12H-T AC Idle Up Question

[elmariachi]

Working on baselining the truck and can't adjust valves til new valve cover gasket arrives. In the meantime I adjusted the base idle up from 600 rpms to manual spec @ 770 rmps (AT). But I am not sure AC idle up is working. Per the manual I remove and plug both vacuum lines going to idle up actuator. I apply vacuum as prescribed but the vacuum bleeds off in about 15 seconds and the system doesn't behave the way the manual suggests. No movement of actuator with or without lines attached and no idle up when AC is switched on. I assume the vacuum bleeding off when using hand pump suggests faulty/leaking actuator?

Thanks.

 

[elmariachi]

I love it. After 16 years of wrenching on Cruisers I finally found something no one has the answer to.

Working on baselining the truck and can't adjust valves til new valve cover gasket arrives. In the meantime I adjusted the base idle up from 600 rpms to manual spec @ 770 rmps (AT). But I am not sure AC idle up is working. Per the manual I remove and plug both vacuum lines going to idle up actuator. I apply vacuum as prescribed but the vacuum bleeds off in about 15 seconds and the system doesn't behave the way the manual suggests. No movement of actuator with or without lines attached and no idle up when AC is switched on. I assume the vacuum bleeding off when using hand pump suggests faulty/leaking actuator?

Thanks.

 

[9180ltd]

Diaphragm is done, can't see it being anything other than that. I'm assuming its more or less the same as the one I have on my 1hdt and they are pretty basic....

 

[HJ61 12HT]

Sounds like the diaphragm has died, double check by putting a clean pipe on there and sucking with your mouth, you will easily suck enough to see it fully move.
My 12HT has a single idle up diaphragm, the AC valve on the inner fender let's vacume through when AC is on, there is also a cold engine valve that opens and let's vacume through, as well as the pressure sensing valve on the side of the power steering pump that also ups the idle.
You probably need to check each of those separately!

 

[elmariachi]

Interesting that after monkeying with it, when I started the truck cold today it rev'd up to 1000 rpms because the AC button was on. I quickly adjusted it back to the spec 880 rpms, but no sooner had I done so than it let go and rpms dropped back down. So yes it appears the diaphragm is leaking. Where the hell will I find this part? Looks to be delisted....

 

[HJ61 12HT]

Interesting that after monkeying with it, when I started the truck cold today it rev'd up to 1000 rpms because the AC button was on. I quickly adjusted it back to the spec 880 rpms, but no sooner had I done so than it let go and rpms dropped back down. So yes it appears the diaphragm is leaking. Where the hell will I find this part? Looks to be delisted....

You are sure it was the AC and not the cold engine switch that was idleing it up?

Usually if the diaphragm has a leak it just wont work at all, its unusual for it to work one minute and not the next.

Stick a pipe on it and suck, if you can suck air through then it proves its leaking.

 

[bigbrowndog]

I tried to buy one of these in Oz for an American a few years ago and they were not available from Japan then, Goodluck.

 

[elmariachi]

You are sure it was the AC and not the cold engine switch that was idleing it up?

Usually if the diaphragm has a leak it just wont work at all, its unusual for it to work one minute and not the next.

Stick a pipe on it and suck, if you can suck air through then it proves its leaking.

You are correct, it must have been the cold air idle up. It only lasted about 5 -7 seconds....likely because ambient temps here were nearly 90F when I started it. I removed the suspect actuator and it does respond to and hold vacuum well. I think the reason it was bleeding off before was I didn't have a tight seal in the tubing. So best I can tell, I don't have any vacuum pulling on the actuator when the AC is switched on (neither line going into the actuator has any vacuum readings with the truck idling with AC on.) So now I need to pull and test the VSV. All of the vacuum lines are soft and in tact. The truck still has the little wraps and clips around the lines so it doesn't appear that the vacuum plumbing has ever been messed with.

 

[elmariachi]

My 12HT has a single idle up diaphragm, the AC valve on the inner fender let's vacume through when AC is on, there is also a cold engine valve that opens and let's vacume through, as well as the pressure sensing valve on the side of the power steering pump that also ups the idle.
You probably need to check each of those separately!

That was it. Somewhere along the way someone installed one of the small orange/black one-way check valves backwards in the line from the AC VSV back to the idle-up actuator, so the VSV wasn't able to pull the diaphragm open. I reversed the check valve and voila, it started working. Adjusted per factory spec and it works like a charm. And along the way, I now know how the entire frigging vacuum system works on this truck. Also learned that the truck won't shut off if you unhook any of the several vacuum lines that supply the vacuum kill circuit. Nice to know that.

For those who come later and are chasing this problem:

On the 88 (and assumedly 89) 12H-T with AC and Auto, there is a vacuum circuit that, among other things, controls the truck's ability to idle itself up either when cold, or when the AC is switched on. The cold start idle-up event is achieved vis-a-vis a sensor in the block (JDM driver's side) just back from the injection pump. It has two vacuum lines connected to it. When the truck is cold, the sensor switches open and allows vacuum to pass through it and on to the idle-up actuator, which is a yellow cad plated 3" diameter plunger diaphragm mounted at the rear end of the injection pump. When vacuum hits that, it closes the plunger and pulls a tab against the idle arm and increases the idle. The amount of increase is determined by the threaded adjuster screw on the pull tab.

When the AC circuit is activated, a VSV located on the JDM driver's inner fender well opens and allows vacuum to pass through it and draw on the same idle-up actuator, but on a different port. The vacuum pulls on the plunger and it pulls the same tab as the cold idle-up to increase the idle. The adjustment for the AC idle up is on the forward side of the actuator and consists of the threaded diaphragm shaft and a lock nut. By loosening the nut and threading the shaft in or out, you determine how much idle-up is achieved when the AC is on. Its complex but ingenious because both circuits use the same actuator to move the idle position lever, and the adjustments are both on the same actuator shaft (one on the pull tab end and the other and the opposite tail shaft end).

 

[Strick]

Hi all, huge thread dig here. Im going through this problem now. 1988 12ht auto

My vac system is hooked up correctly.

My engine doesnt idle up with ac or ps.

How does the vsv that goes to an actuator on the back of the injector pump, with the adjustment on the front of the actuator mentioned above, idle up the engine?

 

[BJ4x4]

That was it. Somewhere along the way someone installed one of the small orange/black one-way check valves backwards in the line from the AC VSV back to the idle-up actuator, so the VSV wasn't able to pull the diaphragm open. I reversed the check valve and voila, it started working. Adjusted per factory spec and it works like a charm. And along the way, I now know how the entire frigging vacuum system works on this truck. Also learned that the truck won't shut off if you unhook any of the several vacuum lines that supply the vacuum kill circuit. Nice to know that.

For those who come later and are chasing this problem:

On the 88 (and assumedly 89) 12H-T with AC and Auto, there is a vacuum circuit that, among other things, controls the truck's ability to idle itself up either when cold, or when the AC is switched on. The cold start idle-up event is achieved vis-a-vis a sensor in the block (JDM driver's side) just back from the injection pump. It has two vacuum lines connected to it. When the truck is cold, the sensor switches open and allows vacuum to pass through it and on to the idle-up actuator, which is a yellow cad plated 3" diameter plunger diaphragm mounted at the rear end of the injection pump. When vacuum hits that, it closes the plunger and pulls a tab against the idle arm and increases the idle. The amount of increase is determined by the threaded adjuster screw on the pull tab.

When the AC circuit is activated, a VSV located on the JDM driver's inner fender well opens and allows vacuum to pass through it and draw on the same idle-up actuator, but on a different port. The vacuum pulls on the plunger and it pulls the same tab as the cold idle-up to increase the idle. The adjustment for the AC idle up is on the forward side of the actuator and consists of the threaded diaphragm shaft and a lock nut. By loosening the nut and threading the shaft in or out, you determine how much idle-up is achieved when the AC is on. Its complex but ingenious because both circuits use the same actuator to move the idle position lever, and the adjustments are both on the same actuator shaft (one on the pull tab end and the other and the opposite tail shaft end).

Great info, in my 88 61 auto, when the AC is on the idle up doesn't work, do you have a picture of the VSV??
thanks