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No cruise control above 2600 RPM


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I've been gradually rehabilitating an '05 OBXT 5MT, which is not in the greatest shape but has received a bunch of work lately and is running pretty well considering the 228K miles on what may be the original short block.

 

My current mystery is the cruise control. It works below 2600 RPM, but it won't activate above 2700 RPM, and if you accelerate it from a lower setting, it deactivates.

 

My leading theory is the neutral safety switch. I suspect that above 2600 RPM, transmission vibration causes the wedge that operates the switch to kick the switch into a neutral reading. I have tried replacing the switch with a new OEM unit, no help. I also disconnected the switch, but that just triggers a CEL.

 

Before I assume my "old transmission" theory is correct and just give up on this problem, does anyone know of any engine control conditions that might result in an RPM threshold on cruise control?

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Can you set it in 4th and a higher rpm?

 

Is the neutral switch on the transmission tight? Wire good?

 

Look to see if your clutch pedal has a grommet in a hole where the neutral switch or brake switch shaft hits the pedal. Either one of these will cancel CC if the switch is activated. If there's a hole and a rotted grommet, put something else in there made of plastic.

 

If your pedal doesn't have the hole with a grommet, then check to make sure the wires in the harness connector are secure. Make sure the switches are stable.

 

Battery and grounding condition make a lot of difference in how things work or don't work. These cars have no rhyme or reason to what may act up with poor grounding or a weak battery. Ampere output and conductance through the ground cable is the best test.

 

Another thing to look at is the switch on the steering wheel. The switch operates through the spiral cable. Is anything else acting stupid on the steering wheel? Does steering wheel position make any difference?

 

Lastly, checking all the modules on the car for fault codes may give you a direction if the BIU or ABS module is storing a fault without illuminating a warning light. It happens with communication faults.

 

The is nothing with the ECM that would do this continuously at the same rpm. Something else is happening - vibrations, switch, grounding, harness movement, at this rpm that is doing it.

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Are you sure the new neutral switch is the right one? There are 2 variants: normally open and normally closed.

 

Try your old switch again, but with a thinner washer. This will compensate for any wear on the pin.

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Thanks Subarcharged and IJpie for great ideas.

 

Re the neutral switch, I made direct multimeter comparisons of the old and new switches' behaviors, and I did try a thinner washer.

 

Of course, as soon as I made my post, the cruise control started working normally (presumably temporarily). Murphy is always listening!

 

Next time I'm under there, I'll do a full inspection of all grounds.

 

When it was malfunctioning, the behavior was based on the same RPM in both 4th and 5th gears.

 

The clutch interlock switch is behaving normally.

 

Ah, clock spring! The previous owner installed a wood multifunction steering wheel cause he liked the look. Maybe that could be the culprit...

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  • 1 month later...

I don't know the thickness, but I had to thin the washer that came in my 05 years ago. I just took it out, and ground some thickness off on a bench grinder. Cut it down about half way.

  • Like 1

305,600miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. (SOLD).  CHECK your oil, these cars use it.

 

Engine Build - Click Here

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