Rear ABS engages, speedo drops to 0, no light

[rangerlocal]

Not too sure where to put this since it's more complicated than brakes, but here goes.

I had a previous post about weird brake thumping issues. So I've narrowed it down to this: My rear ABS is kicking in at around 10 MPH when stopping gradually, like at a stop sign. I have no ABS lights when braking, or illuminated at all. All fuses are good, ABS diodes are good, all connections cleaned. My truck has stock tires. New pads, rotors, air-free fluid from all 4 wheels. Basically, everything mechanical is fine. When I unplug fuse 14, my truck brakes just fine, so I know it's the ABS. I hear it clicking when stopping and the pedal sinks. My truck is 2WD and RABS only, not 4 wheel. It seems the majority of posts with this are due to a dirty tone ring, which I don't have, hence my predicament.

This is all from my old post just to catch new people up. But I have noticed something new today (I just bought the truck, so I'm finding these things as I go). When starting out, my speedo sits at 0 until I hit about 15 MPH, then it jumps up to about 15 and reads normal. When stopping, it reads normal until around 15 MPH, then drops to 0 and the ABS kicks in at THAT EXACT MOMENT.

From what I can figure, it thinks the wheels are locking up, and thus applies the ABS even though I'm still moving forward and have no wheel lockup. This would lead me to believe it is the speed sensor in the pumpkin dropping out, but I have no ABS code or CEL. I tried to detect AC current by spinning the rear wheels with a multimeter on the sensor, and I got nadda. Resistance of the sensor is around 1700-1800 ohms. I hope to God it's not the tone ring in the diff...don't think it is since the speedo registers correctly at higher speeds. Suggestions? Thanks guys!

 

[Earl43P]

Replace the speed sensor in the pumpkin, it's cheap.

 

[rangerlocal]

That's what I decided to do tomorrow. Wires look good, I was hoping it may have been chaffing against something like in most people's instances. It was throwing me though, since there was never an ABS code triggered. I guess the sensor is on it's way out but not totally busted yet.

Will post back with results! Fingers crossed!

 

[rangerlocal]

Twas the speed sensor! The resistance of the old was 1720 ohms, the new was just over 1800, so I guess you can't really tell from that. What really gave it away was the fact that 1. The speedo was doing something funny at slow speeds. Barely noticeable since it was only at about 15 MPH, so I almost missed it. And 2. No AC current from the sensor when spinning the rear tires. $12 Dorman part at Advance, ABS is perfect.

Moral of the story is, don't go on what the computer tells you! I had no lights or any stored codes, since the computer basically thought the sensor was working properly and I was just having a lock-up situation. Glad I didn't decide to jump into tearing apart the RABS valve since it would be free...$12 well spent!

Thanks guys!

 

[adsm08]

Moral of the story is, don't go on what the computer tells you!

Not always good advice. I could have gotten the computer to tell me that, but I can speak with it better than most of you.

 

[rangerlocal]

What I meant was the computer had no stored codes. As far as it was concerned, everything was peachy. ABS diagnostic connector showed bupkis. The only way to diagnose it would be using a diagnostic tool from ford while you were driving it and monitoring all the sensors, and I don't exactly have one in my toolbox.

The better statement would be "Don't ALWAYS trust what the computer DOESN'T tell you to determine what IS broken."

 

[adsm08]

I know. I just like giving people a hard time.

 

[kunar]

I know. I just like giving people a hard time.

its true, he does

 

[Earl43P]

My ScangageII displays MPH, among other things. Don't necessarily need a Ford tool to see speed.

Gratuitous photo, note that I have it accurized to match a GPS. I have smaller tires than stock.

 

[rangerlocal]

I know. I just like giving people a hard time.

I know. That's why we all love you!

Yea, I have a scangauge as well. Just never hooked it up to watch the digital gauge. Thought about it, but I figured it out before I bothered.

I was going to ask why you were doing 82 MPH, then I realized Rangers can't go that fast!

 

[Earl43P]

Pshaw, mine will bounce off of the top speed limiter all day long, as long as the hill isn't a steep mountain. That was taken somewhere in AZ or NM, maybe TX, who knows.

With my offset (from the smaller 235/75-15 tires), when speedo indicates 93, I'm really only doing 85-ish.

 

[rangerlocal]

Seems most of Ford's speedos were a little optimistic. Maybe to keep the customers out of trouble. I noticed all my family's ford vehicles up until the newest Focus we have (2009) were a little fast by 2-3 MPH. Nothing like yours, but they were a bit on the high side. Of course, like you said, having smaller than stock tires will cause your speedo to read high anyway.

 

[Andy D]

the Rat rolls on 235 75 15s the speedo is OK until 60 against a GPS. At 65, it is actually doing 66