HZJ75 HVAC issues (first AC went out and now heater blower) (1 Viewer)

[likethewheel]

'93 HZJ75. On a recent road trip, my AC up and stopped working. The blower motor was still blowing air but it was not blowing cold, just ambient temp. It's a new OEM Denso compressor and is fully charged up with refrigerant for what it's worth. Fast forward to a couple of days ago and now the heater blower won't come on at all. Nothing. Dead as a doornail. Wondering if the two issues are somehow inter-related perhaps? Have searched extensively and come up dry. Any help is appreciated!

 

[RhinoRig]

Sounds like a short somewhere. Or bad ground. Start with the fuse box and check for the obvious. Grounds are on the fenders and foot lock area. I had a tiny fire that wasn’t visible until I pulled the whole box and looked behind. Never did figure out what caused it but the blinkers worked again.

What’s your experience level with electrical diagnosis, diagrams, etc? It’s a simple system, but…

Lots of relays to check. My old girl has one sensing water temp just to make the compressor turn. They don’t go bad often, except for when they do.
Blower motor has one. Compressor has several.

Does the compressor come on? If not, Can you hot wire the compressor and check for cold? Is there still pressure in the system?

 

[jblueridge]

You should be able to run a wire direct to the compressor clutch, apply electricity and watch the clutch to see if it engages.

 

[JuanJ]

Hello,

This assumes you have no leaks, no blown fuses and no blown fusible links.

The fan switch tends to accumulate dirt and lose electrical continuity in old age. So does the AC controller. As a result, they cannot operate properly.

A soak with contact cleaner helps.

Take the switch out and check electrical continuity. Connector pinup is

2 1
8 7 6 5 4 3

Pins 1 through 6 control the fan.

Pins 7 and 8 control the AC circuit.

Check for continuity between

1. Pins 2/3/4.
2. Pins 2/3/5.
3. Pins 2/3/6.
4. Pins 1/2/3.
5. Pins 7/8.

If there is no continuity, replace the switch.


It is also possible that the pressure switch is not working.

Remove the glove box. Disconnect the AC harness, it is a two-pin connector. Check continuity between them. Replace the pressure switch if there is no continuity.

Hope this helps.





Juan

 

[Honger]

Hello,

This assumes you have no leaks, no blown fuses and no blown fusible links.

The fan switch tends to accumulate dirt and lose electrical continuity in old age. So does the AC controller. As a result, they cannot operate properly.

A soak with contact cleaner helps.

Take the switch out and check electrical continuity. Connector pinup is

2 1
8 7 6 5 4 3

Pins 1 through 6 control the fan.

Pins 7 and 8 control the AC circuit.

Check for continuity between

1. Pins 2/3/4.
2. Pins 2/3/5.
3. Pins 2/3/6.
4. Pins 1/2/3.
5. Pins 7/8.

If there is no continuity, replace the switch.


It is also possible that the pressure switch is not working.

Remove the glove box. Disconnect the AC harness, it is a two-pin connector. Check continuity between them. Replace the pressure switch if there is no continuity.

Hope this helps.





Juan

That pressure switch is no fun to change out... you'll need to empty the refrigerant and remove the AC box out from under the dash.

Agree with all the other ideas here. One other culprit if everything else is checking out is your AC controller module/computer. That mounted inside your dash above your glove box... can't really access it without removing your glovebox.

 

[likethewheel]

Still chasing this down but wanted to give an update...

I don't mind replacing things when in doubt because the more new parts on this truck, the better off I am in the long term.

• Jumped the compressor and it turns on - great
• Checked all relays/fuses, all are good
• System is fully charged, all good there
• Replaced the switch on the dash - that wasn't it
• Next step is to replace the computer ("amplifier assembly") because that's easier to do than replacing the pressure switch
• Last ditch will be the replace the pressure switch

Hopefully can get this wrapped up soon ... just in time for winter

 

[likethewheel]

Found the culprit! one of the wires to the pressure switch temp sensor is severed right where the wires go into the connector that attaches to the temp sensor that goes into the block right in front of the injection pump. See attached pics. Anyone know a part # for this little wiring harness? It's just one little bit of harness about 2' long with only 2 wires and quick connectors on both ends, making it very easy to replace. For now I've managed to re-splice the wire back together but given how close the break is to the connector I wasn't able to patch it up reliably and the wires are brittle anyway so best to just replace and be good for another 30 years.