A/C request to have the engine idle up?

[Bigpatsz]

I have A/C on my 1991 c1500 with 5.3L. Harness and computer are out of 99-04 silverado. I have my clutch signal wire triggering a relay which runs my efans anytime my compressor is on but the engine acts a little funny with the load from the compressor. While normal driving the rpms will go up and down, when coming to a stop the truck usually tends to want to die on me and it seems to be from the rpms dropping so low when at low speeds. Is there a way to fix this issue in the TUNE or will I need to find another wire from my factory harness to run to the LS harness to have the ECU recognize and compensate for the load. Runs just fine under normal conditions with A/C off. Thanks.

 

[L31MaxExpress]

I have A/C on my 1991 c1500 with 5.3L. Harness and computer are out of 99-04 silverado. I have my clutch signal wire triggering a relay which runs my efans anytime my compressor is on but the engine acts a little funny with the load from the compressor. While normal driving the rpms will go up and down, when coming to a stop the truck usually tends to want to die on me and it seems to be from the rpms dropping so low when at low speeds. Is there a way to fix this issue in the TUNE or will I need to find another wire from my factory harness to run to the LS harness to have the ECU recognize and compensate for the load. Runs just fine under normal conditions with A/C off. Thanks.

I am assuming you used the R4 compressor from the 91 or an equivalent high mount like a sanden. Basically a cycling clutch setup rather than the LS low mount variable displacement constant run compressor. If you are using a cycling clutch you can set the a/c type to cycling clutch in the tune. You will need a signal wire run to the PCM. Also I would recomeend adding the a/c control relay. The a/c relay will allow the PCM to cut-off the compressor at WOT and also cut off the compressor if it ever tries to stall the engine.

 

[Bigpatsz]

• Yes it is a high passenger mount r4 compressor. What type of signal wire would be run to the pcm, 12v or ground, I assume changing the ac type to cycling clutch that it will help with the load problem? And the ac relay would be wired up between the compressor and pcm with a pcm output controlling the shut off of the compressor. Would you happen to know what pins either of these signals would be wired to, I'll have to add a pin on the input side and a pin to the output side. Thanks for the help man.

 

[L31MaxExpress]

LT1SWAP.COM for the wiring diagram information. The PCM receives a +12VDC signal for the a/c request and controls the ground side of the relay. Relay should be wired between the cycling switch and the compressor. The a/c request signal should come from after the cycling switch. I use a/c bump torque spark advance in my Express van and do not even feel the a/c compressor cycle on or off.

 

[Ken K]

When you get into 2003-2004, you start find DBW. This would only try to keep the RPM's at a set idle point, and adjust for P/S and A/C load, not change the throttle while driving.
With DBC, the Intake Air Control stepper motor would take over. It was not clear what throttle control you have. If it is DBC, I would go with the above PCM controlled inputs as mentioned above.
Make sure the passage for the IAC and throttle plate a clean, but test the TPS for issues as it may wear at the common spot found at road speed, drop out slightly without setting a code.
With part throttle at road speeds, I would think the IAC would not have enough extra air volume to change RPM's. But, how knows a this point. Let us know the fix.

 

[jdla140]

I am assuming you used the R4 compressor from the 91 or an equivalent high mount like a sanden. Basically a cycling clutch setup rather than the LS low mount variable displacement constant run compressor. If you are using a cycling clutch you can set the a/c type to cycling clutch in the tune. You will need a signal wire run to the PCM. Also I would recomeend adding the a/c control relay. The a/c relay will allow the PCM to cut-off the compressor at WOT and also cut off the compressor if it ever tries to stall the engine.

AFAIK, the 99-07 trucks never used a variable displacement compressor or even a TXV. Both my 02 and 03 are CCOT systems.

However, GMT800's don't have pressure switches. They have high and low side pressure sensors so they can adjust idle accordingly and the computer then decides when to cycle the compressor. Without these signals, your computer is not able to accommodate the additional load of the AC

With the compressor and fans coming on at the same time there's a lot more load on the engine that the PCM is not accounting for. You'll have to wire in a signal wire to the PCM and set the desired idle settings for when the A/C is on, as stated above LT1swaps.com is a great website for all sorts of information.

 

[Supercharged111]

As soon as your PCM gets a signal and knows AC is on, there's a decent chance it won't hunt for idle when the compressor cycles. As Fast mentioned, there are spark correction tables to account for the load bit there are also idle airflow tables that take this into account as well. I forget what's available for the fan, but I imagine there's something. The 411 on my 1500 controls my MKVIII efan based on ECT and AC on/off. With AC on, the fan runs based on refrigerant pressure. You can select from auxiliary, single, or dual fans.

 

[Bigpatsz]

Thank yall for the input. So if I can add an ac signal wire to the pcm it may recognize the load and correct without retuning? If I were to add a signal, what is the function called, ac request? Its pin 17 on the red connector. I'm just not sure what pin to bring 12v to on the connector.

 

[Bigpatsz]

When you get into 2003-2004, you start find DBW. This would only try to keep the RPM's at a set idle point, and adjust for P/S and A/C load, not change the throttle while driving.
With DBC, the Intake Air Control stepper motor would take over. It was not clear what throttle control you have. If it is DBC, I would go with the above PCM controlled inputs as mentioned above.
Make sure the passage for the IAC and throttle plate a clean, but test the TPS for issues as it may wear at the common spot found at road speed, drop out slightly without setting a code.
With part throttle at road speeds, I would think the IAC would not have enough extra air volume to change RPM's. But, how knows a this point. Let us know the fix.

Drive by cable TB

 

[Bigpatsz]

LT1SWAP.COM for the wiring diagram information. The PCM receives a +12VDC signal for the a/c request and controls the ground side of the relay. Relay should be wired between the cycling switch and the compressor. The a/c request signal should come from after the cycling switch. I use a/c bump torque spark advance in my Express van and do not even feel the a/c compressor cycle on or off.

So I can bring my 12vdc signal to A/C request signal on my pin out and what would my output be called for the ground of the relay? I have the pin out diagrams just not 100% sure what all things are called. Thanks for the help