Hey fellow 4runners - greetings from North Texas - (Dallas Ft Worth).
Appreciate your help catching where I may have missed the obvious needed repair. Followed many of the recommendation here on T4R.org as well as Timmy The Toolman videos (love those 3rd gen videos of his).

Vehicle:
1999 Toyota 4Runner, Base model 2.7L, 5 speed, 2 wheel drive, MANUAL a/c (has the AC Amplifier not a "second relay" as many folks seem to think).

runner has 245,000 miles - dusty, hot texas miles - use it to run out to west texas ranch and back to DFW area all the time.
2nd owner since 130,000 miles This is one of 6 (yes 6) 3rd gen 4runners I have. My kids drive them as well.

I have used my "trusty" Harbor Freight Manifold gauge set a number of times to check, evac, draw vacumn, and recharge a/c on my other runners. ... point is, i know enough to be successful but certainly not an expert on A/C systems.

Problem for 1999 4runner:
A/C wasn't cooling below 66.2 degrees.

I didn't specifically check ac compressor clutch engaging or not, just assumed it was ok as system would cool down to 66.2 degrees but no further. just Assumed. yeah I know. Guessed i had a R134a issue. Simple recharge and done - right?

But no....never that simple is it.....

today 6/26/21, i am guessing the root cause is within the a/c pressure switch circuit - or could it be too much PAG oil in system? guessing here ...

The Symptom:
Compressor clutch will not remain "engaged".
The compressor clutch will engage for 5-10 seconds, then cut out and will not re-engage - unless vehicle is shut off and restarted.

The twist: This is AFTER I replaced a/c components (using Denso) as called out below.


BACKGROUND / Process Steps I followed:
For the 1999 -
Decided i would replace the A/C components due to amount of use and simple fact we'll keep it another 245,000 miles.

Bought new parts via Amazon - all Denso products - as noted here - Denso units actually shipped direct from Denso warehouse in SoCalif...

- Condenser - Denso
- Receiver - Denso
- Compressor - Denso
- Evaporator Core - Denso
- Expansion Valve - Denso
- Thermister and clip (Toyota OEM)
- Pressure Cut Off Switch - (Toyota OEM)
- O ring set (4 seasons green set) just in case denso didn't send enough
- misc parts all OEM from Toyota i.e. packing for Evap Case etc.

Replaced all parts easily enough. Helps to remove the airbox.
Evap case was a slight annoyance but all together pretty easy process to replace components.

Got everything in. Triple checked connections, torque values etc. Looked good.

Charging System: aka - The place of broken dreams....

1/ Pulled vacumn for 45 minutes, remove line from pump
2/ Checked for leaks by waiting 1.5 hours - no issues, vac remained at 30 psi
3/ connected one can of r134a to gauge set
4/ puncture can, bleed the "yellow' line to ensure no air in line
5/ started engine; ac on, max cool, recirculate mode, all windows down
6/ boosted RPM to 1,500-1,600
7/ opened low side manifold knob expecting good things but no....
8/ low side pressure started reading at 70 psi
9/ after a few minutes low side pressure started building, first 70 psi, then 80....then 90.... closing in on 100 psi .... so I closed off manifold (low side valve), killed engine.

** I didn't catch the HIGH side psi reading as I worried about low side

** Ambient weather in DFW area on 25 June was about 40% humidity, 98 degrees F, (god I miss North County San Diego weather....)

I repeated my steps (1-9) 3 more times and got same results each time. ....didn't see cycling, the RPM's on engine would drop a bit on occasion but not what I see on my other runners when ac is cycling, not same load/rpm drop.

I put a high visibility mark on the ac clutch to check if it was spinning, again it would spin once then no more. when it spin low pressure psi reading are what you would expect/see when charging a system with r134a.....

I bypassed the new pressure switch by jumping the switch connector (using paper clip) - same patten, clutch engages, for 5-10 seconds, can see low pressure gauge drop down to say 20 psi, then clutch cuts off and will not reengage unless engine shut off then back on. i.e. one clutch engagement per engine on/off cycle.

At times, I could hear engine RPM drop slightly....but not what i see with my other runners when ac cycles/clutch engages and no movement on gauges (low side dropping to 20-25-30 psi for a normal charging cycle)

No idea why a new dense compressor clutch would kick on 1 time for 5-10 seconds, systems function as it should for those precious seconds then clutch doesn't engage again without resetting. Guessing issue on electrical circuit...

Could it be:
1/ AC amplifier switch failure?
2/ Too much PAG oil? Son thinks we may have put too much into system:
New Compressor had 3.9 oz - we drained new compressor then put back that amount - or did we add more - we didn't document ...

3/ per denso recommendations we added PAG : to condensor 1.4 oz, receiver, 0.70 oz, evap core 1.4 oz - trying to find an answer we are speculating that we may have added these amounts to the compressor in addition to the original 3.9 oz....essentially adding 2x the condenser, receiver and evap core volumes.

4/used Denso OD-Oil 8 (PAG 46)

Lord knows we missed something....Appreciate the help on this.

I've attached the wiring diagram for the manual AC for a 2000. I can't promise the wiring is the same for the 1999, but the function should be. Based on your description, it sounds to me like this section is going to be pointing you to your problem "The A/C operation is shut off when a signal indicating low evaporator temp., or abnormally high or low refrigerant pressure, is supplied while the engine high speed signal exists. When one of these signals is received, the amplifier shuts off the A/C operation." It sounds like power cycling the car resets things, then after running for 5 to 10 seconds the problem reappears and sends the signal to shut off the power to magnetic clutch.

Attached Images

244 Air Conditioning (Manual A-C).pdf (61.6 KB, 80 views)

I've attached the wiring diagram for the manual AC for a 2000. I can't promise the wiring is the same for the 1999, but the function should be. Based on your description, it sounds to me like this section is going to be pointing you to your problem "The A/C operation is shut off when a signal indicating low evaporator temp., or abnormally high or low refrigerant pressure, is supplied while the engine high speed signal exists. When one of these signals is received, the amplifier shuts off the A/C operation." It sounds like power cycling the car resets things, then after running for 5 to 10 seconds the problem reappears and sends the signal to shut off the power to magnetic clutch.

Skulking-
Thanks for attachment -
Headed to garage

1/ re checking all connections on compressor - amplifier- blower resistor - and pressure switch

2/ will test the circuits per attached- which at first glance is same as my 1999

3/ for good measure my local Toyota dealer has the AC amplifier switch - picking up in a coule
Of hours -

4/ what is throwing me - is the charging - specifically the high pressure readings on low side - hadnt seem that behavior during other recharges - that is reading of 70
Psi then building to 100 psi on low side

Anyway off to trace circuit

Thanks

Update on findings while checking connectors

1/ Attached photos are the wire harness connector that snaps onto compressor connector

2/ does it appear that the connector is missing a piece? specifically is the "top half" of plastic connector broken off? could that be an issue.

3/ Not sure what this connector should look like.

Thanks

Attached Images

   

Thanks to @Skulking for the wiring diagram -

Tested the various segments and isolated to the AC amplifier

Interesting fact -by chance I disconnected and reconnected the ac amplifier while running AC / and twsting if it would take a charge (hope endure:”s against all
Odds)

Each time I reconnected AC amplifier connector- clutch on ac unit would cycle for about 5 seconds

New amplifier on the way -

Looks like evaporator
Core has to come out to replace amplifier - unless anyone knows a hack to replace it

It certainly could be the AC Amplifier, but it could also be the AC Dual Pressure Switch or the AC Thermistor since those are the sensors which would generate the readings that would cause the AC Amplifier to cut power to the AC Magnetic Clutch. I would expect that if one of those is giving a bad reading (Or a good reading showing you actually have a problem) that cycling the power to the AC Amplifier would fix things for a brief time until the reading triggered it to shut down again.

Edit: Reread your post and it does look like you have already replaced both the AC Dual Pressure Switch and the AC Thermistor, so unless there is a wiring issue for one of them, replacing the AC Amplifier does seem like it should fix your problem.

Yes, I did replace thermistor

Is the dual sensor switch the high pressure switch attached to smaller diameter AC line? Switch is near the ac line coupler (the coupler that requires a SST to remove) - if that is the dual sensor switch you referenced than Yes, I replaced it.

Ac amplifier is only thing I haven’t replaced :-)
Even the 10a a/c fuse has been replaced.

Thanks for the support

Waiting on ac amplifier part to arrive at dealer today - will provide an update

I figured out how to remove the ac amplifier - straight forward once glove box reinforcement bar was out of the way

I've never needed to deal with my AC, so I don't have first hand experience with it. I've attached the two relevant sections of the FSM again, of a 2000 rather than a 1999. Hopefully that will give you enough to determine if it is the switch you think it is.

Attached Images

	AC-13 Location (2000 4Runner) Air Conditioning System.pdf (79.4 KB, 128 views)
	AC-79 On-Vehicle Inspection (2000 4Runner) Pressure Switch.pdf (42.9 KB, 96 views)