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Cruise Control - Individual Component Troubleshooting

ChildIdenticant

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Purpose of thread is to hopefully provide some troubleshooting advice in detail not otherwise yet available via this forum for 1st gen explorers. Approach to advice is made assuming that no other issues are present. I have no reccommendation as to the order in which components are tested (except make sure your brake lights work first), as circumstance can change reasoning for prioritizing a test. Example; cruise control worked before you had trans work done, but not after? check that vss & connection. Only works in the Heat/Cold but still reliably? Could be vacuum or electrical. etc.

My vehicle:
1994 2wd 4.0L Automatic
Trans: A4LD
170,000 miles

Component: Vacuum Dump Valve
Location: Drivers side, under the dash, mounted above a plastic shelf which is clipped on to the brake pedal. It is
connected to a hose that goes through the firewall, connects to the cruise control servo. The "shelf" should
be flat, and hold the valve in when the brake pedal is not pressed. If it is broken or bent, this can prevent
the valve from properly closing. The distance between the valve and the shelf can be adjusted, and also fall
out of adjustment, by pushing the valve itself up or down through its mounting retainer.
JrPrSnJ.jpg


Testing: Testing this part is most easily done from within the engine bay. Disconnect the hose from the servo , and
connect it to a vacuum tester. Applying ≈ 15inHg is enough. The important part is that the vacuum is
maintained. Apply the vacuum -wait 30 seconds to verify vacuum persistence - press brake pedal to dump
vacuum. If vacuum persists, dump valve is good, move on. If the vacuum does not persist, and valve is
verified to rest in closed position on its shelf, test the hose itself by removing the end connected to the valve
and plugging it. If the vacuum persists, experimenting with dump valve depth might help, but replacement
is most likely nevessary.
lJjDJo1.jpg



Component: Speed Control Amplifier/ Cruise Control Amplifier
Location: Multiple locations possible. Either under the dash, or behind the glove box. Mine was behind the glove box
mounted with 2 t-25 torx head screws. Be very careful when removing the connectors. The plastic box has
has gone through a lot of thermal cycling by now and is extremely brittle. Mine crumbled out in the back.
sjoi29b.jpg

dnlVFFQ.jpg



Testing/Diagnosing: First things first. Ground yourself. Get rid of that nasty static charge. There is an IC in that box. Detailed troubleshooting here has to be limited, if for no other reason than I don't know what chip
that is. I couldn't even identify the logo of the chip maker.
Second things second: check that trace and those joints. This is what actually turned out to
be the problem for me. Visually inspecting each of the joints with a magnifying glass and checking
continuity. Specifically I had a broken joint in the ground jumper. It was so minute that a change in
air tempurature could make the system work again intermittently.
IfqMdyI.jpg

Do you see it?
FVrFvIC.jpg

1 down....
F5AyClz.jpg

There was also insufficient quantity of solder on some joints as well. If anything looks frosty, pitted, or cracked,
just resolder it.

The approach to make regarding actual component replacement here is to make the same assumptions as one would on similar electronics of similar age. Basically swap replace the electrolytic capacitors, and maybe the diode. That pot probably hasn't been adjusted since it was installed so it is still good but easy enough to test anyway, and unless you see scorch marks it is incredibly unlikely (in my experience) that one of the other basic components will be faulty. You must visually identify the values of that which you replace. I would not assume all of these amplifiers are the same .
sqz4maS.jpg


If you have something to add, do feel free to share. I will edit-add a section on troubleshooting the servo (solenoids, diaphram), steering wheel controls, and possibly VSS a little later.
hopeithelps
 



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roscoe 0202

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thank you I believe my gem is at fault will get it out and check again thank you
roscoe
 






ExploreHudson

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Definitely want to look into this. Thank you for starting with some serious leg work
 






AGodlyCanuck

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1992 Explorer 4dr 4x4 XLT
I need to definitely try what you reccomend.
Appreciate giving me a place to start ^~^
 






mcfixstuff

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Any idea what the pot on the board does?
 






ChildIdenticant

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Any idea what the pot on the board does?
I read a thread a while ago indicating it will address the issue you are trying to fix (Cruise control works, but not ideally.)of incorrect speed being being retained. Haven't had that issue though so can't be sure. Will try to find the post.
Some very informative posts on the cruise control and speed control amp can be found at garysgaragemahal.com.
 






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