A/C Issue

So about two years ago the compressor on my '95 stoped working, Last week
I replaced the evap,compressor,drier,expansion valve. Pulled a vacuum ( held at -28) and hooked up some gauges. im able to add refregerent charge to the truck but the new compressor doesn't kick on. I jumped the compressor to the batter and it engages just fine and the PSI on the low side drops to 1-5 from 45. however no matter how much charge i add the compressor wont turn on.

The a/c light will not engage when i push the button, but if i disconnect the ac amp the a/c button lights up.

Is the amplifier the most likely cause? or thermistor/low pressure switch? Is there a way i could bypass the amp on the harness and see if those components are the root cause?


Thank you,

 

First, your low-side pressures suggest it's undercharged. With the compressor engaged it should be about 40 - 50. Without knowing the high side pressure it's going to be hard to diagnose. With the compressor engaged, it should be around 220 on the high side.
As for the pressure switch, if it's like my '96, the switch is inside the evap-case, so any pressure-switch bypass would need to be where it's wiring harness connects to the AC control board.
Another note - the new R134 cans have a check-valve. You may need an adapter to use them with an older charging-hose/gauge setup.

Happy Motoring, Mark

 

I'm fighting a similar issue with my '99 Tacoma. A/C button switch light does not come on, compressor clutch does not engage. Blower motor does run. I've checked the Heater relay and the A/C button switch, both are good. With the A/C button switch removed I'm not getting 12V at pin 6 of at harness. A/C fuse is also good. Can the pressures be checked without the compressor clutch engaged?

Found service manuals in the "DIY" section maybe it will help you.

 

So with the compressor off it's 70/80 low/hi. Engaged it's 5/115. The charge is Def low, but the compressor should kick on to allow feeding into the rest of the system by then right? I'm thinking something electrical is in the works with either the AC amp/relay or low pressure switch

 

this sounds like there are 2 problems, one electrical, duh and from your readings it still looks like and undercharge. LOTS of bubbles in the sight glass?

 

Yeah the charge is low with bubbles in the glass. Is there a psi threshhold on the low side that the compressor should kick on? surely by 70 it should be engaging?

 

yes, what it is I do not know atm. Usually mid 2nd can the compressor kicks in for me. Is this accurate... no but it works. I understand charge levels but for the one time a year that I have to evac, repair and recharge watching the site glass as the compressor kicks in is good enough for me.

 

The pressure switch is set at 10 psi. If its less than that the clutch won't engage. I think you are low yet.

 

Before the compressor kicks on the pressures should be close to the ambient temp give or take a few psi. Low side should be between 10-30 psi with the compressor on. You may need to use the compressor to suck more refrigerant into the system. Or have a shop vacuum and charge it for you if you're not sure what you're doing. Running the compressor with low refrigerant can cause it to fail, I would not bypass anything. It sounds like the low pressure switch is not allowing the compressor to kick on because you are low on refrigerant

 

You need to make sure the low side gauge is open while you're charging and the compressor is on so that the system can suck the refrigerant from your gauges

 

High side is closed low side is open, the readings without the compressor running is 70/80 and drops to 10/115 when i Jumper the compressor to the battery. The compressor should kick on to draw more charge in without having to manually jump it right?

 

yes. As you add refrigerant, the pressure will increase enough to allow the clutch to engage, thus drawing in more refrigerant.

 

yes, if there isn't enough in the system, the compressor won't turn on in order to keep it from grenading itself.

 

Not if the low side switch is keeping it from turning on.

 

I vote the switch/sensor as well, but where is it located again?

 

On '96 - '99 Tacoma, it's inside the evaporator case. Haven't checked one in a '95 so I can only assume it's the same.

Happy Motoring, Mark

 

well i got a replacement low pressure switch, will change it out and have a shop charge it this weekend. Fingers crossed getting into 90+ here allready

 

Switch changed and Recharge done at shop. PSI is sitting at 31low and 205 high. Low pressure switch changed and tested, Thermistor tested and cleared. Ac Amp tested as working per the FSM. The only two parts i havent checked is the ac switch and the heater/blower relay. The switch is sending 10V through the amp harness when pressed and and when deactivated is sending 0 volts.

Last thing it could be is the relay at this point? Im about out of ideas and its going to be 100+ next week :-\

 

So, is the compressor AND the blower not working?
I just checked my '95 - '96 Tacoma heat/AC wiring diagram.
It looks like a bad heater relay would kill the blower, along with the AC.

10 V sounds low - unless that voltage reading is coming from the third terminal of the AC switch - which is the indicator LED & resistor to ground-circuit terminal.
Full battery or alternator voltage goes through the heater relay, through the heater fuse directly to the blower and the AC fuse, then through the AC switch directly to the AMP.
The AMP sends that voltage to the compressor-clutch.
AC switch to AMP - BLK+BLU wire - 12-14 V.
AMP to compressor-clutch - BLU+YEL wire - 12-14 V.

I don't have voltage or test values for the two AMP to ECU wires, or the three thermistor wires.

Happy Motoring, Mark

 

The blower and speed selectors all work, the only thing I'm not getting is the compressor to engage.