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Air Conditioner

Vnomous1

By Vnomous1,
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Vnomous1 Newbie

Vnomous1

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Gang,

 

Been churning away at the 88 Durban automatic. Everything is going well, well except for the A/C (of course). All connections, relays and grounds seem to be fine. If I add power directly to the compressor, it fires right up and will run and the system produces cold air, which of course is good. Bad part is, the system (compressor) will not engage automatically as it should. The A/C fuse is good and there is power to the blue+red wire on the compressor relay, but none on the black+white wire.

 

The FSM depicts an interface with the ECI just before the compressor, and I am thinking that the ECI somehow allows the clutch to engage (get power via the black+white wire). I was wondering would the ECI prevent the compressor from engaging? Has anyone else had this symptom/problem and resolved it?

 

The wiring harness is not hacked at all. As always thanks for your input.

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vbrad511 Newbie

vbrad511

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Check the high/low switch behind the header panel in the line. There are two of them, one with a removable plug..that's the beast. Unplug it and put a jumper wire between the terminals in the harness end. If the compressor fires, there's your trouble. You might get lucky and just have some dirty connections. Mine's touchy, and the one I had in it before this one quit completely.
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mikec Newbie

mikec

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Two sensors typically keep the compressor clutch from engaging:

 

1: "dual pressure switch" which is what vbrad511 described. It's at the very front of the car, screwed into the refrigerant pipes near the headlight. Look for a 2-wire connector in the typical Mitsu "T" shape. With the A/C system set to "on" jumper this connector; if the clutch engages then:

a: either your system is really low on refrigerant pressure or really high on pressure

b: your dual pressure switch is junk.

 

The Dual pressure switch protects the compressor from too-low refrigerant pressures when the A/C is off; that happens when there is low/no refrigerant (and thus no oil circulating with the refrigerant). It also protects the A/C pipes from excessive pressure if something goes stupid in the A/C system: blocked expansion valve, excess pressure, etc.

 

2: The other sensor/switch, and the one that fails most often, is the engine coolant over-temp sensor. This sensor supplies the ground to the compressor clutch relay. It's located in the thermostat housing - there are several sensors in that area; this one is the highest UP one (and the only one actually IN the thermostat housing). Unplug the connector going to this sensor. Use an ohmmeter to measure the sensor resistance to ground; it should be zero ohms. I'll bet it's OPEN though. If so, try twisting the connector tang... if it turns the sensor has rotted internally - typical failure. For now, just ground the connector to re-enable your A/C system. This switch is supposed to open when the engine is about to seriously overheat, cutting of the A/C.

 

The A/C system is supposedly tied into the ECU; when the A/C computer commands the compressor to turn ON a signal is sent to the engine ECU to warn it - a big load is about to hit. If the engine is at idle, the ECU is supposed to raise the idle RPMs via the normal ISC mechanism. While it's doing this, the ECU "holds off" the A/C compressor for about half a second to give the idle a chance to build up. That's what the factory service manuals claim anyway. The factory service manual schematics are wrong in this area; they show a transistor in the compressor clutch wiring; this transistor is controlled by the ECU. Except the way the schematics are drawn can't be correct: if this transistor were ON it basically shorts out the clutch on/off signal to ground. That'd fry that transistor and/or blow fuses. I've never tried to trace the actual wiring.

 

mike c.

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Vnomous1 Newbie

Vnomous1

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Thanks guys!

 

I did the jumper wire routine and the clutch did not engage. When the compressor is on, there is the typical difference in engine RPM, to handle the increased load, which seems ok. I will double check everything and replace the temperature (over heat) switch, (I got a new one around here somewhere) and see what I get. I will post the results today.

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strang3majik Newbie

strang3majik

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when you manually run the compressor, is the air working (ie, cold)? Do you have a pressure gauge monitoring both sides?

 

When I finally got mine back together it did that, turned out the expansion valve was bad and the clutch relay on the driver's side next to the battery.

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Vnomous1 Newbie

Vnomous1

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Well,

 

I went back out this morning and replaced the temp sensor on the t-stat housing and still no luck. I tried the jumper wire on the lower pressure switch again, and wha-la, the compressor kicked on. So, it looks like you guys were right, the pressure switch is the culprit. The sensor on the t-stat housing was bad also, as it would spin, luckily I had one of those. I do have a spare pressure switch, but it is old and condition unkown, so I will try to install a new one first. I found two at Rock Auto for 1.78 each (close out), so I ordered them, but will continue looking.

 

Anybody know another source, cross reference or like type that I can substitute with? Thanks guys.

 

Well, surprise, surprise, Auto Zone has two of them, so I am going to pick them up and install them and see what happens, will let you guys know.

Edited by Vnomous1
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Vnomous1 Newbie

Vnomous1

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Gang,

 

I evacuated they system and installed the new pressure switch. The system is functioning properly and producing ice cold air...around 38 degrees on the thermometer, just sitting in the garage at idle. The system is cycling too. Thanks for the help guys, I truly appreiciate it.

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t hall Newbie

t hall

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Question...

 

you evacuated the system... that's fine.

 

was there a shrader valve under the dual pressure switch that would isolate the system from atomsphere

and eliminate the need to evacuate the system?

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Vnomous1 Newbie

Vnomous1

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Question...

 

you evacuated the system... that's fine.

 

was there a shrader valve under the dual pressure switch that would isolate the system from atomsphere

and eliminate the need to evacuate the system?

 

Nope. if there is one, I didn't see it. Never hurts to evac the system anyways. working like a charm now.

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Contsi Enthusiast

Contsi

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Question...

 

you evacuated the system... that's fine.

 

was there a shrader valve under the dual pressure switch that would isolate the system from atomsphere

and eliminate the need to evacuate the system?

 

What do you use for Freon? Was the system converted?

Mine is original and not sure what to do; there is Freon in there but needs some charge.

87 TSi

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Vnomous1 Newbie

Vnomous1

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What do you use for Freon? Was the system converted?

Mine is original and not sure what to do; there is Freon in there but needs some charge.

87 TSi

 

 

On the premise your system is already fuctioning properly, but could use a recharge....

 

I used the new stuff (134); throughly evacuate the system. I added one can of 134 leak sealer and conditioner (refrigerant) followed by one can of regular 134 refrigerant (got both cans at walmart). That did the trick for me.

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  • 2 weeks later...
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wrngwae Newbie

wrngwae

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i too am having similar issues. i jumper the high/low switch and the fan turns on but not the compressor. i can put pressure gauge to the suction side of the compressor and it holds. i then jumper the compressor and the suction goes down as it should but no cold air????check the fsm and read in detail the a/c trouble shoot procedures. just stumped???? Edited by wrngwae
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Kenny G Newbie

Kenny G

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HELP! I'm in need of a TEMP SWITCH OEM PN# MB272705. this is the temp switch that sits in the thermostat housing and cuts the AC off when the car over heats. does anyone know where I can get one please? This part is no longer available through Mitsubishi or Chrysler. Any other PN's out there please!!!!!!!
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FORGOTENSTAR Newbie

FORGOTENSTAR

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question,, My A/C works fine but my 3rd fan does not come on anymore, it used to but not now, ran power to the fan and it comes on, any ideas what sensor makes it come on that I can check or replace, thanks
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Shelby Newbie

Shelby

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a/c system relays are by the igniter but the fan relays are under and beside the air filter houseing ,behind the pass head lamp , all fans are full time batt powered but turn on and off by relays that control the ground for the fan motors , you'l have to id the frt fan relay by wire colors,

all three fan relays look just alike

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