GeorgeM

I get no increase in RPM to compensate for the A/C compressor kicking on. The compressor is new and the system otherwise works very well.

When I turn the A/C on, the RPM drops, then barely recovers back to normal idle. When coming to a stop/pushing the clutch in with the A/C on, as RPM drops back to idle, it'll drop far below idle and almost stall, and has on a few occasions, unless I hit the gas.

The TPS is working.

My question is: Where does the signal to increase RPM to compensate for the A/C compressor come from? Is the RPM supposed to rise above normal idle when the A/C is on, or just compensate enough to keep it at normal idle?

Any insight would be appreciated. Thanks!

bureau13

My understanding is the LH uses AC on/off status as an input into the control algorithm for the ISV. Having recently been through a round of attempting to diagnose possible ISV issues, I have realized that that algorithm is a lot more complicated than I thought it was.

FredR

As I am aware when the a/c is engaged the additional load will drag the rpm's down if the air flow stays the same- thus the ISV opens up to compensate for this as it seeks to maintain the operating set point. What I am not too clear about is whether the system algorithm applies a predetermined correction to the ISV knowing that switching on the a/c is going to drag the system down [predictive control] when the compressor takes load or whether it simply responds to the falling rpm's [feedback control].

captainOCD

I can say that my 86.5 increased idle when hitting the ac button inside even when the ac had no refrigerant in it (so no compressor running).

siscogts

If I remember well, a/c signal to the ecu comes from the relay inside hvac control panel. Check for bad solder joints and bad printed circuit.

Mrmerlin

your ISV could be getting corroded inside and sticking,
you might spray some WD 40 into the line see if that changes anything

GeorgeM

Thx, guys. Will have to check that relay. I did an intake refresh (new ISV) 15k and 5 years ago, so I figured it was okay.... perhaps not.

I broke the Hammer out, went the LH inputs menu and found a test for the ACS switch and compressor engagement. Wasn't sure what ACS Switch was until I got to the next menu with the compressor engagement check and it says to press the ACS switch.... ACS = Air Conditioning System, I guess. I'm obviously still trying to figure this thing out.

Anyway, the compressor test checks out, with the test showing the switch closing, the compressor engages and the fans come on. But with the ACS switch check, the Hammer doesn't show the switch closing when I turn on the A/C.