Cruise Control ON Switch Function

[sdpatt]

I have searched, but not found the answer to my question. The cruise control on my 1991 (MTX) stopped functioning while engaged at 65 mph one morning on the way to work. This was after 380,000 miles of operation. I felt the gas pedal "pop" up under my foot that was hovering over the pedal and the system jas not operated since that time.

With the steering wheel removed, I checked the continuity and resistance of the cruise control buttons at the connector inside the steering column hub. The Resume, Set/Accel and Coast buttons all measured to the correct impedance. The OFF button measured as closed circuit when pressed and open when released. The ON button, however, made no difference on any of the conductors. Only open circuit.

The question is, what is the required change of state of the cruise control ON button as measured with a DVM?

 

[mustangracer91]

did you check the vaccum diaphram of the cruise control? It sounds like it ruptured.

 

[LJRuddy]

The OFF button measured as closed circuit when pressed and open when released. The ON button, however, made no difference on any of the conductors. Only open circuit.

The question is, what is the required change of state of the cruise control ON button as measured with a DVM?

did you check the vaccum diaphram of the cruise control? It sounds like it ruptured.

This is obviously an electrical malfunction, not a mechanical one.

As for your question, I have no idea. But a friendly bump wouldnt hurt I suppose.

 

[Mr Anonymous]

I don't have a '91 EVTM, but here is the system diagram for a '92.

Since I know that a '92-'95 steering wheel and airbag will retrofit into a '91 and the cruise works properly including the On/Off switch,
I would have to assume the configuration is the same. The primary difference is probably the wire colors.

I've never had a need to troubleshoot the On/Off switch, but if you need me to I can pull apart a '91 steering wheel tomorrow and
meter out the On/Off switch for you.

Note that ckt #57/461 does not show as a ground in this diagram (schematic would be far too generous of a word...) but in fact it is.

 

[sdpatt]

INote that ckt #57/461 does not show as a ground in this diagram (schematic would be far too generous of a word...) but in fact it is.

That would make more sense. Thanks for the schem..., er, picture.

 

[hawkeye18]

Not to be a smartass, but the fact that the wire is black is usually the dead giveaway that it's a ground. I know there are exceptions (and plenty of them), but generally if there's no marked ground, the black wire is it. Just sayin'.

To their credit, this is pretty creative engineering on Ford's part to keep the wire count through the clockspring low. Nowadays, I would imagine all of the controls on the wheel are all tied into a microprocessor on a serial bus, allowing nearly unlimited controls on a 4-wire clockspring (+5v, GND, +SIG, -RET). At least, that's how I'd do it. But hey, I don't work for Ford, so I'm just armchair quarterbacking here...

 

[Mr Anonymous]

Not to be a smartass, but the fact that the wire is black is usually the dead giveaway that it's a ground. I know there are exceptions (and plenty of them), but generally if there's no marked ground, the black wire is it. Just sayin'.

In addition to the lack of the customary ground symbol, the wire is not black for the entire circuit which is why I wanted to emphasize that it was in fact a ground.

 

[93rev2sev]

You did make sure the cable didn't break, right? I guess I'm just not sure what makes this an "obvious" electrical problem.

To find the correct change of state you will need to be going over 30MPH as the CC is disabled at low speeds and you WOULDN'T see a state change while testing...unless you found a way to do it at 35+MPH.

 

[93rev2sev]

in addition,

The fact that it "popped" indicates to me that it didn't just lose power, but that something broke, allowing the pedal to fall down...just like if you let go of the cable after detatching it from the throttle in preparation for intake removal.

 

[sdpatt]

With the electrical diagram in hand, a re-check of the switch function in the steering wheel hub shows the ON and OFF switches to be functioning properly and the 12V supply to be as designed. The horns had always worked and the clockspring had been replaced about 5 years ago. Additional continuity checking down the circuit is in order.

My initial check on the side of the road after the failure had me look at the throttle linkage. I had in the past used a tie-strap to secure the cruise connector to the post on the throttle cam and would have been surprised if it had come loose. A tug on the disconnected linkage found the cable to be intact.

A later check of the vacuum supply to the servo found there to be no apparent vacuum leaks. I suspect that an electrical connection or conductor has failed over the 18 years and 387,000+ miles. More testing will follow.

Thanks for your feedback.

 

[SHOZ123]

I would think the on/off would be normally open and just sets latch logic in the CC module.

 

[Mr Anonymous]

I would think the on/off would be normally open and just sets latch logic in the CC module.

In cars with dedicated cruise modules that is usually the configuration.

Just another MTX SHO oddity.

 

[hawkeye18]

It does seem that the ECU controls most of the cruise control functions in the MTX. Odd.