RPM pulsating during my morning warm up (2005 AC Tundra)

Since it has been colder in Louisiana I have been warming my truck up in the AM.

I've noticed when my heater/defrost is on my RPM gauge surges up and down. Once I turn my heater off it goes away. Going down the highway I have no issues. Any thoughts on this? Cause for concern?

I searched the forum but did not find this subject discussed.

Thank you

 

Surges up and down continuously? or as the AC compressor kicks on and off? The defrost utilizes the AC to help dry the air it's blowing to make it more effective at drying and clearing a frosted windshield without fogging up.

 

I just got done reading this thread below. Tomorrow I will adjust to the foot/lower setting and see what happens!

https://www.tundras.com/threads/odd-idle-at-start-up-but-only-when-motor-is-cold.64772/#post-1688974

 

The foot setting does not utilize the AC automatically. That will help discern if its just the AC kicking on and off that's causing the problem. Just make sure the AC button is also turned off.

 

Situation Normal in Defrost Mode.

 

A dirty throttle body will cause an rpm surge as well.

 

As mentioned, the AC system is automatically turned on when when in defrost or mix mood. This causes RPM surges.

It's pretty easy to disable if you like. There is just a switch in the climate control module in your dash that you have to unplug.

 

How high are the surges?? My only experience is with the 1UZ grandpa version, and it usually needs a coolant top off.... doesn't hurt to check, before you start it.

 

WHOA!!!

He is back!

 

I understand why the compressor comes on in defog mode, but OP says the RPM is pulsating during defog. Why doesn't the compressor engage and stay engaged until taken out of defog mode?

 

The Freon level might be just low enough to trigger the low pressure switch in the AC system, so it continually cycles on-off-on-off-on, etc..

That would give the truck that "Surging" sensation he experiences.... Maybe...

 

There is some intervention somewhere, whether it's the computer or just a timer relay. Every vehicle I've ever had only cycled the AC every 30ish seconds when defrost was used.

Service manual spec is to bump the idle speed from 750 to 800 when the compressor is engaged.

 

Yeah, air conditioning was never my strong suit in school....

 

The AC compressor always cycles on and off to stay within its assigned pressure window. When AC is called for, the compressor clutch engages and builds pressure until it reaches its high limit, then shuts off. As pressure drops, it will eventually reach the low limit, then cycle back on. The air compressor in your shop/garage works the same way. It cycles on and off to maintain the pressure between 80 and 100 psi (or thereabouts)

 

Idle-up while the compressor is engaged is definitely normal operation.

Compressor on-off-on-off operation (and idle-up) always indicated low refrigerant in my old 4Runner. The truck didn't automatically select AC with defog mode, you had to push the compressor button. I don't remember any on-off-on-off idle-up during defog/defrost, only normal on-off cycling. Maybe they had "improved" the design by 2000.

 

How old was your 4Runner? AC auto-on with defrost has been SOP for decades.
I’m sitting in my truck right now waiting for something to be delivered curbside. I just turned my AC on to time its cycles. The compressor cycles on/off about every 45 seconds and the rpms rise/fall accordingly.

 

It was an '86...but it did have AC, just no cup holders.

Climate control was two slide controls (one for temperature, one for air flow distribution options), a multi-position switch for blower speed (Off, +3 speeds), and a lighted button to turn on the AC (compressor). If you didn't push that button manually, you got no AC and/or no dry air for defog.

I read the OP's question of "RPM pulsating" as rapid on-off cycling of the compressor, each cycle maybe 2-4 seconds in duration. That would not be normal.

 

You sorta got it, the hi limit is only a safety, not exactly a control.
The low pressure switch is dual switch, for control. Once the pressure rises(from heat being added to the Freon, when it absorbs the heat from the air flowing over the coil) the compressor pumps and pumps until the pressure drops below a set point; opening the low pressure switch turning off the compressor.

 

Normal. I've owned 2 first gens, years 02 and now my 01. Both did it/do it

 

Heh.....never really left.

Its just that once I got caught up on things to fix on my tundra, I didn't have much to talk about.

 

Understood.

 

You have just sent a hand stamped invitation for the auto gremlins to visit you late in the night in the near future......