust on the bottom and can be fixed or a generic used to replace. I already know new OEs are not available.
If leaking bad enough to trigger cutting out compressor you should also notice the air inside default to defrost ducts or I doubt a vacuum leak is it.
The black wire yellow tracer from relay to to low pressure cut out can be run direct if a load cut out is on this vehicle sensing intake manifold vacuum as the info that there's a load on engine. The wires run out of sight down the back of engine in harness so it's hard to find what device is opening the flow of juice to cut out switch. I own an 88 that factory cuts out but takes a lot of load and just ran a wire neatly as mentioned. VERY few 87-89s had that load thingy but some did or could have been added and device failed or a leak - either possible.
Also, if the car was retrofitted to 134a a high pressure switch may be cutting out. OE wouldn't use one but you're supposed to add one on conversions to 134a. If so a clog in O tube would cause excessive high pressure simply with RPMs but regardless of load.
Need to know whether this is only load or raised RPM and pressures taken if not the first issue mentioned,
T
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Hammer Time
Ultimate Carjunky
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Jul 29, 2009, 3:59 AM
Post #4 of 4
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Re: 88 Grand Marquis AC problems when in drive
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After a little research, it turns out that this year does use vacuum to control the air doors only if it's an auto temp control system. The manual system uses a cable.
Here is the vacuum diagram
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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