How Do You Do a Cruise Control Servo Vacuum Test ? - : Jeep Wrangler JK Forum
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post #1 of 2 Old 03-24-2012, 06:07 PM Thread Starter
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How Do You Do a Cruise Control Servo Vacuum Test ?

To cut to the chase, how do I perform a cruise control "servo vacuum test" on my 2000 TJ Sahara?

As background, I started developing some classic symptoms of a vacuum leak. What would happen sometimes (especially when I had the cruise control on and the A/C on at the same time) is that the HVAC blend control doors would start acting up, and other times, the cruise control would stop controlling the set speed (or both).

After getting a decent vacuum gauge for dynamic vacuum testing as well as a hand-pump style vacuum pump/gauge for static vacuum testing, I have completed the following tests:

- Manifold vacuum ... good & steady at around 12 to 13 inches of Hg.
- Visually checked all accessible vacuum lines & fittings ... everything looked good.
- Isolated vacuum reservoir vacuum leak check ... held 15 inches of Hg for over 5 minutes with no leaks.
- One-way vacuum valve ... tested good.
- Isolated vacuum supply line going through firewall into backside of dash ... held 17 inches of Hg for over 5 minutes with no leaks.
- Isolated cruise control servo and attempted to pull vacuum ... would not hold any vacuum & immediately bled down to zero (atmospheric pressure).

I have a 2000 Jeep Wrangler Service Manual and all it says to do (if the rest of the vacuum system checks out), is to, "Do the servo vacuum test". The trouble is that no servo vacuum test is actually described in the "Speed Control System" section.

My problem is this. As described above, when I pull a vacuum on the cruise control servo, it won't establish or hold any vacuum whatsoever. On the surface of it, this could well mean that the internal vacuum chamber or the diaphragm in the cruise control servo is shot and that the entire servo assembly needs to be replaced. However, without the actual servo vacuum testing steps, I don't know whether this vacuum issue is indicative of a failed servo, or whether the test requires the servo to be powered up and dynamically tested in some way, where a vacuum might be able to be achieved and maintained. In other words, I don't want to spend the money to get a new cruise control servo and after installing it, finding out that it is normal to not achieve & hold a vacuum when it is unpowered and in a static state.

Can anyone help clarify this for me? I would sure appreciate it.

Thanks in advance,
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post #2 of 2 Old 03-29-2012, 04:17 PM Thread Starter
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False alarm.

I had originally tested the cruise control servo by way of the 3 ft vacuum line going to it. I subsequently vacuum tested the servo directly and it passed with flying colors. I then tested the vacuum line and it failed miserably. After I pulled the vacuum line out of its protective sheath, the whole center section was embrittled and cracked in several places. Replacing the bad section with some small diameter rubber vacuum line I had cured the problem.

I am happy to report that road testing showed that I could again run the A/C with the cruise control on with no issues.
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