Resolved Cruise control diagnostics

[FastDriver]

I just finished my Cobra brake install & cruise control isn't working. I am 100% confident it did prior to the Cobra brake install. I checked a few electrical things, and they looked good. All fuses were good. What I'm thinking is that now that the brake pedal flicks the brake lights on a hair trigger... I literally cannot move the pedal in the slightest without the brake lamps coming on. So, I got to wonderin' whether it's possible that whatever triggers the brake pedal switch might be slightly separate from what triggers the lights. Is it possible that the brake pedal is preventing the cruise control from activating, and if so, would pulling the #1 fuse be a way to ensure that the pedal isn't deactivating the cruise? Problem is that according to this diagram "Speed Control" is also tied in. So, I'm worried that the #1 fuse would also kill cruise control. Does anyone know, and/or would anyone with working cruise mind checking what happens to your cruise control if you pull the #1 fuse?

[Note/edit: answer is yes. Pulling the #1 fuse kills both the brake lights and cruise control which is why "speed control" is noted for it in the diagram below. Therefore, one cannot remove the #1 fuse to test whether the brake switch is preventing cruise control]

 

[Noobz347]

Thanks, TMOSS!

 

[FastDriver]

Ah, man... Wiring diagrams make my head spin. Is the brake switch disabling the cruise control on there somewhere? Not seeing it.

 

[General karthief]

Check the dump valve (vacuum operated) on the brake peddle and the clutch switch

I have another wiring diagram for the cruise control but I don't wanna give you another headache. Let me know if you want a shot of it.

 

[Noobz347]

First... Does your horn work?

 

[Noobz347]

And according to this (Thanks @Mustang5L5 ):

Pulling the #1 fuse would disable the cruise control just like pressing in the clutch.

 

[Noobz347]

D'oh...

I should just leave this here too:

Foxbody Mustang Cruise Control information, location, wiring diagrams

Done a little research into this and figured what I learned should be put into a thread and added to the tech sticky. As you can imagine, nobody really gives much thought to cruise control on these cars, so many searched threads had dead ends. So many this will help someone out down the road...

stangnet.com

 

[FastDriver]

Thanks, guys. I better check that dump valve. Anything related to the brakes.

 

[Mustang5L5]

Do you have LED's in the brake lights? If so, those would prevent the cruise from working as well.

Also, faulty VSS sensor.

 

[FastDriver]

I do not have brake LEDs, only in the headlights. The VSS, Dakota, & Cable-X are all working, as they are pushing the signal/cable that drives my speedometer. The same output from the Dakota runs back to the computer and I've tested for continuity all the way to the wiring harness. I have not tested it at the EEC, yet, but not sure that's necessary yet as it worked before messing with the brakes.

Yes, to Noobz. The horn works. Also, the cruise worked great for the few weeks prior to the brake work. I used it every day on the way to/from work.

I did notice on the drive into work, this morning, that the brakes don't have that telltale whoosh sound that at least my black turbo car definitely has. Not sure if it should. Makes me wonder about that vacuum dump described above. I've never known about that, but I guess that's where that sound comes from. Black Jack, on the other hand, does not make any sound and has no pedal travel before lights kick on (like zero, when I tested the pedal by-hand laying upside down under the dash looking at the pedal with a family member watching the lights). I also feel braking pretty instantly, and I will say that I think there's a bit of drag to work out as I can feel it stop the car when backing out of the garage. Is this where I need to adjust that rod in the booster? So, would an open vac dump valve prevent the vacuum control to the server thereby preventing the CC from being able to control the throttle cable? If so, this might be the culprit.

I'll crawl under the dash and take a looksie for the dump valve. Still, is that what drives the cruise shutoff? In the diagram above it looks like it kills any vacuum modulation to the servo, but not sure it would disable cruise. I figured it was electrical, and the fuse killing the CC supports that idea. I suck at electrical, and I'm having trouble making heads or tails of how that brake switch closes when depressed, looks like the signal then runs to the clutch pedal switch which opens when depressed. So, I can't figure out if a closed signal or an open signal is what kills cruise from that diagram. Or maybe it's two separate signals. I was just trying to figure out a way to bypass one line at a time to help assess where the problem is.

 

[General karthief]

The dump valve releases the vacuum that holds the throttle at speed you selected, hit the brakes, vacuum is released releasing the throttle cable. Otherwise you ain't stopping.

 

[FastDriver]

Yeah seems right, and if left open, presumably it wouldn't allow the modulating valve to control the servo.

From Mike's thread, this is the correct wiring diagram, and now the brake switch & clutch switch integration make more sense - they're separate and both monitered by the Speed Control Amplifier.

 

[Mustang5L5]

The vacuum dump on the pedal only connects to the cruise servo. It normally wants to be open, and the brake pedal pushing back against it pushes the yellow button in to close the valve and allow the servo to hold vacuum and operate.

Often times i see them installed not deep enough, so the yellow plunger isn't fully depressed. Thjis gives you a vac leak into the booster.

You just need to make sure its seated all the way forward in the clip it's in. This valve is normally open, and required pushing the yellow part in to work. It can also fail in terms of not sealing.

 

[FastDriver]

for the MFW!!!

Boys, all credit to ya and especially to @General karthief for calling it first & to @Mustang5L5 for all of the info... another great learning experience for me! You guys killed two birds with one stone for me. First of all, the dump valve was askew & there's a plastic clasp on the brake pedal that provides a flat surface for the dump valve. It was rotated such that the flat surface pointed downwards. I guess I screwed it up during the booster install. Getting that set right and the dump valve angled correctly solved the cruise control issue. It's working again!

BONUS!
I really thought I'd chosen poorly going with the 93 Master Cylinder. The pedal has been firm and feeling great around town, but when I've pushed the pedal to threshold braking, I'd say it's taken maybe 90% of my total leg's capability to keep it there. That makes it hard as hell to be consistent and would have been totally exhausting on an HPDE or track day. Well... I guess the vacuum leak created by the dump valve was the culprit for that, as well. Now, I'm at threshold with about 25% leg-effort, which is completely comfortable and manageable for a whole day.

Stoked! Thanks so much, guys! I don't know how long it would have taken me to figure that one out alone.

 

[Noobz347]

I hit the [Resolved] button

 

[General karthief]

I just made a lucky guess