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850 Cruise Failure

You could also exchange your existing module with one from Midwest ABS, ~$80 isn't a bad price to pay for a known good part and you're up and running the same day. Not sure I'd wanna mess with the junkyard lotto on a part that's pretty much guaranteed to fail.
 
All you can do is verify power, ground, and signal at the module and figure out which is missing.

Furthermore, I have been becoming more of a fan of using test lights and noid lights to verify proper CURRENT to a circuit. I got nailed recently on a window regulator where it had 12v going to the motor, but after a switch, and a regulator install it still didn't work. Using a noid light we realized there wasn't enough current, but it would show up as 12v on a meter. It turned out to be a poor connection at a connector!
 
Which means figure out a way to check for a VSS to the cruise module while driving since that comes from the ABS module, and make sure there's no missing pulses.
 
Ah...now there's a piece of information I've been looking for. The trans speed sensor does not provide the pulse to the cruise? There was one post on the interweb about the cruise signal coming from the cluster, which means it would be from the trans. Thanks Gary. More support for the ABS fault. Also, now that I think about it, there have been a couple of instances in the last few months when the pedal gave an unusual pulse.
 
Wiring diagram!

Then look up what kind of signal it is and find a way to measure it whether it be a AC voltage test on a meter, LED test light should blink, or graph it. I have my $80 Hantek scope in the electrical drawer to hook up to the laptop if I really wanted to graph something like a speed signal.

It would be a shame to put an expensive part in like a computer and have it NOT be the problem.

Another way would be with a factory level or equivalent aftermarket scan tool. One could go into the data parameters of the different modules and see if there is something like a valid speed data PID. And if the module is communicating, obviously there is power and ground to it! And of course, look for DTCs and do research as to what commonly causes the code. That is step one when I get a problem: pull codes, look for bulletins and research common points of failure.
 
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Zach...seriously...did you read my earlier post? ONLY the VST will read the cruise, the aftermarkets WILL NOT. This is from numerous sources and personal experience. Also, a wiring diagram is not needed to know the cruise is fed from the ABS module if you read the system descriptions.

Dave...up thru 95 they ran a speed sensor on the trans that fed the speed signal to the cluster (why you can use a pre-96 trans on 96+ but not the other way around). Starting in 96 they went to the ABS module for the speed signal to the cluster. The cluster filters that signal, sends it out to the cruise module, pin 8B, sine wave signal.
 
Ok so lacking a proper scan tool, that leaves us with checking power, ground, signal in and signal out before condemning a module.

Gary, how would one measure a sine wave signal and detect that it's correct?

Is not the ABS light on?
 
No ABS fault light, but the light is working (diagnostic key mode). The only ABS indications are a couple of brake pedal flutters recently. Oscilloscope would be the way to read the wave, but sold mine probably fifteen years ago when I got out of RF service. Circuitry to the pump and brake pedal switch has, at this point, been checked from the module base. The stalk is proving a bit more troublesome. Looking for a better schematic at the moment.
 
Thanks Gary. Missed that last message of yours. So the module is reading the sensors at the wheels, generates the signal to the cluster and the cruise from there. But the cluster is processing the signal first. If the signal from the module was the fault, wouldn't I loose cluster functions?
 
You can still have a fully functional cluster, but the cruise may not function if it sees an inconsistent signal from the cluster. The cluster will be buffered so a single or a few missed pulses randomly won't be visible to you, but for a module looking at the signal, it can be an issue. Brake pedal flutter huh Dave? Key detail right there.

Zach, you could get a ballpark with a dvom to see if the level is in the ballpark but to see if you have missing pulses, really gotta throw the scope at it.

Personal experience multiple times, has shown no ABS light, no cruise, all systems normal otherwise. Repaired the ABS module after it did finally throw a random light 1 time, cruise began working again.
 
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