Erratic Speedo 1992 F-250

[reklund]

On the way home from Salt Lake City a week ago, my speedometer began behaving erratically. At random times, the needle will drop roughly 10% of whatever speed the vehicle is moving, and then bounce back to the normal speed. So far to diagnose I have:

Observed the waveform of the rear axle speed sensor using a picoscope and it checks ok.

Tried a known good rear axle speed sensor (new from Ford) with no change.

Visually inspected the tone ring on the rear differential ring gear through the sensor mounting hole, and it appears fine with no damaged, bent, or funny looking teeth.

Verified circuit integrity from the rear axle speed sensor connectors all the way up to the cluster. All check ok.

I'm thinking it's a malfunction in the speedo head unit itself, and in my googling to see what's up, I came across these guys: www.clusterfix.net

It appears my condition is a somewhat common concern, and it usually manifests itself on automatic trans vehicles sooner as it causes all sorts of issues with the E40D shifting.

Anyone here have any experience with these guys, or have you successfully repaired one of these yourself?

Thanks,

Ryan

 

[towcat]

ryan- i have a extra OBS cluster if you want to do a plug-n-play to see if the issue replicates itself.

 

[reklund]

Thanks, Towcat. That would help confirm my diagnosis. What will it take to ship it?

 

[towcat]

pm me your addy. i'll send it your way and let you know what the post office ***** me for.

 

[Shadetreemechanic]

I had to replace my cluster about 10 years ago to correct a e4od issue. At the time it was $300 to get a rebuilt cluster from ford with your correct ODO reading on it.

 

[zebrabeefj40]

When was the last time you changed the oil in the rear axle? My experience is that trash (specifically metal bits) in the oil causes speedo errors too. You might try cleaning out the diff area and refilling with fresh oil before you dig into the dash.

Nick

 

[reklund]

When was the last time you changed the oil in the rear axle? My experience is that trash (specifically metal bits) in the oil causes speedo errors too. You might try cleaning out the diff area and refilling with fresh oil before you dig into the dash.

Nick

That's on the list prior to having the cluster repaired, but as I said, the waveform looks good on an oscilloscope, so that leads me to believe the signal being generated is clean and good enough for normal operation.

This time of year its hard to find time to go tinker with the truck with all of lifes other obligations. I need to service the diffs, replace rear driveline u-joints and support bearing, install new battery cables, and just this morning I noticed a drip of fuel from the fuel pump. Guess its time for one of those too...

 

[Tyee II]

Not to steal your thread, I have a 92 with manual and have had intermittent erratic speedometer. I have been under the impression that the sensor in the rear differential is the ABS brake sensor, does it also signal the speedometer ?

 

[snicklas]

Not to steal your thread, I have a 92 with manual and have had intermittent erratic speedometer. I have been under the impression that the sensor in the rear differential is the ABS brake sensor, does it also signal the speedometer ?

Yes it does.

 

[reklund]

So I finally got back on this today. I was due for a rear diff oil change anyway, and wanted a closer look at things before I had Towcat ship me a cluster.

I have a PML diff cover that has a drain and fill plug and allows for an extra quart or so of capacity. I drained the oil and heard chunks come out. Oops. The magnet was full of shavings too.

Upon removal of the cover I found one chipped ring gear tooth, and some other small chunks in the bottom of the diff. Looks like something is trying to come apart.

The tone ring looked surprisingly good, but had one or two teeth that looked like they had contacted something. I cleaned them up with a file a bit. I cleaned the chunks out of the bottom of the diff, hosed it out with brake clean, and resealed the diff cover with some good Toyota orange FIPG.

I refilled it with 75-140 and took it for a spin. The speedo operation is very much improved, although the needle still fluctuates a bit.


Sooooo, looks like it's time for a R&P and some bearings at a minimum. sunovabitch.

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[icanfixall]

Wow.... Thats a bummer of a find. When it rains it usually pours like hell has opened up on you. Sorry you have to dig into another big repair like this. I too really have to replace my ring and pinion because of lots of play. Never done this kind of work and kind of intimidated by this. I plan using 3.73 gears and a shim pack instead of the crush sleeve. No idea what is going to be bad inside besides the gears. Don't even know what to look for....

 

[reklund]

I've done tons of gear setups on Toyota 4x4s, this truck was supposed be the one I DIDN'T have to regear

You'll know what to look for when you disassemble. You'll likely find pitted or damaged bearings that will require replacement. Really, you should replace them anyhow since you're taking it apart.

You may want to have a look at the clutches in the diff too if yours has a limited slip. They'll be worn from age, and that will be the time to change them.

The shims and a solid spacer are preferred over a crush sleeve but do require a touch of finesse to set up. I've done them in the past and personally like them for off-road, but on road in normal applications, the crush sleeve usually suffices just fine.

Ryan

 

[Selahdoor]

So... The end diagnosis is that the chipped ring gear tooth, is the cause of the erratic speedo readout??? Or am I more cornfuzzled than usual?

 

[Selahdoor]

I disconnected the RABS control box behind the glove box, and the erratic speedo needle problem went away.