HDC CTO (???)

[Gil]

So, my neighbor and I were working on one of many leaks on my Cherokee when we noticed there was a hose missing coming off the distributor area:



Looking at the vaccuum diagram, it looks like the hose is supposed to go to the HDC CTO:





So, what is the HDC CTO? My neighbor says it helps regulate timing under acceleration. So why was the hose removed? It's not like it fell off because where it connects to the HDC CTO is capped (yellow cap).

What are the pros and cons of having it connected vs. disconnected? Clearly someone did it on purpose. If I need a new HDC CTO, are they available?

Please help...

[will e]

A good explaination from an customefi page. Even though you don't have a custom EFI the explaination is good.

basically, yes, you should at least get it working well enough for the mechanical advance to work. This would mean connecting it to ported vacuum (this is the one that starts at or almost at zero when you idle and goes up as you apply the gas).

Looks like your device is set up to allow the use of manifold vac to advance the distributor timing at idle when the engine is 'too hot'.

http://www.customefis.com/emissions.html

The vacuum advance mechanism is a different story. There are 3 different devices controlling the vacuum advance on a Jeep distributor, the Non-linear valve, the Spark CTO, and the HDC CTO. Lets follow the vacuum hoses to understand what is going on. It's probably easier to go backwards to see how the vacuum signals end up at the distributor. First, follow the vacuum line from the distributor (location "C") back to the middle port on the HDC CTO. This means that the other two ports on the HDC CTO are incoming lines, and the center port is an outgoing line. The HDC CTO switches vacuum to the center port (distributor) depending on the coolant temperature. The HDC CTO is located in the thermostat housing after the thermostat, so it is a high temperature overide. If you follow the bottom port (in the diagram its the highest one; on the CTO its the one closest to the engine), it is teed into an engine manifold vacuum source (hexagon with the M in it), so the HDC CTO switches the distributor vacuum direct to engine vacuum when the engine is very hot. I have found that this temperature is 220 degrees F. The purpose of the HDC CTO is to advance the timing at idle to the maximum vacuum advance it can have. This helps to cool the engine by making it more efficient. It also increases nitrogen oxides, but since this is sort of an emergency condition, the pollution is tolerated. So the HDC CTO is only used to switch the distributor vacuum to full engine vacuum when the engine is very hot. Any other time, the vacuum comes from another source.

Now, follow the other HDC CTO port (the top one; bottom in the diagram) and you will see that it goes to the middle port of the spark CTO. We know that the middle port on a 3 way CTO is an outgoing port, so lets see what the other two are coming from. The bottom port goes to the carburetor at port S. (There is also a tee in this line that goes to the non-linear valve.) The top port goes to the non-linear valve. I have tested the spark CTO and found that below 135 degrees, the vacuum is directed from the top to the center port and above 135 degrees the vacuum is directed from the bottom port to the center port. So when the engine is below 135 degrees F, the vacuum that operates the distributor comes from the non-linear valve. Above 135 degrees F, the vacuum comes from a carburetor ported vacuum port. Since I have already discussed how ported vacuum works, we now know how the vacuum advance works between 135 F and 220 F. When idling, there is no vacuum advance. When the gas pedal is pressed ported vacuum is applied to the distributor and the vacuum motor advances the timing according to the vacuum motor spark timing curve. But what about when the engine is below 135 degrees F? This is when the non-linear valve comes into play. I have tested the function of this valve. It provides about 8-9 in HG. of vacuum at its outlet port (the one going to the spark CTO) at idle when there is engine vacuum on the middle port and no vacuum on the bottom port. But as soon as the gas is presses and port vacuum is applied to the bottom port, the vacuum at the outlet immediately jumps to whatever the engine vacuum is. So the function of this valve is to send about half the engine vacuum to the distributor when it is cold and idling, but as soon as the gas is pressed, the vacuum at the distributor tracks engine vacuum.

What does all this mean? Well, it means that the vacuum advance depends on engine temperature and is controlled differently depending on what range of temperature you are operating in. Below 135 F, you get more vacuum advance at idle than when warmed up, but not full engine vacuum advance. Above 135 F but below 220 F, you get no vacuum advance at idle because there is no vacuum on the ported vacuum port S at the carburetor. And above 220 F, you get full engine vacuum advance. The big question is why is all this necessary? Well, when an engine is cold, you want a lot of advance to help the engine run well, plus you get more power with a lot of advance on a cold engine. When the engine is fully warmed up, you also run better with about 30 degrees of advance at idle, but this creates hot combustion temperatures. So to limit the creation of nitrogen oxide pollution, Jeep engineers took away vacuum advance at idle on a warmed up engine. But this makes the engine run hotter because it is less thermally efficient. In other words, late spark timing makes the engine run hotter. So, the HDC CTO puts the vacuum advance at its maximum when the engine is running hot to help cool it down.

[Gil]

I just found that same site via Google as you posted it...I still don't understand why someone would cap it (and still a little confused by the article, but I am trying).

So, still looking for pros and cons. Again, it was done on purpose.

[will e]

Maybe a misguided person who doesn't understand what it is for???
Maybe the distributor diaphram is broken and it caused a vacuum leak so they just disconnected it?
Perhaps they were getting pinging from the timing getting advanced and this was an easy way to fix it?

I would say your best bet is to test to see if it is working.
Check that you have the proper vaccum at the source ports (one from manifold, one from ported). Then check if you get ported vacuum while idle and the engine is cool. Then check if you get manifold vaccum when the engine gets 'too hot'. (This might be difficult to check). I would say if you get ported vacuum when the engine is at normal temp, hook it back up.

If you get pinging, try better gas and/or adjusting the initial timing. (Note where the timing was before you started this whole adventure). If you can get the engine to run better WITH it then do so. If not, disconnect it and put the timing back to where it was.

[fulsizjeep]

If you don't need emissions equipment where you are, the vacuum line can go from the dizzy to the intake or carburetor base where you have unported vacuum.

Reasons why the line was missing... Why ask Why?

[Gil]

If you don't need emissions equipment where you are, the vacuum line can go from the dizzy to the intake or carburetor base where you have unported vacuum.

Reasons why the line was missing... Why ask Why?

Just trying to learn . If my neighbor didn't see it, I never would have known. So, if it ain't broke, don't fix it...and if you don't know it's broke, you don't know you need to fix it. So all is well

[DanHS]

After toying around with adjusting the vacuum advance (little hex key screw inside the vacuum port on the distributor) and connecting the vacuum through CTOs and ported and manifold vacuum, I got the best performance and pretty much eliminated the spark knock/pre ignition (bad for your engine) with the vacuum advance connected to manifold vacuum with a vacuum delay valve from some old Ford in the junkyard. Your timing may have been altered to compensate for the lack of vaccum advance, but I doubt it. To test if the vacuum advance works, connect a hose to the port on the distributor and take off the distributor cap. Suck on the hose and see if the distributor advances. If your neighbor has a vacuum guage, then set the advance to start moving at about 7 inches of vacuum, and work from there.

[Gil]

Thing is, I never had any pinging or knocking. I didn't even know the hose was not connected. I stopped by my mechanic and he said HE disconnected it. He said AMC engines are real sensitive to timing and getting it just right is tough...so he disconnected the tube, capped the vacuum valve and upped the timing. Per my neighbor, now it's a constant timing, so if I try towing or going up hills, I'll notice issues.

So, tomorrow we are going to reconnect it (might just get a new vacuum valve just for fun -- just to make sure it's not bad) and see what happens.

[BarryL]

Going up a hill or towing a good load would be near the same as having it disconnected. High vac advances the timing. Vac is highest when the throttle plates are closed. As you press the gas pedal and open up the throttle plates, vac drops and the timing advance drops with it. The distributor mechanical advance is the primary advance mechanism while running with power. I have no idea why your mechanic disabled it. Perhaps there was some spark knock and he is unfamiliar with the adjustment. I would re-connect and adjust it all to spec.

[shimniok]

In case it helps: http://tc.wagoneer.net/sblog/static....sj_vac_routing

The HDC CTO is really for overheating conditions to deliver manifold vacuum to (as I understand it) increase rpm by increasing timing advance at idle. I went ahead and bypassed mine. I'm curious to know if it really helps cool down the engine.

Michael

[GWChris]

I would say you should look around for a mechanic with more knowledge.

[JERSEY JOE]

HEAVY DUTY COOLING CTO

THE ONLY THING IT DOES IS ADVANCE THE TIMING VIA MANIFOLD VACUUM AND SPPED THE ENGINE UP TO INCREASE THE COOLANT FLOW AND THE FAN SPEED.

[Gil]

Well, I was afraid that would happen...more than one opinion . Some say reconnect it, some say don't.

So far, my engine runs cold...needle never makes it into the green area of temp guage. I was told it's because of the aftermarket aluminum radiator. But that doesn't make complete sense to me. So, I will be checking the thermostat tomorrow to see if it's the right one and if it's working.

Who knows, while some say I should find a new mechanic...maybe he knows exactly what he's doing.. (?)

[GWChris]

Vacuum spark advance will give you better throttle response and mileage, which is why all street engines had it - at least up until modern computor controlled ignition. Once you are under load it doesn't do much, but real engines spend a lot of time at light/part throttle. Maybe your mechanic is smarter than all the engineers that designed the stuff....

[Gil]

Maybe your mechanic is smarter than all the engineers that designed the stuff....

I sense a little sarcasm. I certainly don't want to get into an argument about who designed the stuff...the FSJ was certainly never an engineering marvel, light years ahead of its time . All over this web site are threads of how people modified/bypassed/added components to improve the functioning of the FSJ. In fact, within this thread some bypassed the HDC CTO with no issues.

I certainly don't NEED the HDC CTO connected to the distributor...proven by others and the fact that I didn't even know it was disconnected and noticed no problems. If hooking it up helps, then I will do it. If hooking it up makes adjusting the timing a nightmare, I will disconnect it again.

I am all for experimenting as that's how I am going to learn to work on this beast and what all the components do. I guess that's part of the madness of FSJs.