Hi all,

91.5 W250, need some AC & OD wiring pointers

1/ Where do I wire an additional momentary switch (second-gen shift lever) to act in series with the factory dash mounted OD switch? This is the red-headed stepchild 91.5 with the OD control module behind the glovebox, and the truck doesn't have the normally used OR/WH wire on the factory OD switch.

2/ Looking at the schematics - there seems to be no AC relay and ECM pin used for the compressor clutch cycling, unless I'm missing something.

Relevant FSM wiring diagrams attached; thx in advance.

 

The A/C clutch cycling switch that has the probe on it is a temperature controlled switch that controls the compressor clutch. no relay required.

 

I don't think you can wire a new switch in series using just one wire because the OD switch looks like a double pole single throw type switch. So pushing the button as it appears to be in the picture closes the manual contractor and opens the four way looking contact in the corner of the switch. And I would guess it rotates back to complete the connection for OD on when the button is released.
I think you will need to activate one and disable the other.
So making the connection between U5 and U50 with a new switch at the same time removing the connection G5 and E10. Then reversing it in off.
Does this make sense to you?
I think this is the only way to keep the overdrive control module happy and the ECC.

 

 

I don't want to add another switch as the factory one works fine.
Just want to use the second gen shift lever switch alongside the factory dash mounted switch (future expanding waistline-proof)
The second gen OD switch at the end of the shift lever is a SPST momentary, with 2 independent wires.
So is the factory OD switch on the dash.
Given this, which 2 wires should I connect the second gen lever wires to on the factory OD switch and/or OD control module ?
Thx on the AC answer as well :-)

 

wire U5 or U50 either one. looking at the OD switch in the diagram it looks like a 4 wire plug.
If you do wire it like this the switch on the dash will need to be in the state shown in the diagram.
The big difference in the trucks with the orange and white wires is that wire grounds the ECM and that triggers the ECM to send power to the OD solenoid I think.
With your truck there isn't a ECM per say.

 

Here is the 1998 2nd gen wiring.


transmissions-a-t-circuit-1-of-2.pdf

 

Alrighty. I did the following -

New gaskets & o-rings everywhere.

5oz oil in the compressor, 2oz in condenser, 1oz in evaporator & 1oz in drier.

Nitrogen in the system at 300psi and it held for almost 2 hours - no drop in pressure at all.

Ran 26psi vacuum on the system and it held for 2 hours.

Sucked in 40oz of R12 with the compressor cycling at 1300rpm + Max AC + blower fan on high.

Compressor OFF: 50psi low side and 220psi high side
Compressor ON: 25psi low side and climbs in a few minutes to 450psi high side.
Faceplate vent temperatures - 46-50 F

Raising the rpm to 1200-1300 turns the compressor OFF and high side drops almost instantly to 200-250psi

Why am I getting such high side readings, and is it normal for the compressor to be ON constantly until it de-clutches at 1200rpm?

 

Wow this just happened to another guy on here last week. let me see if I can find the thread.
https://www.dieselbombers.com/24-val...ac-issues.html
What kind of oil are you using Pag, Ester?
do you have good airflow from the fan? is the fan shroud in good shape?

 

Hi I'm using an ester oil 7oz in the system compatible with mineral 500.
Fan shroud is good. Fan blades are true.
Spraying water against the condenser instantly makes the compressor cycle and high side pressure drops to a normal 240psi.
Without water, the compressor is always on and pressures climb quite quick to 450-500.
The fins on the condenser and rad are 95% straight.
Idling, paper is held against the rad but not against the condenser.
The fan barely blows a breeze hot or cold hood open.
I've never heard the clutch engage but truck never overheats.
Perhaps the fan clutch is barely engaging but the overbuilt cummins cooling system keeps temps down anyway ?
Switched off hot, the fan has some resistance - spins a half turn by hand and stops.