How to MarkVIII your OBS on a budget..!

[SAATR]

The functions of the low-pressure cut-out and cycling switch are one and the same; in both cases, they disable the compressor when the pressure is low. In '96 and later it's one switch which performs both functions, and likely in earlier models too.






For 1996 in particular and later, AFAIK:

The switch on the manifold isn't a low-pressure switch; it's the high-pressure switch that forces recirc "on" and turns on the aux electric fan (if so equipped). This is easily demonstrated by removing the connector on that switch and shorting the terminals; recirc will engage, etc.

The high-pressure cut-out switch is mounted on the back of the compressor.

The low-pressure cutout and the cycling switch are one and the same.

See highlighted areas in the attached.

ALL these functions can be demonstrated by disconnecting the switch's connector and shorting the terminals together, so if in doubt

Reviewing the information you provided, I see that my misunderstanding was the function of the AC Compressor Low Cutout Switch. It does not have anything to do with shutting the system down when low pressure is present. Thank you for the clarification.

My original point, however, remains the same. The switch on the accumulator is called out as a Cycling Switch, and controls cycling of the compressor clutch. The switch on the compressor manifold is specifically the A/C Compressor Low Cutout Switch. No mention of pressure (my mistake), but causes significant confusion when the names are used interchangeably on the later systems.

 

[1998_K1500_Sub]

The switch on the compressor manifold is specifically the A/C Compressor Low Cutout Switch. No mention of pressure (my mistake), but causes significant confusion when the names are used interchangeably on the later systems.

Just to amplify @SAATR's comments:

The switch on the manifold is mistakenly referred to in some parts of the FSM as an "A/C Compressor Low Cutout Switch", although it's function is NOT a low cutout switch. This labeling error "Low Cutout Switch" is evident in the wiring diagram @SAATR shared.

Rather, the switch on the manifold is a "high-pressure switch" which is called different things in the FSM, i.e.,

- "compressor pressure switch"

- "Auxiliary Cooling Fan A/C Pressure Switch"

- erroneously "A/C Compressor Low Cutout Switch"

The true function of this switch is given in my earlier post.

This mislabeling of the switch in the FSM might cause a problem when searching for service parts.

 

[Redsled]

If I understand what you are trying to accomplish, you want your low speed electric fan operation to
track with the A/C when it's running. Taking a look at the A/C wiring diagram out of the '91 Electrical FSM,
here's the circuit we have to work with:

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Given that the A/C compressor is energized when cooling is being called for, according to
this diagram it makes sense that the DK GRN wire (circuit #59) is the one that you could
use to tell the control side of the low speed fan circuit to come on coincident with the
A/C compressor.

NOTE: If you already have the Factory Service Manuals for your '91 by your side you are good
to go. If not, then follow THIS LINK and download those manuals for future reference.
(Particularly the '91 Wiring manual that p. 112 above came from.)

Looking at this, you could connect to circuit #59 either upstream or downstream of the
A/C Presssure Cycling Switch, depending upon exactly how you want your fans to track
the operation of the system.

Hope this is helpful. Let us know how this cooling project works out.

Cheers -

That is exactly what I was asking! Thank you. I saw the dark green and black wires on the compressor and had a hunch that circuit could be used. I will definitely download the manuals too. Again - thank you very much!

 

[Redsled]

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This is how I would wire it up so now that I think about it, would I not want to tap it to the black ground wire coming off the compressor instead of the green one? And then splice it into the low speed line. The reason I ask is because the temp switch grounds that line in order to activate the relay. So therefore the ground coming off the compressor would do the same thing would it not?

 

[Road Trip]

This is how I would wire it up so now that I think about it, would I not want to tap it to the black ground wire coming off the compressor instead of the green one? And then splice it into the low speed line. The reason I ask is because the temp switch grounds that line in order to activate the relay. So therefore the ground coming off the compressor would do the same thing would it not?

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OK, let's first reason our way through the circuit above as drawn.

1) From your description, the 'dual temp thermo switch' grounds either the 'low' or 'high' input wires to the Volvo fan relay.
Since we don't have the specs, I'm going to use made up temp numbers for the truth table based off of a 192° F thermostat:

* <200° F = neither input wire grounded.
* 201°<>219°F = "low" input grounded = low speed power to electric fan
* 220°+ = "high" input grounded = high speed power to to electric fan

So the light blue relay is drawn so that it enables/disables the Volvo 2-speed fan relay. If we were to
connect pin 86 to the DK GRN (A/C compressor enable) wire, this might work at cross-purposes to the
temp-driven behavior of the aux cooling fan? In other words, the extra cooling (low or high speed) would
only be available when the A/C compressor is running?

I know that if I feel like I am flirting with an engine overheat I don't hesitate to turn the A/C off in order to stop
preheating the cooling air headed into the radiator via the condenser mounted in front of it. In English, if I
borrowed your vehicle I would be (unknowingly) disabling the electric fans when I need them most?

****

Given the above, I think a better solution is to wire pin 86 of the relay to one of the circuits that's powered
off of the Ignition Switch when it is in the RUN position. Here's the FSM schematic showing our choices:

(NOTE: Look for arrow showing Ignition Switch power circuit in the RUN position.)

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The diagram shows that the RUN (only) position of the Ignition Switch feeds the CRUISE, 4WD, BRAKE, & HTR-A/C fuses.
From other discussions in the forum I think that the 4WD fuse has the lightest load/most headroom available on it, so the additional amp or so from
the control side of the relay should not present any kind of a loading issue.

IMPORTANT NOTE: Feeding the high amperage side of this circuit would be sourced from the 'Hot Always'
Underhood Fuse Block located on the firewall, using appropriately sized wire and inline fuse holder.

****

At this point you would end up with the electric fan circuit only allowed to operate when the key is in the RUN position.

1) If it was me I would start here and make sure that this circuit is working as advertised first before adding extra functionality. (!)

2) THEN, if I still wanted to add the functionality of having the low speed fan come on whenever the A/C compressor
was operating, I would wire in a 2nd relay in parallel, configured to supply a ground to the "Low" Input 1 of the Volvo
fan relay.

Control side of 2nd relay:

* Pin 85 - Ground
* Pin 86 - DK GRN wire at A/C compressor clutch

Load side of 2nd relay:

* Pin 30 - Ground
* Pin 87 - Spliced into "Low" Input 1 wire to Volvo fan relay

By doing this, you should end up with the temp-driven 2-speed fan operation AND
the low speed cooling fan operation whenever the A/C clutch is energized. And if I
were to borrow your truck, watch the temp climbing, and shut off the A/C, then I
would *still* have the temp-driven fan operation pulling (cooler) air through the radiator.

****

I'm hoping that the above helps to clarify how to best implement the circuit you provided into
a '91 GMT400 vehicle. (Including the driver not having to know special rules during operation
in hot weather.)

Disclaimer: The HI/LOW electric fan circuit from the Cherokee forum using BMW, Volvo, and
Taurus bits is new territory for me, and therefore I can't confirm or deny that it has enough
CFM suds to provide the cooling your GMT400 rig will require. (But no doubt this setup has been
real-world tested in whatever forum you found it. YMMV.)

If you have any further questions on the above, don't hesitate to ask. I consider this exercise
valuable practice in the wiring diagrams -- this time in the vicinity of the RUN circuit off of the
Ignition Switch.

Best of luck with this cooling exercise. Be sure to reply back with your real world experience.
(And feel free to include photos of the parts/install.)

Cheers --

 

[GoToGuy]

The accumulator/receiver-drier mounted switch is the cycling switch. Not sure where the low pressure cutout switch is installed on the earlier models, but the switch mounted to the accumulator has always been the cycling switch.

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RTFQ , I answered the question asked by the owner of a 1991, as in the post before mine. Are you using the 91 service manual? You wanna be understood, use the correct terminology. I answered correctly, with description, I didn't say it was a hi/ low cycling switch.
And its 1995 trucks and earlier.
The 1996 trucks are Vortec, are OBD 2 , the HVAC was redesigned / changed.
Try to remember you have TBI through 95, and with 96 forward are many changes.
A big problem is owners doing upgrades modes, but don't bother to use correct terminology.

 

[SAATR]

The low pressure switch on the receiver dryer is the switch that opens the circuit to the compressor clutch when the 134 pressure drops below the minimum around 30 psi, indicating a loss of freon charge. It is not a cycling switch.

It IS a cycling switch. From the '91 Factory Service Manual, in the stickied thread below this one:

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RTFQ , I answered the question asked by the owner of a 1991, as in the post before mine. Are you using the 91 service manual? You wanna be understood, use the correct terminology. I answered correctly, with description, I didn't say it was a hi/ low cycling switch.
And its 1995 trucks and earlier.
The 1996 trucks are Vortec, are OBD 2 , the HVAC was redesigned / changed.
Try to remember you have TBI through 95, and with 96 forward are many changes.
A big problem is owners doing upgrades modes, but don't bother to use correct terminology.

You have the wrong term and, worse, the wrong function. The '96 and later trucks may have a different system, but that switch, with that function, has been in that location since day one.

I had a '98 manual at hand, and certainly went on a tangent describing the other switches, but knew from my own experience what it was called and what it did, regardless of year.