Cruise control won’t hold speed.

[Dickinson County Comanche]

Hey all, added cruise to my Comanche and I’ve run into the problem where it wanders in terms of speed. If I set it at 55, it’ll speed up up to 60-65 then slow back down, over and over. It’ll also occasionally sometimes slow down to 50 before taking back off again. I’ve tried adjusting the module screws for about 2 hours and used 2 separate control units to no avail. 
 

Also, if anyone has a Comanche with a manual and cruise, I’d like to see how your screws are set to get a baseline of sorts. 

[Ωhm]

cruise control system.pdf

[eaglescout526]

Check vac too. At the dump valve when the pedal isnt depressed, you want no leaks there of any kind. A simple blow into the hose that paired with the harness will work as a leak test. But the odds of two modules doing the same thing leads me to another issue that isnt the module.

[Dickinson County Comanche]

3 hours ago, eaglescout526 said:

Check vac too. At the dump valve when the pedal isnt depressed, you want no leaks there of any kind. A simple blow into the hose that paired with the harness will work as a leak test. But the odds of two modules doing the same thing leads me to another issue that isnt the module.

Should the Servo hold vacuum at the dump valve connector? The hose with the pedal on the end is fine, no leaks, but the servo side just blows thru with little to no resistance. The other port (little line) holds vac pressure just fine. 

[Dickinson County Comanche]

I also tried a vac restrictor just for grins and it didn’t seem to help. It’ll hold speed perfect going up or down steeper hills, just flat ground or smoother hills make it do the whole on-off-on-off type feeling it makes. the servo also makes a lot of noise at low speeds

[eaglescout526]

Well if I recall, the internals of the servo should have a diaphragm and a solenoid that work together to pull the throttle wire. I still think there’s a vac issue somewhere either at the reservoir or a break in the line. I have some servos I suspect are good. I haven’t heard of one failing yet. 

[Dickinson County Comanche]

17 minutes ago, eaglescout526 said:

Well if I recall, the internals of the servo should have a diaphragm and a solenoid that work together to pull the throttle wire. I still think there’s a vac issue somewhere either at the reservoir or a break in the line. I have some servos I suspect are good. I haven’t heard of one failing yet. 

I have a vac gauge. Do you know how much mercury it should pull? I’ve tried 2 separate vac bubbles so far and nothing has worked.

[eaglescout526]

If the servo can not hold vac then the diaphragm inside has probably deteriorated. But the servo should be able to hold a vac. So you will have to disconnect both lines, plug one and test it. 

[Dickinson County Comanche]

13 hours ago, eaglescout526 said:

If the servo can not hold vac then the diaphragm inside has probably deteriorated. But the servo should be able to hold a vac. So you will have to disconnect both lines, plug one and test it. 

Ok, so I tested it using plugs and here’s what I found:

 

The larger (Bleed?) hose going to the pedal will hold vaccum. The larger port on the servo side will not hold a vacuum. 
 

Both smaller sides, going to the vac bubble and the servo side, hood vacuum just fine. 
 

Just to recap, the one with the hose connected (not the same truck but still applies) does not hold a vacuum within the servo itself. The hose does with the pedal switch, however. 

[Dickinson County Comanche]

I did notice while poking around that the cruise cable wasn’t fully clicked into place, and that the larger bleed hose might have been leaking when plugged in, but not when testing. I’ve remedied both so we’ll see if that happens to fix anything.

[eaglescout526]

Glad you found something. Sorry been busy all day pulling some fuel lines. 

[Dickinson County Comanche]

2 hours ago, eaglescout526 said:

Glad you found something. Sorry been busy all day pulling some fuel lines. 

Something, but not what’s causing the problem. Is the servo ok vacuum-wise though? I feel like the other side should hold vacuum but doesn’t. If you pull enough vacuum tho it will start to move the diaphragm within it.

[eaglescout526]

If it’s moves the diaphragm then that means it’s pulling like it should and the brake pedal end is just to dump vac when braking so it doesn’t remain engaged. So it sounds like the servo is functioning. 

[Dickinson County Comanche]

55 minutes ago, eaglescout526 said:

If it’s moves the diaphragm then that means it’s pulling like it should and the brake pedal end is just to dump vac when braking so it doesn’t remain engaged. So it sounds like the servo is functioning. 

Brake pedal end will move the diaphragm, the other side won’t. 

[Dickinson County Comanche]

I almost feel as if the servo isn’t holding the speed proper. It’s like it’s letting it slow down too much and then trying to catch up, not the other way around.

[eaglescout526]

Do you maybe have the ports swapped?

[Dickinson County Comanche]

18 minutes ago, eaglescout526 said:

Do you maybe have the ports swapped?

No, swapping them would be quite difficult given their sizes. One is a 1/4” and ones probably a 5/16” or 3/8”. 

[eaglescout526]

Okie dokie, making sure there.

[Ωhm]

Looks like the SERVO has an internal vent, controlled by the Vent Valve Solenoid. Drawing shows valve venting to atmosphere. Could be the default setting (CC OFF).

 

 

[eaglescout526]

2 hours ago, Ωhm said:

 

 

 

 

 

Is that from the electrical manual? 

 

[Ωhm]

33 minutes ago, eaglescout526 said:

Is that from the electrical manual? 

 

Yes. 88ElectricalManual pg48.

[eaglescout526]

Neat!

[Ωhm]

Now, I'm going to make a guess here, but I'll bet when the CC Clutch SW (Manual Trans only) is depressed and only the CC Clutch SW, this will vent the SERVO.

[Dickinson County Comanche]

46 minutes ago, Ωhm said:

Now, I'm going to make a guess here, but I'll bet when the CC Clutch SW (Manual Trans only) is depressed and only the CC Clutch SW, this will vent the SERVO.

So you think the clutch switch is defective then? I can try bypassing it with the little jumper thing the autos use

[Ωhm]

19 minutes ago, Dickinson County Comanche said:

So you think the clutch switch is defective then?

 

Truthfully, I was only talking about how the CC Clutch SW would vent the SERVO, no more than that. But now that you mention it, make sure your CC Clutch SW is adjusted properly. I would think once the CC Clutch SW is engaged that would totally shut down CC and CC would have to be SET again. Try your jumper wire idea.