Can I safely disable the 1985 HAC?

[tercle]

Looks like my best vacuum advance pod has a bad port that goes to the High Altitude Compensation valve.
The car is a 1985 Federal 4wd automatic.
Can I disable the HAC system?
I am always below 1000 feet.
How do you do it?
Can I just plug the line that goes to the vacuum advance pod?

[xirdneh]

just plug it at the vac adv
and plug the removed hose
i am at low elevation
and run both cars without it

[dlb]

i think you should disconnect and plug the two lines that tie in with the EBCV, as well as one that goes to the vac advance unit. shouldn't affect anything else. wouldn't even be an issue for high altitudes unless the car was spending lots of time up there.

what makes you say the HAC port on the vac advance unit is bad? i recently got a new one and found that it shares a diaphragm with the main upper port, the one that goes to manifold vacuum. so if both those ports are disconnected, you will suck air right through it—that's normal. so if you plug either of those ports and suck the other one, it should hold vacuum. if it doesn't, that means that diaphragm is busted and you also need to disconnect and plug the main port from manifold vacuum, and then adjust your idle to compensate for the lack of advance at idle.

the lower port on the unit, the one that goes to ported vacuum, has a separate diaphragm and should be tested separately.

[Ace]

It would be helpful to consult a Factory Service Manual for the diagram of the vacuum lines. It should show the connections for cars with and without HAC.

I added HAC to my car years ago because I often drive over 4000 feet. It provides no benefit under 3500 feet, or whatever the trigger point is.

[TURTCEL]

I myself live at 4000 feet everyday so my HAC is a neessary evil. My vacuum adavance was bad so I got a new one and put it in the dizzy as well as cleaned and regreased the dizzy as well (per DLB) works great now. Before the car was having a rough time up here but now the the advance has helped greatly.

I woud recommend that everything be working properly on the vehicle but lets be real, that HAC unit is not for you low elevationers. Just don't plan on a visit out my way unless you are ok with runability issues once you would get up here. But regardless a new vacuum advance is not that expensive and the dizzy regrease is nice to do. Get a new one and replace the bad one is my opiniion.

If the advance is bad then I am sure the dizzy could use a little attention in the lube department too. You will even learn a lot of cool stuff about the Terc on the way too.

Be careful if you do tear into the dizzy though, the FSM has some mistakes in its reassembly process. I just did mine and took a lot of pics that I wil be posting with the process soon. In the meantime I have found that DLB is the dizzy guy on this forum. Listen to his and others advice based on their experiences they know what's up.

Peace

[tercle]

Thanks, all.
Um, dlb? I tested the vacuum advance pod inputs one at a time, without plugging the other two.
I will be testing again soon...

thanks!

[TURTCEL]

You can test the bottom port by itself without plugging the other two ports to comfirm if the diaphram is good for the advance. The lower port controls timing adavance upon higher intake manifold vacuum during acceleration.

When checking the other two ports you must plug the opposing port otherwise it will cause a straight through passage causing no vacuum.

Yeah, dlb...I have found him to be the most helpful on the dizzy topic as well as Petros. They have many posts on the dizzy and can easily help lead you through the process of working on it. But like I was saying the FSM I have learned has some mistakes so pay attention to what you are doing and take close up pics until my post comes up or another.

Good luck...darn vacuum leaks and vacuum controlled components.

Peace

[Petros]

minor point; during acceleration the vacuum drops down when you open the throttle, so the vacuum advance advances the spark timing when you are at part throttle condition (when the vacuum is high). This improves cruise economy, the leaner and lower density fuel/air mixture at part throttle conditions burns slower than the rich high density mixture at wide open throttle, so advancing the timing will improve efficiency to get the peak pressure in the combustion chamber earlier in the piston stroke, improving economy.

One of the other ports is to advance the timing when the High Altitude Compensator (HAC) is engaged above 3900 ft. this will lean the mixture and advance the timing. The third port is to actually retard the spark timing for emissions purposes, the vac line is linked through a thermal link that allows it to advance only when the engine is hot. this last feature you can complete disable without any detrimental affects. The other two can also be disabled without any change in drivablity, it will only affect economy slightly, and if the HAC is actually working, slightly less power when above 3900 ft.

[tercle]

On a simple vacuum advance system, the ported vacuum port near the base of the carburetor goes to the vacuum advance pod.
I read a lot about it years ago when working on a 63 Plymouth and sort-of understand it.

I am trying to figure which of the three inputs on the Tercel vacuum advance pod is which.

First, I am working on a 1985 Federal car. All three inputs to the pod are connected.
It looks like the line with the check valve (the line that is disconnected and plugged on page IG-14 of the service manual pdf) must be the old-fashioned vacuum advance.

I am unable to trace it back to the ported vacuum, though.

If that is the connection that advances the timing at part-throttle high vacuum, what function is the other inlet on the same level?

[Petros]

likely the other is for the HAC since when activated it leans the fuel/air ratio and advances the spark timing. The one on the other side will retard the spark timing for emissions reasons.

[tercle]

"The one on the other side" is the one on the end farthest from the distributor?

[Petros]

I do not remember which side it is on. There are three ports on the vac advance, there are two on one side of the diaphragm that advance the timing, and one on the other side of the diaphragm that retards sparking timing. You have to look at it so see on what side they are when mounted on the distributor, or you can take the cap off and put a vacuum on each of the ports to see which advance and which retard the timing.

[tercle]

http://i48.tinypic.com/x6ep82.jpg

I - vacuum advance
II - HAC (advance)
III - vacuum retard

Right?

[dlb]

I - primary vacuum advance
II - HAC
III - vacuum retard

Right?

no, not quite. it's

i - manifold vac - for advance at idle and hwy cruising
ii - HAC
iii - ported vac - for advance at hwy cruising

the 'i' and 'iii' ports create different amounts of vac because they are meant for different things, although they are combined at hwy cruising. 'i' only advances about 8* while 'iii' advances around 13*. i don't have experience with the HAC but looking at the FSM, it says "this system insures proper air-fuel mixture by supplying additional air to the primary low and high speed circuit of the carb and advances the ignition timing to improve driveability at high altitude above 1100 m (3900 ft)." so it appears that none of the ports on the vac advance unit are used to retard the timing.