86 4Runner AC and Idle Speed

I am having an interesting problem with my AC and engine idle speed. Everytime I have he AC on and running, and then come to a stop sign (ie., put the brakes on) the engine idle speed cycles up and down hi>low>hi>low at about 1Hz (1 cycle per sec). At first I thought this was due to a vaccuum leak, but y/d we located it to be an electrical problem - if you take the fuse out of the brake lights, it now longer cycles the idle speeds, it stays at the high idle just like it is supposed to. So, somehow the brake light electrical system is messing with the engine idle speed, and ONLY when the AC is ON.

I have had the AC recharged, with same problems, and I have had to replace the alternator (for different reasons) with the same results.

Anyone have any ideas on what this might be due to?

Thanks!

Reply to
docuw
Loading thread data ...

You may have the A/C idle turned up too high, adjust the large white plastic screw on top of the idle up valve on the intake plenum down to around 1000 RPM max when the A/C is on. Here's what the valve looks like:

formatting link
If the RPM is too high, the ECU will cut the fuel injectors off when you hit the brakes (it assumes you are braking to a stop, why apply power to the engine). I find mine will oscillate if the RPM is about 1050-1100, so keeping it under 1000 seems to be the key. If you want a high idle and no oscillations, one "fix" is to cut the wire feeding the brake light signal into the ECU:
formatting link
And your removal of the brake light fuse has the same affect as cutting the wire, no brake signal to the ECU.

Reply to
Roger Brown

Hey Thanks Roger. We were actually out working on it when you emailed. We found that the VSV valve (I think thats what it is - thge white plastic screw on the medial aspect of the throttle body - see pic at

formatting link
was out too far. Wetried changing the rpm adjuster that is behind the glove box with no luck,but then went out to the engine and played around with that screw. Itbrought the idle speed down, and stopped the cycling. Thanks again!

Reply to
docuw

The box behind the glove box with the "RPM" knob is called the "A/C Amplifier". It is used to set the low RPM cutoff for the A/C compressor. The idea being that if the idle up valve does not kick up the RPMs high enough and the engine is bogging down under the A/C compressor load, the compressor is kicked off until the RPM comes up higher. You should set that knob such that the A/C cuts off a bit below the normal idle RPM. The idle-up valve should be set to boost the idle to around

900-1000 RPM or so with the A/C running (the compressor is less efficient if run at too low a speed). The idle-up valve is itself controlled by a Vacuum Switching Valve (VSV) that sits atop the valve cover and is turned on whenever the A/C is turned on with the engine at idle.

There is a similar valve on the power steering pump that if operated by hydraulic pressure inside the pump. If you try to turn the wheels at idle, it'll bump up the idle speed in the same way to boost the p/s fluid pressure for less steering effort.

Reply to
Roger Brown

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.