86T4R_project

long time reader first time poster!
I have a 1986 4runner that I just re-installed the air conditioning system back into. The AC went out in the late 90s due to a leaky hose and my dad never bothered to get it working again (even though we lived in southern AZ). I bought it off my dad and the AC was the first project I wanted to tackle. All went well thanks to the TSB, the big repair manual, and multiple forum posts to guide.
The AC works well enough, not as good as a newer car but wasn't really expecting that. Current issues are the following (note: truck does not have a tach):
1. drove around for about an hour yesterday and when sitting in stop and go traffic, the coolant gauge got about at the 75%+ of max point. I replaced my thermostat with the LCE one back in march due to the old one being stuck open and it is working properly.
2. AC compressor doesn't appear to cycle at all. my assumption is that the compressor is signaled to turn off when the sensor at the evaporator says the air is too cold. is this correct?
3. surging idle. this happened without fail everytime i stopped on the hour long drive. when i got home, i went inside to see what could be the issue and then went back and let the truck idle in my driveway while i messed with the idle-up screw on the intake plenum. unfortunately the idle stayed constant (about 5 min). today i did a quick drive and the same surging happened. got home and idled for about 30 minutes (in which the temp again rose to about 75%+ of max in the first few minutes). when the temp started to climb i turned off the ac (hood is open btw) and it settled back to normal. turned the ac back on and the temp didn't fluctuate for the remaining time i was out there.
3a. eventually the compressor turned off. i clicked the ac off and the idle dropped, clicked the ac on and the idle rose BUT the compressor didn't turn on. i pressed the gas pedal slightly to raise the rpm and i could hear it click on. went out to the engine bay and watched it click off then messed with the throttle until it clicked back on. the ac didn't turn back on while idling for the rest of the time.

I've read about the ac amplifier and am a little confused as to what it actually does but what it sounds like is if the idle drops below 600 rpm (ideally) it'll turn the compressor off and when it comes back up it'll turn it on. i also don't understand that while driving and then stopping at a light to idle, the idle will surge but sitting in my driveway it will not.
if someone could also point out where the thermal relay is, that'd be great. the picture in the manual leaves much to be desired. i know where the thermo switch is there appears to be no connectivity so that will be my first thing to replace.

any advice or comments are appreciated. i know this forum has a lot of knowledge based on what i've already read to get this far and have the AC working. thanks!
David

86T4R_project

anyone know where to buy the thermo switch? PN is 83430-96440 (straight from the toyota diagram) but everywhere i look its discontinued. is there any hope rying to repair a switch like this?

Paul22RE

There is an adjustment knob at the AC amplifier (behind the glove box) that allows you to change when the compressor kicks on, based on the RPMs. The AC switch must be cycled to see the results of the adjustment.

The idle will surge up and down if it gets too high, your idle may be set too high causing the surge loop when you're idling with the AC on.

86T4R_project

i've been trying to adjust the knob at the amplifier and it seems like its a rubber knob and isn't actually turning anything. i didn't notice any effect when turning it but i also don't have the surging problem when just sitting in my driveway, only after stopping at a stop light.
adjusting the idle is one of the things i'd like to try but currently my cable is too slacked and i have no more play in the adjustment nuts. i'm not sure i understand why it surges if the idle is too high though.

Paul22RE

Adjusting that knob will allow the compressor to kick on at that adjusted RPM setting. You should see a difference after you recycle the AC switch.

The AC idle-up vacuum-actuated diaphragm under the hood can go bad too and not hold the idle up but it sounds like you are not having that issue. My truck is also from AZ and many rubber parts, including that diaphragm, dried up. I replaced the diaphragm by following what Tom did here in post#36 since our diaphragm is discontinued: https://www.yotatech.com/forums/f116...70/index2.html

scope103

Have you tried holding your foot on the brake in the driveway? A problem with the brake booster will cause vacuum issues; that usually causes idle effects.

RAD4Runner

Mine had same hunting idle upon stopping from freeway speed while AC is on. Adjusting to proper RPM fixed it.
How did you measure RPM?

Are you talking about the switch on top of tstat housing?
Did you test it? Should be open when engine's cold. Lower resitance when coolant's hot. Sorry I dont have figures handy.
Also it's threads and mounting should be bare shiny metal because that's how it gets its ground.

86T4R_project

i will go down and try this. is this an issue that the ac is exasperating?

that is the plug i'm talking about and i think i know the issue but will go test again. first test had no continuity and when i removed it, theres teflon tape around he threads.

86T4R_project

no way to measure RPM but by adjusting the amp screw, i figured the compressor would switch on or off. adjusting the idle-up screw does change the idle speed so that piece appears to be working properly. going down now to try the braking suggestion and clicking on and off the ac button after making a change to the amp.

86T4R_project

hot tip on holding the brake in the driveway. here's what i found:
1. turn on the ac with foot of the brake: OK
2. with the ac on, press the brake pedal: OK
3. while pressing the brake pedal, turn the ac off: OK
4. while pressing the brake pedal, turn the ac on: PROBLEM
5. leaving the ac on, release the brake pedal: OK
6. press the brake pedal after releasing: OK

it seems if the compressor cycles while the brake is pressed, the idle surges. releasing the brake and pressing again stops the surging. i was able to lower the idle with the idle-up screw low enough to shut off the compressor so it seems the shut off is pretty low but it works.


i checked continuity between the housing threads and ground and its good. cleaned up the threads on the thermo switch and reinstalled and no continuity. i may have sourced one that isn't $130 shipped from japan but still waiting to hear back.