Jumpy speedometer, throttle being cut. (5 Viewers)

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MongooseGA

Learns things the hard way
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Had some issues with a jumpy speedo in the last few weeks. Was kind of rare, and only happened over particularly harsh bumps.

Over the last 2 weeks, it's become more common over bumps. Speedo jumps 20-30MPH and it'll actually kill the cruise control until the speedo settles. It's consistent enough that I'm able to anticipate losing cruise when I see certain bumps in the road.

Went to Cruiser buddy's house a few hours away last night. On the way home today, we went through a ton of heavy rain. Eventually it got to the point that the speedo would just fluctuate from 0-130mph, with no real pattern or anything. Just wildly swinging. In addition to not being friendly with cruise control, this behavior also cuts the throttle on my truck until the speed reading settles down again. Big issue, not having throttle when you're doing 80 on the interstate. We made it off the highway to our neighborhood with no real issue, but at the light exiting the ramp, I had erratic throttle from take off. We pulled in to a parking lot and I had the Ms stand on the brake in 'D' while I jiggled sensor wiring under the truck, looking for a needle bounce. No dice. Got the remaining 3 miles home with lots of trouble, having throttle for 2-3 seconds at a time. I did find that when I was able to get to speed, keeping the throttle engaged was easier.

So here's what I've deduced:

This is not a cluster issue, but a sensor issue. My SGII reads the same as the speedometer when it's acting up. So, both of those reporting devices are receiving bad signal from somewhere.

I replaced a speed sensor on the transmission last week as the old one was weeping. Connector was full of ATF when I took it out. No change in behavior with the new unit installed.

I installed new rear ABS sensors and harnesses at the end of last year. One of the original connectors had nearly disintegrated, and I wouldn't be surprised if it's had some water intrusion. I specifically jiggled this connector to see if it would throw a false reading, but no luck.

The truck has thrown no codes. I imagine this will settle a bit after a day or two of drying out, but it's not safe and I can't let anyone drive it until this is resolved.

Video for fun:

 
CDL is on because I'm RWD at the moment.*

I'll start with checking all of the speed sensors on the trans and the case. Many will be aware of my transmission saga last year, so I'll revisit everything that got touched recently.
 
@MongooseGA and myself were texting about this earlier in the evening. I suggested he remove his speedo correction box that he has installed, and then see if the problem persists. If it does, the procedure to check both of the speed sensors is below (sharing for posterity):

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Updating:

Was feeling kind of alarmist at the end of this drive yesterday. Cooler heads prevailed after some time home and Alex was spot on, it was my speed calibrator on the fritz. Disconnected it today and took a drive. I aimed for all of the harshest road defects to get the needle to jump, but she was solid.

So I'll order a new calibration unit. Seems I only every get about 2 years out of them 😄
 
Updating:

Was feeling kind of alarmist at the end of this drive yesterday. Cooler heads prevailed after some time home and Alex was spot on, it was my speed calibrator on the fritz. Disconnected it today and took a drive. I aimed for all of the harshest road defects to get the needle to jump, but she was solid.

So I'll order a new calibration unit. Seems I only every get about 2 years out of them 😄
Ugh. And here I was all excited to get to deep-dive my diagnostic manuals again...

Glad it was an easy fix!
 

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