1989 A/C issue

[JohnQ]

Need some help.  I tried searching but I haven't found anything to help my issue yet.  I have a 1989.  The AC has not worked since I've owned the truck (6 years) and probably for a lot longer.  I have all new hoses, compressor, condenser, dryer, expansion valve.  Factory evaporator.  My AC has been leak tested and charged by a local AC shop.  They could not get the AC compressor to come on by normal means.  They had to jump it to charge the system.  Through my trouble shooting I have verified that I am getting power through my low pressure switch and power is getting to the AC relay.  From there nothing.  I can not get power to the Compressor plug.  Relay has been verified to be good.  I swapped the thermistor with a spare.  Same issue.  Bought a new thermistor (I hope its the right one) but waiting for it to come in the mail.  I can jump the terminals in my AC relay plug and the electric fan and compressor turn on.  Any idea what else could be wrong?  

[jdog]

Have you installed another relay, what it sounds like is the relay is bad. Just cause it test good doesn't mean it's good under a load

[MJCARENA]

So you have power to both terminals of the relay 85 and 87.

[JohnQ]

57 minutes ago, MJCARENA said:

So you have power to both terminals of the relay 85 and 87.

I only get power to 87. 

[JohnQ]

Just now, JohnQ said:

I only get power to 87. 

Where does power to 85 come from?

[Ωhm]

Pin 85 is a Fuel Pump Relay feed (B+). With KEY ON you may only have B+ for 2-3 seconds only. Try taking reading with the engine RUNNING.

[JohnQ]

5 minutes ago, Ωhm said:

Pin 85 is a Fuel Pump Relay feed (B+). With KEY ON you may only have B+ for 2-3 seconds only. Try taking reading with the engine RUNNING.

Getting power to 86 when running and power to both when the ac is switched on

[Ωhm]

Is AC relay installed? if so, remove it and retake B+ readings on Pins 85 & 87. Engine RUNNING and AC selected.

[JohnQ]

26 minutes ago, Ωhm said:

Is AC relay installed? if so, remove it and retake B+ readings on Pins 85 & 87. Engine RUNNING and AC selected.

No AC relay installed. Power to 87 only with ac switched on. No power to 85. Power to 86 with jeep running. Power stays on with jeep running.  Standing by passenger fender 85 is on the right and 86 is on the left? 

[JohnQ]

Also tried pin on right side no power at start up either. 

[Ωhm]

I'm using the 88 Electrical Manual, which has been known to be wrong. So In order to keep us on the same page check out page 37.

_mj1988electricalmanual_1.pdf

 

If you show Pin 86 having B+, then maybe Pin 85 is the grounding leg for the AC relay.

 

[MJCARENA]

My truck is the same. Power on 86. The manual might be wrong. If you take your test light and hook the lead up to positive on the battery and probe 85. The ECU provides a ground to pin 85. Engine run AC on.

[JohnQ]

1 minute ago, MJCARENA said:

My truck is the same. Power on 86. The manual might be wrong. If you take your test light and hook the lead up to positive on the battery and probe 85. The ECU provides a ground to pin 85. Engine run AC on.

I have tried 5 different relays. Same results.  Maybe the problem is in my ECU?

[MJCARENA]

Could be your thermostat switch behind the glove box. Would have to get to it from underneath the dash.

[JohnQ]

3 minutes ago, MJCARENA said:

Could be your thermostat switch behind the glove box. Would have to get to it from underneath the dash.

I swapped mine out with a spare I had.  Same issue. I ordered a new one but still waiting for it to come in the mail. I don’t have any power going to the thermistor plug.  Ac on/off doesn’t get any power to the connection. 

[MJCARENA]

Looks like your issue. Need power to it on one side. Looks like light green wire according to the manual.

[JohnQ]

1 minute ago, MJCARENA said:

Looks like your issue. Need power to it on one side. Looks like light green wire according to the manual.

Any idea where that power comes from?

[MJCARENA]

Do you have the electrical manual?

[JohnQ]

18 minutes ago, MJCARENA said:

Do you have the electrical manual?

Reading through the 88 version that was shared earlier

[JohnQ]

So if my ECU isn’t sending power to the thermistor then I need a new computer?

[MJCARENA]

Page 37 shows the circuits. Page91 shows C238, C120, and C100.

Page 90 shows C239.

Pages 75 thru 84  shows the connectors and a brief description where they are.

Hope this helps.

[MJCARENA]

9 minutes ago, JohnQ said:

So if my ECU isn’t sending power to the thermistor then I need a new computer?

No, power from the thermostat switch sends power to the ECU.  On page 37 power starts at the top st the fuse. Then goes into your mode select switch. From there it goes to your low pressure switch. And the goes to three places, one of them is your thermostat switch. Your thermostat switch sends power to the ECU. The ECU them will send a ground to your ac clutch relay to turn it on.

[Ωhm]

8 minutes ago, JohnQ said:

So if my ECU isn’t sending power to the thermistor then I need a new computer?

No it is not a output circuit. ECU C201_C2 is a input circuit. It will either see voltage or see no voltage. Its a voltage sense circuit. Same thing applies to ECU C201_D2.

[JohnQ]

Thanks guys. I will try and find out why I’m not getting power to the switch tomorrow. I appreciate the help. 

[JohnQ]

Still can’t get power to the darn green wire on the thermistor. Checked all wires visible without taking the dash off. No damage. Everything looks to be in good shape. Even changed the hvac switch. Nothing. Next step, pull the dash and see what the rest of the harness looks like.