A/C clutch will not engage at idle

Hello all,

I have searched around but no go yet. Here is my issue.

I have a 2001 Tacoma dbl cab with a 3.4L auto transmission. The A/C clutch will not engage at idle. Idle is back to normal after throttle body, MAF cleaning, IAC replacement and ECU reset. System has been vacuumed and recharged with pressures within specs. It does not appear to be RPM related as it still will not engage when idling at 1500 RPMs at cold start. If I barely crack the throttle the clutch engages. I suspected a bad TPS. When I loosen the TPS and turn it ever so slightly towards the driver side, the clutch engages. I can tighten it down in that position and it will engage at idle as it should, increasing idle as it should. However, after I drive it a little bit it stops again. I replaced the TPS with the same results. If after it stops working, I loosen TPS turn it toward passenger side and then back to drivers side it will again work for a short drive and then stop. It is like the ECU learns its new position. I have checked values with OBD II scanner and all looks well. The CEL has not come on and no DTCs are present. I have racked my brains and cannot figure this out. I have searched and searched the Internet and can find a few others with the same symptoms, but no solutions posted. Save one that ended up being an O2 sensor. They found that out after CEL finally came on with an O2 sensor DTC. I checked voltage of my O2 sensor with scanner. It appears to be giving proper voltage. Any ideas would be so appreciated, please?

 

So I noticed with my OBD scanner that the only values I see out of spec is the idle and advance timing at idle. FSM shows the following:
Idle 650-760 RPMs (truck is running 1150 RPMs)
Adv timing at idle 12.5-22 degrees (truck is reading 10 degrees)

When I slowly crack the throttle, the RPMs gradually increase until the timing adv goes up. Exactly when the timing goes up, the A/C clutch engages. Could I be on the right track here? If so, what would cause the timing to be lower than specification?

 

I think the FSM, states idle 750+ - 50, and timing at an idle with the Te1 T1 jumpered , should be 8-12 *
So your idle is within spec, and your timing will be as well if you do the jumper.
On my Super Duty, you can adjust the air gap on the A/C clutch.
I personally don't see why you can't on these trucks, all compressors have a required air gap for the clutch.

 

Have you checked your voltage at idle ??

Then when the rpm increases

Might be on the low side if the clutch is getting tired

Have you checked the pressure in the AC system sometimes the pressure switches cause things to not work correct .

I have had a poor connection that changed as the engine rpm increased .

 

I think this is where you will find your problem, something isn't allowing the right voltage or amperage until you raise rpms.

might be the switch in the dash or a wire issue

a pressure switch would be acting in reverse and only working at idle but that's not to say the contacts in the pressure switch might be where you are not getting good voltage from

 

Thank you very much for your input and thanks for the information. It's good to know these values are normal. I have checked the air gap with a feeler gauge and it checks in specs.

 

Thank you very much for your response. I had been mulling a voltage issue as the last time I had the battery tested I was told it is testing weak though it still cranks the truck fine.

So where should I check the voltage values? I am certain the clutch is engaging the moment the relay sends the voltage as I can hear the relay click the on the a/c head unit (in dash) the moment the clutch engages.

 

Thank you very much for your response. I had been mulling a voltage issue as the last time I had the battery tested I was told it is testing weak though it still cranks the truck fine.

So where should I check the voltage values? I am certain the clutch is engaging the moment the relay sends thevoltage as I can hear the relay click theon the a/c head unit (in dash) themoment the clutch engages.

 

check voltage at the a/c relay in fuse box under the hood and test it for resistance on the contacts (you will need to power the relay coil to check the switch contacts) but first check voltage at the compressor clutch, if its good there then the rest should be good and not need testing. if all checks out ok then it could be the internal crankshaft sensor on the compressor that shuts it off if the compressor rpms don't match the engine rpms. I hate this safety feature but its designed to not let the compressor run if the belt or clutch is slipping. I know of several people who had to replace perfectly fine compressors because that switch wasn't working right. if it is the internal crank sensor doing it you should see the a/c light in dash not come one until you raise rpms and hear the clutch kick in.

 

Thanks, I will investigate this tonight when I get home.

 

After work I went to buy a new volt/ohm Meter as my old one bit the dust. Got home and the LED screen on new meter did not work. I will have to take it back tomorrow for a replacement. Meanwhile, I have a curious question. If only one wire goes to the a/c compressor with voltage to engage the electromagnetic clutch, how can a crank sensor within the compressor compare itself to the RPMs of the engine?

 

Frankly....I had never heard of anything such, so I was waiting to hear more.
A/C operates of only a couple factors.
As long as the system is full , it has an ambient temp switch, and a high pressure switch that controls it after you turn it on in the truck.
Ambient temp switch looks like a disc, watch battery thing, manufacturing lamented all over, usually don't see them,
High pressure switch is on the high pressure line of the system does exactly what it insinuates, monitors pressure, line pressure, so the compressor does not blow the system to help .

Once you get the system to take 131a or in the old days before the Patten expired R-12
Then you can find the leak , and fix it yourself .

 

System has been vacuumed and recharged twice. High and low pressures are within limits. Once I can get my new meter I can check the voltage at the a/c clutch to eliminate bad pressure switch. My truck has a dual high,low pressure switch. But I do not think it has an internal to compressor crank sensor. I see no reason for it based on my wiring diagram for my truck. There is simply one light green wire going to compressor to engage electromagnetic clutch.

Any idea how to test the ambient temp switch?

 

I have check the voltages. All voltages look good. I even went ahead and checked the voltage at as well as jumped the a/c dual pressure sensor. Voltage on both terminals were equal and did not change when I pressed throttle to make clutch engage. I checked the thermistor. It checked out as well. Resistance remain constant when clutch engaged vs not engaged.

This pretty much eliminates voltage issue on all safety features. So as I see it this leaves 1) a/c control unit. 2) ECU. 3) a sensor the ECU monitors to decide to make decision to signal a/c control unit to engage clutch. Though I am uncertain how #3 would not set off CEL/MIL.

Question. Where and how can I check the signal from ECU to A/C control unit?

 

One of the VSM's is a step up motor , raises the idle speed with the A/C turned on.
Is your idle coming up when you turn the a/c on?

 

Only when the clutch engages. If I hold the throttle barely open enough to make clutch engage, the idle will go up as the clutch cycles

Also, the IAC has been replaced.

 

On my 95 it is the front vsv, closest to the timing cover.
It is adjustable , with a 4mm allen wrench.
Just for shits and giggles.
Mark the present location, start the truck, turn the a/c on, and turn the adjustment up or down.
I dont remember which way to turn it, but it is supposed to engage , when you turn the a/c on.
I think it is supposed to raise the rpm 200.

 

Considering the below information from the FSM and considering that I have checked the A/C dual pressure switch is at no time opening during the issue and I have also checked that the value does not change at the thermistor during the issue. Doesn't this point to an issue with the A/C control unit? This truck does not have a magnetic clutch relay in the fuse/relay box. However, I hear a click in the dash where the A/C control unit is located when the clutch engages and disengages. That would indicate to me that there is potentially a bad relay on that unit. Does this sound probable or am I still missing something here?

From 2001 Tacoma FSM:
5. AIR CONDITIONING OPERATION
When the blower SW is set to on, current from the HEATER fuse flows through the HEATER relay (Point side) to A.C fuse to
TERMINAL 2 of the A/C dual pressure SW to TERMINAL 1 to TERMINAL 3 of the A/C control assembly. The evaporator
temp. signal from the A/C thermistor are all supplied to the A/C control assembly. When the A/C SW is turned on, the A/C
SW on signal is sent to activate the A/C control assembly. Current flows from the A/C control assembly to the magnetic
clutch, turning the compressor on. 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 assembly shuts off the A/C operation.

 

A/C control unit (in dash) has been swapped and had no affect. No on on here has any other ideas?

 

So that no one else looking and searching for this issue goes about finding a few posts all without resolve, I am posting what my resolve was. It took my local dealer a little while to figure it out because they checked things out and eliminated everything until they got to the last resort. It was the ECM. Replaced it with a salvaged ECM and now idle is normal as well as the clutch engaging at idle.