2nd gen 4runner AC wiring

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MoabPorkChop

MoabPorkChop

Joined
Sep 23, 2025
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Location
Moab, UT
I swapped my 3.0 for a VW TDI that has been running great for the last 2500 miles but it is time to get the AC running. I connected the lines and it holds a vacuum just fine. My issue is with the wiring. I left all of the non engine wiring in the truck but the wire that went to the AC magnetic clutch was removed. Can I just wire a switch directly to the AC compressor and run it or does it need to be controlled by the pressure switch? I did hot wire the compressor and it made cold air so the system works but I dont know enough about it to know how it should be working and if just adding power to the compressor will end up causing problems with overpressure or heat.

Matt
 
I ran two fused relays from my AC push button (only hot when key on). I disconnected it from the original truck wiring and ran it to another fused link in the driver's fuse box that was hot with key on. It feeds power excite of the relay so there's no real amperage draw here. The relay takes all the amperage.

One relay grounds through a trinary switch that screws into a high side AC charge port. I added this extra port when I adapted my Passat TDI AC compressor to my Toyota condenser. The trinary switch (eBay/Amazon/jegs/summit/Hot rod shop) kills the power when the pressure gets too high or too low. This relay powers my AC compressor.

I left out the fan control portion of the trinary switch wiring.

My second relay that activates when I push the little blue AC button is for the electric fan. I found the AC worked better when the fan ran all the time vs just when the trinary switch reached its set pressure range.

I used standard 5 prong relay harnesses found on eBay/Amazon but trashed the cheap Chinese relays for some HELLA relays.

The relays are mounted beside the fuse box under the hood and feed off of a fused connecting running directly to the battery feed inside the fuse box.
 
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I had to read that a couple of times but I understand it now. I assume that I could use the high side port that is already connected in my system. I don't guess that I would have to add one.

I was hoping to be able to use what the truck came with but I haven't figured out which is the power wire to the mag clutch. I found it in the wiring diagram but it doesn't get power when I turn on the system.
 
If the second gen is like the 1st there's an amplifier system that I didn't want to figure out. The 1st Gen is an expansion valve system and our TDI compressors are built for an orifice tube system so I'm not sure the amplifier system would work for it anyway. There's, i believe, an idle control to prevent stalling out when the AC is turned on that runs off the system and iirc goes to the ECU which is also gone after the TDI swap.


You can reuse your existing high side charge port for the trinary switch , but it's easier to have more than one.

You'll have to bypass the switch while removed to activate the clutch when charging/checking pressure. Not the end of the world but another inconvenience


I purchased a hydraulic crimping tool so I could get bulk hose and install the fitting I needed. There are hundreds of different fittings, bends, ports etc. Made it so the custom AC adaptation was simple once I converted the yota and VW compressor lines over to standard SAE fittings.
 
I've been searching and I think I can cut the wire from the factory thermistor and wire it to the switch and bypass the ac amplifier. The amplifier seems to just adjust the idle when one of several ac related conditions are met.

My problem right now is finding the thermistor. I can see it on the wiring diagram but im not sure where it is on the truck. If I can find the ac amplifier I can use the wires there to bypass it but I cant find that either. I think the amplifier is behind the blower that is behind the glove box and that seems like more work than I want to do.
 
I finally understand how a car ac system works after reading this. I understand the ideal gas law but never really got the basics of each component.

Thanks,
dohcdelsol93 said:
I ran two fused relays from my AC push button (only hot when key on). I disconnected it from the original truck wiring and ran it to another fused link in the driver's fuse box that was hot with key on. It feeds power excite of the relay so there's no real amperage draw here. The relay takes all the amperage.

One relay grounds through a trinary switch that screws into a high side AC charge port. I added this extra port when I adapted my Passat TDI AC compressor to my Toyota condenser. The trinary switch (eBay/Amazon/jegs/summit/Hot rod shop) kills the power when the pressure gets too high or too low. This relay powers my AC compressor.

I left out the fan control portion of the trinary switch wiring.

My second relay that activates when I push the little blue AC button is for the electric fan. I found the AC worked better when the fan ran all the time vs just when the trinary switch reached its set pressure range.

I used standard 5 prong relay harnesses found on eBay/Amazon but trashed the cheap Chinese relays for some HELLA relays.

The relays are mounted beside the fuse box under the hood and feed off of a fused connecting running directly to the battery feed inside the fuse box.
Click to expand...
 
I spent a good deal of time reading threads of how guys with old muscle cars (expansion valve AC) got the AC working after installing and LS engine (orifice tube). It's a similar concept. I'm sure there are ways one could incorporate the factory yota electronics but I wanted something quick and easy as our winters turn into scorched earth summers in less than a month so time wasn't on my side
 
If the A/C is similar to the 2nd gen pickups, the A/C amplifier is mounted on the A/C box and so is the thermistor. It sticks into the evap coil and the wire goes through the box so it can be connected to the amplifier pigtail. The pigtail is routed on the outside of the A/C box in plastic clips molded into the box. Basically behind the glove box on my truck.

You can bring up pics of the A/C box posted up on many forum posts with a Google search. It's not super hard to get to on my 84-88 trucks. IDK how difficult for your 4Runner.

There's also a pressure switch mounted on the hard line feeding your expansion valve inside the box. Those wires also go through to the outside of the box so check the wire colors to decide which are the thermistor vs the pressure switch.

Again, I'm "assuming" it is similar to my 84-88 truck. Maybe Toyota completely changed all this for your 4Runner. If not, I hope this helps.
 
Ok, this far surpasses my intelligence, but if you install a manual switch to run your compressor, which I have seen done, you would have to manually switch it on and off every 20 or so seconds because of Ice up. The thermistor tells the compressor to shut down when the evap starts to get iced up. The amplifier is really the computer controlling every signal in the system. In my 60 series, the po had a manual switch for the ac comp. 'Twas a pain in the arse. It took disassembling the wiring harness to find the ac comp wire hidden in there to be able to solve the problem.
 
aztoyman said:
If the A/C is similar to the 2nd gen pickups, the A/C amplifier is mounted on the A/C box and so is the thermistor. It sticks into the evap coil and the wire goes through the box so it can be connected to the amplifier pigtail. The pigtail is routed on the outside of the A/C box in plastic clips molded into the box. Basically behind the glove box on my truck.

Again, I'm "assuming" it is similar to my 84-88 truck. Maybe Toyota completely changed all this for your 4Runner. If not, I hope this helps.
Click to expand...

I had the glovebox off this week and was looking around and I think I know the pig tail your talking about. I will look when I get home.

mongoose2231 said:
Ok, this far surpasses my intelligence, but if you install a manual switch to run your compressor, which I have seen done, you would have to manually switch it on and off every 20 or so seconds because of Ice up. The thermistor tells the compressor to shut down when the evap starts to get iced up. The amplifier is really the computer controlling every signal in the system. In my 60 series, the po had a manual switch for the ac comp. 'Twas a pain in the arse. It took disassembling the wiring harness to find the ac comp wire hidden in there to be able to solve the problem.
Click to expand...

My amplifier is looking for a rpm signal which went away with the exciter when I pulled the old engine. I either need to find a way to give it the rpm signal back or I read that I could cut the wire from the thermistor to the amplifier and wire it to the clutch switch and it will bypass the amplifier. I just havent found the right wires yet. If I cant make this work I'm going to buy a highside trinary switch and try the way dohcdelsol93 did it.
 
The thermistor monitors the evap temp to avoid freezing up. It basically "signals" (electrical path to) the A/C amplifier to cycle the clutch off. The amplifier also maintains the proper RPM when the clutch is engaged. On the earlier EFI engines it actuated the vacuum switching valves.

I had to modify the wiring on my 1988 A/C box to a 3.4 amplifier. For my 3.4 it tells the ECM to raise the idle. No vacuum switching valve in this case.

The expansion valve is also a variable orifice. The thermal bulb that clips to the end of the evaporator senses the temp and the pressure in the bulb and tube acts on the orifice in the valve to control the flow. So there's a little built in regulation there.

Aftermarket binary or trinary switches CAN work to control freezing of the evaporator if you can adjust it to cycle the clutch off before it freezes based on the low side pressure. I think it's main purpose is to prevent compressor damage from cycling on when you have too little or no refrigerant in the system and as protection from high pressure.

The factory pressure switch in the A/C box just before the expansion valve is a binary switch. It opens at high pressure or low pressure and is also a "signal" or electrical path to the amplifier to allow the clutch to cycle on....or not if pressure is too high or too low.

The A/C amplifier is the gate keeper to the power getting to the clutch from the dash control AND raising idle speed.

Using your stock A/C, you have what you need to control the on/off of your system. Without freezing. IDK how you would integrate the idle up to your VW engine though.
 
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Knock on wood I've never had icing up issues. With original condenser I did have over pressure issues when temps were above 80.

I upgraded to an aftermarket condenser with much more cooling surface and those issues went away. At the same time I wired ac fan to come on with clutch

The AC isn't as good as the new bronco, the fan speed alone of the 4 runner is laughable compared to new bronco. But it does blow about 48-52 degrees at the vent on a 90 degree humid day with the black soft top that is about as air tight as Grandpa's 20 year old tighty whiteys.

With the r132 conversion plus VW compressor throw the recommend weights out the window.

I add r132 slowly until temps stop dropping. Low side is some 30-40 and high 200-225 on a hot humid day. These numbers, mostly high side, will vary based on temp and humidity. 1can from full vacuum gets me started and I'll use a partial second can.

I tried both straight propane and a propane/butane mixture to lower pressures, which they did but cheap Walmart r132 gave me the lowest dash temps.

New denso dryer and expansion valves were used to try to make the most out of a less than perfect situation.
 
I should also note the TDI is a sophisticated throttle by wire engine. There's not a need to raise idle while AC is on, the ECU senses the load and adds fuel to maintain the set idle speed. The only time a TDI has a higher idle is cold start up. You can program a TDI to have a lower idle to use with an automatic transmission swap application or a high idle like that of a wood chipper.

The TDI already idles at 950 rpms. Which is why it isn't friendly at stock idle with a proper stall torque converter used in a 4l60l/aw4/ etc swap application. It idles so high it engages the torque converter. In 4l it slams between gears harshly at a stop and you'll need to stand on the brakes with both feet to hold her at a stop.
 
I finally got some time to work on my AC. I found the thermistor and the hi and low pressure switch. They all run to the AC amplifier which talks to the ECU. Because I no longer have an ECU I need to bypass the AC amplifier.

Both the hi and the low switch are normally closed (there is continuity when the truck is off). So I assume if I run the AC clutch power through them in series and the pressure gets too high or too low it will disconnect the power to the AC clutch. Let me know if this is correct.

I also found the thermistor but it doesn't have continuity through it so I cant just add it in series. I dont know how to use it. Any ideas?

Can I just skip the thermistor and hope it doesn't ice up? If it does ice will it cause one of the pressure switches to flip or will it just blow mildly cool air?

Matt

(I edited this because I mixed up a closed and open switch)
 
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I just realized that if the low pressure switch is normally closed with the truck off then it either has some pressure left from when I ran it last week or it doesn't work the way I thought it did.
 
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If you are talking about the factory switch on the tube going to the expansion valve, it's a binary switch.

It opens if the pressure is lower than the setpoint. IDK what the spec is but it should be closed so your clutch can engage. A charged system has enough pressure to close it. Even if the engine isn't running. It's designed to protect the compressor if you lose your charge.

Since it's a binary switch, it will also open if for some reason the pressure gets too high. IDK what that setting is.

So it's actually normally closed. Normally meaning a charged system.

The compressor also has a pressure relief valve that will lift if pressure gets too high.

The evap can still freeze if the compressor doesn't cycle off. The OEM binary switch isn't really designed for that.

There are adjustable switches that can almost do that though. It would take some experimentation to set it and that can vary depending on temp and humidity. Basically you adjust the low pressure switch to open just before it freezes. I tried it on a Suburban I had. Wasn't consistent.

The thermistor is more of a variable resistor so I don't think it's designed to open or close in our system. More of a reference signal.

Maybe one of the aftermarket A/C companies has a product or recommendation to handle this.

If your evap ices up, it's literally a block of ice and not much air gets by it.

The expansion valve will throttle back but it doesn't shut off. It will still ice up unless something turns off the clutch.
 
You can try it as is and see how it does. If you have very low humidity, the air will blow super cold. Once there's enough humidity for condensation to form it will ice up. Been there, done it.
 
the ac amplifier runs independently of the ecu other than telling it that it's on so it ignores the idle up. you can still use it to control the ac function. the only real issue the tach(igniter) input. you would have to simulate the signal it's looking for. you could possibly do that with a signal generator like this set to 50% duty cycle and about 100hz which would create an approx 3000rpm signal.

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www.amazon.com

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I just asked Google if there are universal auto A/C thermistors and there are. There's your solution. One of these in series with your factory binary switch. Here's one, NAPA TEM 207512 Or this one from Classic Auto Air A/C Thermostat-Rotary A10-64090

I'm sure there are others out there.
 
I'm no expert but the 17.99 jegs or summit trinary switch as a present low and high pressure sensor. I think low is approximately 20-25psi. The AC freezes when the charge or pressure gets too low.

The factory yota compressor is a fixed compressor while the VW is a variable. Technically the variable style compressor found on newer vehicles like the VW TDI and a 5.3 LS engine should not work in an expansion valve system.

Luckily for us some Chevelle, nova and other 70s/80s model muscle car/truck owners decided to give it a try and it worked.

I wouldn't over think it. There's no shortages of humidity or 90 plus degree temps here in the south east and I've never had my system freeze up, even on 2 plus hour road trips.
 
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