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Cruise Control not holding speed

B

bill.sts72

NAXJA Forum User
Location
texas
Need a 2nd opinion.

First i did search, most post have to do with not working at all.
I have 99 XJ, 4.0, auto, 4x4, with 130k on it, all lights come on, I can set it, I feel the peddle do it little dip, I can maintain speed on level ground, but when I hit upgrades it falls off. Bad, from 65 to 45 going down the other side, it holds 65 again. Also, after driving for some distance it seems to work better. I got home, disconnected the vacuum line about 1 hour later still holding a vacuum so no leak. I think its the vacuum control unit, but before I go blow a 100$ for a new one, I want to get a 2nd opinion.

Also is it worth it to get one from a junk yard, do these go bad a lot?

thanks guys.
 
Well, based on the symptoms, I agree you have a vacuum related leak. Have you noticed if you running your heat/ac that the vents shift to defrost when this happens as well?

If you follow along the vacuum lines, especially around the vacuum reservoir, you'll find a couple of disk-like gizmos in the lines. These are check valve and should only allow air to pass one way. Carefully disconnect them and then either blow/suck through each end. One way should flow, the other not.

If the check valve is back, it will also allow you to loose vacuum under periods of low engine vacuum (like climbing hills, accelerating while pulling a trailer, etc.)
 
Its holding vacuum, when A/C heater all work good, changes positions. I pulled/clean the speed control unit. When I disconnected the vacuum line going into it, you could hear it suck air. (with motor off)
I read somewhere it could be the speed sensor, that tells the computer how fast your going, could that not be reading right.
Is there any checks for either of these?
thanks
 
Check the connector on the speed sensor to see if there are any obvious problems, clean the terminals and apply a little dielectric grease before reconnecting. If that doesn't resolve the problem I can check the '98 FSM to see if there is a troubleshooting procedure for the speed sensor.
Speed sensor is located on the transfercase on the driverside.
 
If the speed sensor signal was defective then the cruise control would not be reliable at any time. The computer would not be able to set the initial speed consistently (sometimes it would read low or high and the set speed would vary accordingly), nor would it be able to hold a speed on flat ground or downhill slope. The fact that speed only varies in limited conditions says that there is a mechanical issue. Maybe the mechanical problem is that a wire is broken and changes position going uphill to cause intermittent signal but that is not the speed sensor itself
 
Try ptting the jeep in drive(not overdrive) an try it on the same hill..If it holds speed then you know it's not that..Also have you adjusted the kick down cable?
What gears and tires you running??
 
ehall said:
If the speed sensor signal was defective then the cruise control would not be reliable at any time. The computer would not be able to set the initial speed consistently (sometimes it would read low or high and the set speed would vary accordingly), nor would it be able to hold a speed on flat ground or downhill slope. The fact that speed only varies in limited conditions says that there is a mechanical issue. Maybe the mechanical problem is that a wire is broken and changes position going uphill to cause intermittent signal but that is not the speed sensor itself
Click to expand...

Your reasoning is sound.... ,but I didn't mean to suggest there was a problem with the speed sensor. The wiring going to the speed sensor or cruise module could be the problem. Checking the connector terminals for corrosion is just another step in troubleshooting the problem. After 20+ years of troubleshooting electrical I have discovered never assume anything.
 
At the control unit, when I disconnected the vacuum cable it sucked air. So no I don’t know the pressure, but it was a strong suction.
 
I have not checked or readjusted the kick down, but something I will look at.
Is there a check for the vacuum control, a way to jump it so that when you apply a vacuum you can see if everything moves right?
thanks guys
 
you did not mention gear ratio or tire size, or if it is down shifting or converter unlocking.
 
Sorry, stock tire 235s, stock gears 355s, not down shifting in cruise control. but when I give it gas to get it back to speed it will sometimes (normally)
 
Info from FSM 99

"(1) Disconnect vacuum hose at speed control servo and install a vacuum gauge into the disconnected hose.

(2) Start engine and observe gauge at idle. Vacuum gauge should read at least ten inches of mercury.

(3) If vacuum is less than ten inches of mercury, determine source of leak. Check vacuum line to engine for leaks. Also check actual engine intake manifold vacuum. If manifold vacuum does not meet this requirement, check for poor engine performance
and repair as necessary.

(4) If vacuum line to engine is not leaking, check for leak at vacuum reservoir. To locate and gain access to reservoir, refer to Vacuum Reservoir Removal/Installation in this group. Disconnect vacuum line at reservoir and connect a hand-operated vacuum
pump to reservoir fitting. Apply vacuum. Reservoir vacuum should not bleed off. If vacuum is being lost, replace reservoir.

(5) Verify operation of one-way check valve and check it for leaks.

(a) Locate one-way check valve. The valve is located in vacuum line between vacuum reservoir and engine vacuum source. Disconnect vacuum hoses (lines) at each end of valve.

(b) Connect a hand-operated vacuum pump to reservoir end of check valve. Apply vacuum. Vacuum should not bleed off. If vacuum is being lost, replace one-way check valve.

(c) Connect a hand-operated vacuum pump to vacuum source end of check valve. Apply vacuum. Vacuum should flow through valve. If vacuum is not flowing, replace one-way check valve. Seal the fitting at opposite end of valve with a finger and apply vacuum. If vacuum will not hold, diaphragm within check valve has ruptured. Replace valve."
 
As mentioned above-

See long thread titled -
"Anyone that hasn't adjusted there Throttle Valve cable do it. WOW. "
(near by or back a page or two usually)

Can really make a change sometimes. Real easy to do.
-----------------
Run at least 20 minutes to check trans level. parking so XJ is level. NO overfill.
Run thru all gears stop in Park check while idleing. Dex 3 only.


Good Luck,
Orange
 
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