ac idle advance

[Ptarmigan]

On my 1995 2nd Gen 3L, you adjust the idle stop with one screw but there is also a vacuum actuated second screw that is pulled into place when the AC gets turned on. You can adjust the length of that screw for it to increase the throttle position to precisely counteract the drag of the AC pump so that the idle speed stays identical. If you get the adjustment wrong then turning on the AC can reduce or increase the idle speed.
What has happened to me is that the vacuum advance gets stuck in the on position so when you turn the AC off, the rev advance stays engaged so the idle is now higher than it should be.

The advance has a small vacuum device with a hose running to the other side of the engine, into a black box with electrical power and a single vacuum feed from the engine. It also has another nozzle which is N/C. If I disconnect the vacuum hose to the vacuum valve next to the throttle, the advance falls away immediately. When I reconnect, the advance stays off until I engage the AC, then it clicks on. But turning of the AC doesnt let it spring back as the vacuum is still present, holding it in position. Does anyone have a diagram for this part please or could perhaps have a 30 second look and tell me how many tubes are connected to the black box on the side of the engine?

The problem has been like that since the cylinder head came off so I can`t help assuming that there should be another hose connected somewhere to that isn`t, like maybe there should be three hoses on the blackbox, not just two?

Currently it looks like:

advance screw--vacuum device--single vacuum hose to box on other side of engine--blackbox--single vacuum hose to engine to get vacuum feed.

Cheers,
Jon

[NiftyNev]

Sounds like the vacuum is not bleeding off when AC is switched off. There will be a vacuum supply to the VSV and a switched port for vacuum to the idle up actuator. There is also a filter on all VSV which also serves as the bleed off port. Make sure this is not blocked. If it is the vacuum will not bleed off. You say that when you disconnect the vacuum hose from the actuator then it operates correctly until AC is switched on again which is what makes me think the bleed off port is blocked.

Nev

[Ptarmigan]

Hi NiftyNev,

The setup is exactly as you described, actuator working perfectly but I`ve not had time to get the VSV off yet, since I`m using the truck practically all day every day. I did wonder if the VSV should have a third pipe connected to it, so at least I`m now not still hunting around the engine bay looking for it!

Although I`ve not found the problem yet, I wanted to say thanks for the reply and will post back once I`ve actually got the VSV off and cleaned.

Cheers,
Jon.

[MattF]

The VSV's generally have just two hoses connected. The third port, (the one which is at the opposite end to the other two and at a right angle to them), just has a filter on it. That's the bleed-off port.

[Ptarmigan]

Solution: AC idle advance not functioning

Hi guys,

After some time outside Brasil, I'm back for a bit so I decided to tackle the A/C problem. I had the VSV off today, cleaned up the electrical contacts and did a basic blow through test to see how the different ports behave when the current is off. I found the third port (also called the filter port) had a rubber cap over it, so basically closed off. I removed it and rechecked the hose connections, gave everything a quick clean and now it's working perfectly. I kept the cap just incase I need it later!

So the end solution was:

1) Remove two hoses (one leading to actuator, one leading to engine block vacuum feed)
2) Remove electrical connector (just pinch it and it pulls out easily)
3) Undo 12mm bolt securing VSV and withdraw (I accessed the bolt from on top of the engine with a normal socket, no extendor bar)
4) Withdraw bolt (it's a biggy, maybe 3 inches long)
5) Remove VSV
6) Check all the hoses by blowing through them and checking there is also a good vacuum seal.
8) Apply a bit of vacuum / air to all the ports on VSV to pull out any crap
9) Clean contacts on VSV and on the contacts on the plug with a small wire brush
10) Reconnect the hoses in what you think is a sensible fashion (one port at one end of the VSV is n/c open, the other two ports at the other end have hoses connected)
11) Start engine, start internal cooling fan from control panel and press A/C button; return to engine bay and check advance is active; if not, switch the hoses on the VSV and try again. Once it is working, turn off the A/C button and check actuator returns to correct position. N.B. Do this with the VSV "loose" in the engine bay; you will find it a lot easier to switch hose positions!
12) Check engine idle speed with AC turned off and adjust screw as required (remember it has a locking nut, so don't force it!). From memory, the 3L should be ~750RPM
13) Engage the AC and the actuator should push forward the throttle advance screw. Depending on adjustment, this will either fall short of the throttle, just touch it or actually push it further forward, therfore "advancing" the throttle position to compensate for the drag of the AC compressor. We want the latter.
14) Adjust the ac throttle advance screw to give an engine RPM of around ~850 RPM; this will mean you get nice cold A/C but also decent fuel economy still in slow moving traffic.
15) Check is all works (AC is off, engine at 750 RPM idle, AC is on, engine at 850 RPM idle)
16) Refit VSV with the single 12mm bolt and tension gently; no need for high torque here as I fear you might strip the thread.
17) Enjoy nice cold A/C

Best regards,
Jon

[NiftyNev]

Hi guys,

After some time outside Brasil, I'm back for a bit so I decided to tackle the A/C problem. I had the VSV off today, cleaned up the electrical contacts and did a basic blow through test to see how the different ports behave when the current is off. I found the third port (also called the filter port) had a rubber cap over it, so basically closed off. I removed it and rechecked the hose connections, gave everything a quick clean and now it's working perfectly.

Sounds like the vacuum is not bleeding off when AC is switched off. There will be a vacuum supply to the VSV and a switched port for vacuum to the idle up actuator. There is also a filter on all VSV which also serves as the bleed off port. Make sure this is not blocked. If it is the vacuum will not bleed off. You say that when you disconnect the vacuum hose from the actuator then it operates correctly until AC is switched on again which is what makes me think the bleed off port is blocked.

Nev

Nev

[Ptarmigan]

Nev,

You da man!

;-P

Thanks again to everyone who took the time to reply,
Jon