350 tbi idles down with a/c on

[Jp87v10]

I've got a 1987 Chevy v10 350 tbi with 700r4 that when I turn on the a/c (that has all just been replaced and refitted for 134A) my rpms drop a little instead of going up and it puts a strain on my engine. I assume the IAC valve works because when its cranked up it idles up for a second then idles back down,  But when the a/c kicks on it idles down instead of idling up to compensate for the load. Any help would be appreciated.

[bd]

There are a few things you can check to ensure there are no malfunctions, such as TPS adjustment and function, timing adjustment, spraying for vacuum leaks, EGR function, IAC response, and the various sensors and actuators.  Ultimately, however, idle bump with the A/C energized is a definable parameter that cannot be changed without the appropriate hardware and software to rewrite the PROM.  Unless the idle dip is intolerable (nearly stalls the engine or the A/C belt produces an annoying standing wave vibration at idle), you may need to live with it.

[Jp87v10]

I know my tps in non adjustable one. Could be vaccum leak but everything else works fine. Throttle body and all sensors have been replaced. New egr valve. The belt does have a wave/bad vibration to it when idle in gear,smooths out at higher rpms or in park. Just dont want to risk messing up the new compressor really. Timing is on point also. Just always heard that it should idle up to compensate and when mine is on it drops anywhere from 200-500 rpms depending on if it's in park or drive, warm or cold, etc.

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[bd]

Verify there are no vacuum leaks and that timing is properly set.  Otherwise, the simplest approach may be inordinately tensioning the compressor belt to help dampen the vibration.  The standing wave is a function of the pulse frequency produced by the compressor, the mass of the belt and the length of the span between the compressor pulley and the water pump pulley.  At idle RPM the belt approaches resonance and a standing wave develops.  It is an obnoxious design flaw. 

Four potential approaches to address the issue that come to mind are: 1) exceedingly tight belt adjustment, 2) introducing a 50-75 RPM idle bump with A/C energized by overwriting the PROM, 3) fabricating and retrofitting an idler pulley and bracket, and 4) a different design compressor with more pistons.

[MIKE S]

An 87 Tbi has a green wire to the clutch. Follow it back to the firewall to the top near the washer nozzle hoses. It should plug into another green wire going towards the a/c suitcase which goes to the cycle switch.  Were that wire plugs into the connector at the top of the firewall you will notice one side has one green wire and the other side has two green wires. Follow the one with two. One goes to the compressor and the other goes to another connector down the drivers firewall to another connector which leads back to the ecm. That tells the ecm that the compressor is on an will raise the idle speed. It may be disconnected.   

[Jp87v10]

Okay thank y'all. I'll look into these things after work and make an update.

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[JohnnyPopper]

Wow, never heard of a v10 350!

Do you still have a mechanical fan?

I ditched mine in lieu of an electric and gained mad horsepower.

BD advised switching it on whenever the A/C is running, works great!

[Jp87v10]

Here's a video of what I have going on.
A/c off truck runs great 1,000 rpm in park and 500-550 in gear. A/c on I'm at 450-500 rpm in gear and stays at 500 in park. Havent gotten to check anything out yet. Probably be this weekend when I get off work. http://cloud.tapatalk.com/s/5d7ad43de77a0/20190911_145739.mp4

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[JohnnyPopper]

the video wasn't very useful, it froze in the middle, looks like your belt is really loose.

Is the motor a vortec, ergo 'v10 350'?  I somehow thought it was a ten cylinder... 

[Jp87v10]

I'm not sure. I was told it was a 1st gen vortec but the heads dont have the vortec markings. No the model of the truck is a 87 v10 4x4, like a c10 but its a v10. Has the 350 tbi with a 700r4 trans. That's how I thought it looked too but the belt is tight. Just wanted to try and upload a video to show what my belt was doing but I dont guess it helped. Haha

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[MIKE S]

You are not going to hurt that compressor by making that belt tighter. I bought an 87 R10 GMC brand new in 87. I also maintained a fleet of about 30 square body’s of various years from 1975 until 1999. At that time I did all the repairs on these trucks. I never replaced any part I couldn’t repair. Starter brushes , starter drives ,alternator brushes, bearings and diodes. Even rebuilt the A/C compressors. I learned a lot from those trucks which led me to working at a GM dealer for the last 19 years. In 87 they had a service bulletin for the compressor pulley being to light weight causing groans and vibrations. I replaced the one on my first 87 with an updated heavier weighted pulley. It took away about 80% of the vibration. But I had to make the belt pretty tight to get rid of the belt deflection. The retrofit pulley is no longer available from GM. The new compressor that I installed on my current R10 looks about like yours and is heavier than original but not like the one that was available in the service bulletin. Also don’t forget that if  is a new belt it will stretch in the first 30 minutes of use and you will have to retighten. 
 

[bd]

I remember that bulletin and concur with Mike.

...The standing wave is a function of the pulse frequency produced by the compressor, the mass of the belt and the length of the span between the compressor pulley and the water pump pulley.  At idle RPM the belt approaches resonance and a standing wave develops.  It is an obnoxious design flaw.

The dancing A/C belt is all too common for engines with that belt and R4 compressor configuration.  It is most pronounced at <650 idle RPM.  You can tighten the snot out of the belt, which will help some.  Removing the hardened, tamper-resistant cap from the TBI base and adjusting base idle to 500 RPM in drive (with the IAC air channel in the TBI base temporarily plugged and ignition timing set to zero with ESC bypassed) will benefit idle quality and off-idle driveability issues and also help with the standing wave in the belt (that I suspect is audible inside the cab and quite annoying). 

The PROM idle parameter is 600 RPM from the factory with no added idle compensation for the A/C energized.  As long as the ECM receives the A/C on signal via the green wire referenced by MIKE S, there exists a 50/50 chance that the ECM will maintain idle at ~600 RPM.  However, idle speed is not precisely controlled by the ECM.  In addition, 600 RPM likely will still produce the standing wave vibration in the belt. 

Bumping engine idle to 650-675 RPM in drive with the A/C energized should null the standing wave in the belt.  Unfortunately, if this is attempted through manual adjustment of the base idle speed screw (as described above) in lieu of using a retrofit idle speed solenoid or the IAC via programming, idle RPM will soar to ~800+ RPM in drive with A/C off and the transmission may slam into gear while the engine may try to diesel when shut off.  On the other hand, adding idle RPM compensation with the A/C energized is a simple task for a programmer with the appropriate definition file.  In other words, there is no easy, comprehensive solution readily available to the average guy.  It was a poor design.  Begin by tensioning the compressor belt some more.  But, don't be surprised if the second P/S belt loses some tension in the process.

[Jp87v10]

Okay I will try to tighten it some. Thanks for the info guys. I have the service manual and fuel/emissions/drivability manual for the 87 year models and that's how I set my idle after I replaced my throttle body. Right now it is idled about 550 in drive and 1000 in park. Didnt want to mess with it too much because i didnt want it to idle too high in park or when it's cold. Checked timing and it's set at 0°. Again thanks for all the help. And even being set where it's at my truck kind of slams into reverse or drive figured it was just the stall in whatever converter I have, which "should" be stock.

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