M1167 A/C trouble shooting

[DREDnot]

Trying to help a friend get his BAE style A/C to activate the clutch on the compressor. I have an electrical schematic im working with



My friend took it to an A/C shop first. They hooked up their A/C gauges to it and told him the hi and lo pressures were good (without the clutch running) but they didn't know the electrical system to diagnose anything further. So, I'm operating under the assumption that its not a hi or lo freon condition.
So far I've determined that...
There is constant 24v into and out of the two circuit breakers.
The A/C power switch works and turns on/off with the start switch in run.
The front blower works hi and low.
The rear evaporator/Blower assembly does not run but has power at the 720A pin at the cannon plug.
The ground wire to the compressor at 7J has continuity to ground.
There is no power at the compressor at 436 with the engine on or off in run.
There is no power to the high pressure switch 436.
There is 24v at the 445B connector at the right rear condenser assembly
There is continuity to ground at the 7H at the RR condenser.

Before I just jump 24v to the compressor as a fellow owner suggested Im not quite clear in what causes the rear relay to close and start the whole thing running when its operating normally.
My gut says the relay is bad but im not sure of the function of the water valve (on the engine?)
Im also not sure what gooes on inside the rear evaporator unit to send power to the main relay(415) that closes the relay
Or anything else I missed?

 

[AAVP7]

I´d focus on the front system initially. The compressor clutch circuit is quite simple. If you have 24 V at the blower switch, just check along the wire towards the compressor. All those switches have to be closed to activate the clutch. If you don´t get 24 V at the high pressure switch, one of the switches before that will be the culprit. Even if you have freon in the system, the low pressure switch could still be stuck.
Also, did you put the temperature switch in a "low" setting ?

The high pressure switch, by the way, works "normally closed", meaning that it is closed at ambient pressure, and only opens once a certain threshold is passed. Such a high pressure is usually only reached with the compressor running, and your buddy`s A/C shop checked the static pressures. Theoretically, a high pressure switch could also get stuck open, causing a break in the compresor clutch chain .

 

[Retiredwarhorses]

I’d find a new AC shop….you can really see the correct pressures on the high and low side without the compressor running.
The system should be evacuated and the appropriate 1.8lbs or 3.8lbs of r134a added back in, but understand, you can’t get that
amount of refrigerant installed without the compressor running, this will require you jump the low pressure switch, the best way to do this is to
plug the thermostatic flow control wires into each other, the compressor will not run and will short Cycle below 1lb of r134.
if the compressor does not run when jumped, you have an issue somewhere else…on the schematic it’s the temperature switch that gets jumped on the body side, just plug them into each other.

 

[DREDnot]

Thanks for the info Steve!
I bypassed the Temp Switch on the side of the front evap unit. It fired up the whole system!

Started blowing cold air out of the rear evap unit. The front was only blowing ambient(85).
I freed up the hot water valve there by the air filter canister but no change to the front temperature, hotter or colder. The front didnt get any warmer with the engine warming up to normal operating temp either.
All 4 rear condenser fans were operating....BUT....they would not shut off when the A/C switch was turned off...AND...they continued to run after the engine was turned off. They did not shut of after 10 minutes waiting for a delay or something. They only shut off when I disconnected the cannon plug at the rear Evap unit. They did not turn back on when immediately plugged back in. They did start back up when the engine was started again and the switch was turned back on.

Does that temp switch on the side of the front Evap unit control the refrigerant flow/orifice in the front unit causing the warm air up front?

Is the rear relay under the fuel filler tube sticking causing the fans to run after shutoff?
or is there something in the rear evap unit that would cause the fans to stay on?

 

[Retiredwarhorses]

Thanks for the info Steve!
I bypassed the Temp Switch on the side of the front evap unit. It fired up the whole system!

Started blowing cold air out of the rear evap unit. The front was only blowing ambient(85).
I freed up the hot water valve there by the air filter canister but no change to the front temperature, hotter or colder. The front didnt get any warmer with the engine warming up to normal operating temp either.
All 4 rear condenser fans were operating....BUT....they would not shut off when the A/C switch was turned off...AND...they continued to run after the engine was turned off. They did not shut of after 10 minutes waiting for a delay or something. They only shut off when I disconnected the cannon plug at the rear Evap unit. They did not turn back on when immediately plugged back in. They did start back up when the engine was started again and the switch was turned back on.

Does that temp switch on the side of the front Evap unit control the refrigerant flow/orifice in the front unit causing the warm air up front?

Is the rear relay under the fuel filler tube sticking causing the fans to run after shutoff?
or is there something in the rear evap unit that would cause the fans to stay on?

The left rear condenser fan relay is inside the rear evaporator, if the fans were stuck on, the relay is stuck, seen it happen before, it may fix itself or will need to replace it, the only way to kill it is what you did., the Right right fans in the condenser are for the front evaporator, that fan relay is on the dryer bracket.

 

[DREDnot]

Ok. That makes sense.

How about this switch that I bypassed?





What does it control and under what conditions? Im trying to figure out how the system functions but I've had no luck finding any documentation on this BAE(?) system.
I see it has a thermocouple .
Would this somehow cause the front evap unit to blow warm?
Does it just shut off the compressor when it gets too cold?

Or is the orifice in the front evap unit clogged causing it to blow warm while the rear evap unit blows cold?

Normally I would find a junk unit to dissect and figure it out but I dont want to start tearing this one open quite yet.

Are there any pics of a dissected front evap unit?

Does the unit need to be evacuated/removed in order to get to the orifice or replace the controller unit?

 

[Retiredwarhorses]

Ok. That makes sense.

How about this switch that I bypassed?

View attachment 906729

View attachment 906730

What does it control and under what conditions? Im trying to figure out how the system functions but I've had no luck finding any documentation on this BAE(?) system.
I see it has a thermocouple .
Would this somehow cause the front evap unit to blow warm?
Does it just shut off the compressor when it gets too cold?

Or is the orifice in the front evap unit clogged causing it to blow warm while the rear evap unit blows cold?

Normally I would find a junk unit to dissect and figure it out but I dont want to start tearing this one open quite yet.

Are there any pics of a dissected front evap unit?

Does the unit need to be evacuated/removed in order to get to the orifice or replace the controller unit?

It senses the temp coming off the evaporator coil, this will turn off and on the compressor when running so your system doesn’t freeze up…
The thermostat with the white label is Mfg discontinued, the gold label is the correct replacement…it fixes issue of the compressor Running before the Gen is at full output.

 

[DREDnot]

…it fixes issue of the compressor Running before the Gen is at full output.

We noticed an issue with the 400A generator. When the engine is first started, the generator charges at the high side of the green arc. The measured output at the batteries is 28v. As the throttle is pressed slowly the gauge reads through and past the red arc. Voltage measured at the battery goes up to 30+ volts. When the engine is revved higher the generator stops charging reading in the middle of the yellow arc and measured voltage at the battery is 24.6 volts. This is all while the flow control switch is bypassed. The voltage remains in the yellow(24) regardless of engine speed. When the engine is shut off and restarted the voltage jumps up again overcharging like before and stops charging when revved up again.

Is this related or a separate problem with the generator?

The only other problem with this truck that Ive noticed is that while all the lights(full original LEDs) work, The turn signals do not come on steady or flashing. There is a flasher unit present. I havent verified the part number though.

 

[Mogman]

Could be a bad regulator, check the voltage at the IGN terminal to make sure it tracks with the battery voltage, the regulator probably uses that as a reference so if it is low it will try to raise the voltage, also of course check all the connections from the batteries to the block and alt including the grounds.