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Weird cruise issue, 86.5 vacuum servo

 
Old 02-24-2019, 02:17 PM
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Christopher Zach
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Default Weird cruise issue, 86.5 vacuum servo

So awhile back I fixed the vacuum issues, replaced the cruise brain, and I had good CC. This is nice. However a few weeks ago the cruise system died and I figured the brain went bad. So I got a repaired one, tested, ready to go.

Same symptoms. Cruise does not work, HOWEVER if I push in the throttle and hold the set/accel button on the stalk it will hold the car's speed. Release the button and it drops out. Press the button again without foot on throttle and it doesn't come back on. Could be the vacuum actuator perhaps?

I just checked the secondary vacuum circuit (the one that runs the cruise, mixer servos, and the vacuum tank). The check valve was bad (again!) but the loop did hold a -15 bar when pumped down so no leaks there. Replaced the check valve, fired up the car, it holds a good -20 when off the throttle and goes to 0 when you floor it (this is measured before the check valve). Cycling the heat servos doesn't change anything really and getting the car to speed and engaging the cruise doesn't change things much either, so I think the cruise servo is good.

Any ideas what might be wrong? Is there a separate switch for the brake circuit and/or the automatic transmission that might be shot? I can't believe two computers would be bad the same way...

Thanks!
C
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Old 02-25-2019, 12:23 AM
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The Forgotten On
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The cruise gets its signal from the third brake light on 84+ cars. Up to 83 they get the signal from the brake pressure switches attached to the bottom of the brake master cylinder.

If you installed a LED in this light the cruise will not work. But what it sounds like is that you have a failed vacuum servo. I would use a vacuum tester on all of them to see.

Most likely your defrost and or recirculation valve(s) have failed. Common issue, but a pain to remedy.
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Old 02-25-2019, 06:45 AM
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Christopher Zach
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Thank you for the reply. I do hear a small "click" coming from what seems to be the central warning computer when I try to engage the CC. Might be a relay setting in there that gets dropped if the brake comes on but I'll check with the harness.

It could be a failed vacuum servo, I only have the 91 version of the WSM and it sometimes has gaps for things like the 86 and prior cars. But someone sent me a PDF with the pages for testing the servo unit apparently if you jumper pins 3 and 5 together then jumper pins 7 and 12 together the servo should move the throttle smoothly to full open. Obviously you want to do this with the car engine off (but key on), which means you need to ensure you have a working vacuum check valve and that you have vacuum without the engine on (reservoir can provide it).

My defrost valve has failed, I have blocked it off with a nipple for now until I take the entire car apart. I do have a good 15-20hg of vacuum at idle measured at the check valve; it's quite possible I have a leak inside the servo. That test above should be the proof in the pudding without having to take the entire inner wheel well apart to get to the servo.
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Old 02-25-2019, 01:00 PM
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928S MN
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Originally Posted by The Forgotten On View Post
The cruise gets its signal from the third brake light on 84+ cars. Up to 83 they get the signal from the brake pressure switches attached to the bottom of the brake master cylinder.

If you installed a LED in this light the cruise will not work. But what it sounds like is that you have a failed vacuum servo. I would use a vacuum tester on all of them to see.

Most likely your defrost and or recirculation valve(s) have failed. Common issue, but a pain to remedy.
Third break light on 84+ cars? I have an 85 and don't have a third brake light.
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Old 02-25-2019, 01:40 PM
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Christopher Zach
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Originally Posted by 928S MN View Post
Third break light on 84+ cars? I have an 85 and don't have a third brake light.
There is one on an 86.5. It's not burned out or replaced.

That said, the cruise control works today. AARRGGH!!! Well at least this morning. I set the speed, hit the gas just to see what would happen with set enabled and when I released it, it held.

Hitting brakes turns it off.

Pressing down on stalk resumes speed.
Pulling back on stalk turns it off.

UG. Maybe the problem is a loose connection back by the cruise connector. Or something intermittent in the brake light sensor. Not sure at the moment, but I will try pressing the stalk forward with my foot just on the brake to see if it holds speed. Assuming it works again :-)
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Old 02-27-2019, 11:41 PM
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Update: Cruise control works perfectly and I know the cause.

The root cause was the vacuum one-way valve between the brake booster line and the splitter to the vacuum tank, servo, and the interior flap servos. What had happened is that one way valve failed, so when the engine speed changed so did the vacuum to the servo. Servo could not get enough vacuum to operate, and thus the failure.

$5.00 part. With it in, the vacuum is constant on the servo side and cruise control works. The vacuum tank helps to keep the pressure on and everything's fine. So if your CC goes out, check that one way valve first (take it off and blow into it, it should only pass air one way)

Thanks everyone, hope this helps someone else!
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