96 Legacy AC works with defrost, but not other vent settings

[JPX]

My 96 Legacy L can turn on the AC compressor and cool when the defrost position is set and the blower turned on. In a 96, the defrost+blower will automatically turn on the AC switch light whether or not the AC button is depressed.

 

But when set to any other vent position, the AC switch will not do anything. No compressor engagement.

 

I did a very thorough continuity check of the PCB of the HVAC control panel. All fuses and relays have been pinned and checked......which would have been unlikely problem spots since the compressor does indeed turn on and cools.

 

I can see in the wiring diagram how the regular vent positions+blower+AC would be gated by the low pressure switch.

 

In looking at the wiring diagrams in the service manual and Haynes books, it appears that the defrost setting might be able to bypass the low pressure switch? Am I reading that wrong?

 

If this were the case, then a low refrigerant charge would fault out the vent+AC, but could possibly still allow the defrost+AC to work?

 

Can anyone think of a reason why defrost+blower=AC, but vent+blower does not?

[Legacy777]

Without the diagrams I can't confirm or deny what you may have found.

 

However, you should be able to get a recharge kit with gauge to check the pressure without the compressor running to see what it is. If it's less than about 30 psig, then it's probably too low.

[JPX]

So I jumpered the pressure switch at the receiver/dryer. No change - still won't try to turn on compressor at vent+AC. Pull it on defrost+blower and it will stop the compressor.

 

I also checked continuity of the thermoswitch on the evaporator behind the glovebox. Pull it out during Defrost+blower and it will stop the compressor. Put in back in and the compressor turns on. Again, no change to the vent+AC which still doesn't work.

 

Outside, this evening's temperature dropped suddenly into the high 40s (San Jose, CA). So it was difficult getting a decent reading of pressure on the low side. Will have to wait until it warms up.

 

I'm having some doubts about the control panel again. During my LED replacement, it could be possible a trace got damaged or there is a short. Will take it back out and test it some more.

[Legacy777]

Yeah....I would check the circuit board again.....sounds more like that could be the problem.

[Fairtax4me]

Yeah I'm looking at the wiring diagram in the FSM for 95 and I don't see anything that would make it work on defrost, but not work if the button is pressed.

The line from the mode control panel joins with the line from the switch, and then goes through an evaporation thermo switch, and the pressure switch before going to the AC clutch relay.

I'd kinda suspect the switch to be honest. But a break in the wire somewhere between the switch and where it meets with the wiring from the control module is a possibility as well. Perhaps the connector on the switch is loose?

[JPX]

Thanks for the interest in this guys.

 

Yeah, I took a closer look at the diagram and can see how the AC relay still "listens" to the evap thermo switch and the pressure switch in either case.

 

The button itself checks out on all three of its terminals.....which I would expect since it does light up when the button is pressed AND the defrost+blower is going.

I've been looking for a good excuse to hit a junkyard this weekend anyway. So this is just enough of a reason to go as well as feed the Rabbit (89 VW Cabriolet).

 

At this risk of going off topic, the yard is next to an alternator shop that carries the electrical contacts for the starter plunger. Replacing these has been working out great for me so I can avoid buying solenoids or starter assemblies.

 

Which points to me being a curious cheapskate because there is still part of me that wants to figure out what is wrong with the control panel. Traces on the PCB are good, but I'l lhave to check them again. Maybe a bad capacitor or the transistor by the recirc switch?....who knows?:-\

[JPX]

Well, I dove back into the dash to pursue this problem further.

 

One thing they don't tell you in the manual:confused: is that the continuity checks might give OPEN per the diagram. But if you swap the test leads on your meter, you will detect continuity. This is because of several diodes (one way valves essentially) are mounted on the board. The vent selector portion of the control panel still checked out okay.

 

But something about that AC switch was not sitting well with me. When I ran the continuity check last time, it was working. This time it did not test okay on pins 1+2 or 2+3.....only 1+3.

 

So I dismantled the switch down to the inner guts - a spring, a 3-way contact plate and the three copper traces on the main board.

 

The contact plate had a bent center contact point while the other two were okay.:-\ This was mostly the reason for the 2+anything not working on the continuity check. And this was the culprit. So I straightened out the bent contact plate pin. The traces for the contact plate were not as bright as the rest of the copper. So I took a pencil eraser and brightened them up for good measure.

 

Upon reassembly, the AC button lights up and turns on the compressor in all the vent settings correctly. Perfect timing as it has become quite warm this week and the wife will be happy to have AC working correctly.

[Legacy777]

Good deal!

 

Thanks for the updates and perseverance on troubleshooting the problem

[Fairtax4me]

Hooray for AC! Glad you got it fixed.