So yesterday the speedometer in my '89 F150 started to jump around a little bit at higher speeds, (65mph) it would show steady, then dip down about 10mph for a second and go back, but it was sort of bouncy the whole time. This morning on my way to work, I tested it by setting cruise control, and when the speedometer dropped, the engine accelerated, even though my speed hadn't actually slowed. So I'd think that would imply that the issue is between the trans end of the cable, the transmission speed sensor, (for the cruise control) of the gear itself. When I pulled the cable out of the speed sensor, it had gear oil on the end of it, so that means fluid was leaking through the speed sensor, which it isn't supposed to do. So I ordered a motorcraft speed sensor, and bought a dorman sensor to use until it arrives.
I pulled the cable completely out of the truck and lubricated it with a graphite lubricant, put the dorman sensor in, and now it is still bouncy, and it still dips, but not as often. Now it also reads about 10mph slower all the time. Both of the ends on the cable looked okay, but could the gear oil getting into it have caused it to become sticky internally and bind? I inspected the gear at the end of the speed sensor, and looked at the gear in the transmission, and both looked fine, but I would also expect gear damage to cause a dead speedo.
The cable seems to spin fine by hand, so I don't really think it is the problem. I might be able to try using the speedo cable from my '90 F150, just to see if the problem goes away, but that one should be a bit longer, so I'm not sure if I'll be able to route it smoothly. Is it possible for the gear in the trans to slip on its shaft? And is there a good way to test the speed sensor mechanically to see if it is just binding internally?

 

Not uncommon, at this age.Did you wash the cable & sheath in solvent first to remove all the old grease/oil/residue? Ford recommends PTFE grease.So when stopped it shows -10mph? And at 10mph, it shows 0?Only if all the teeth are stripped. If only 1 tooth breaks, it'll just get bouncy & lose the same portion of speed as that tooth represents from the total tooth-count.I guess, if it's plastic, but it's not likely. And I think the T18 drive gear is steel, anyway, machined onto the tailshaft.

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It's a straight-through shaft; it can't lose speed from one end to the other. The electronic portion produces 6 pulses per rev, but that has nothing to do with the speedo.

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Okay, so no I didn't think to clean the cable before lubricating it, I'll pull it back out, clean it and the sheath, and re-lube it with ptfe. Is there a specific degreaser you recommend? Or should I be fine with either a brake cleaner or purple power?

When stopped, the speedo sits on the bottom peg, and doesn't move for the first couple rpms of acceleration. The markings on the speedometer start at what would be 10MPH (although it doesn't have a number label) but there is about a half inch gap between that mark and the bottom peg on the speedo. I normally shift from 2nd to 3rd gear about right when it hits that 10mph mark (if I am just cruising, unloaded) which is just below 2000 rpms, but now, 2000 rpms in second gear is still below that first mark. In fourth gear, 2000 rpms is about 45mph, and 65mph is just below 3000 rpms, but I was pretty consistently reading ~35mph at 2000 rpms, and ~55 mph at 3000 rpms, with the occasional dip. (usually down, but sometimes up to actual speed)

I do not intend to replace the cable, but in case there was damage to one of the ends that I missed, I've been looking at replacements. My current cable (E9TA-9A820-KA) is just over 59 inches long, and every aftermarket option I have found online that crosses to that number, or its alternate (E7TA-9A820-A) is 72". If I do end up needing replacement, and I just route it longer while avoiding sharp bends, can I expect reliable operation without binding?

And lastly, when I get a chance, I will pull the cable from the trans and inspect the gear again, last time I sort of just eyeballed it from where I was laying, but this time I will get a closer look and turn the driveshaft so I can see the whole gear. I have a T19 in my part shed, and it has a plastic gear on the output shaft, but I do know that the T19 in my shed, and my T18 have different output shafts, so it might be different. (both transmissions are RWD, but the T18 is a long tailshaft, slip yoke. While the T19 is a short tailshaft, fixed yoke.)

 

I'd coil it as tightly as possible inside a shallow bucket (or jug cut down to a cup) and fill that with naphtha, deodorized mineral spirits, or kerosene; to soak for a while. Then use a baster to pump solvent through the sheath. Maybe pump a cotton string through, and tie it back to itself with a swab in the knot so you can pull it back & forth through the sheath to break up all the old, dried grease. It might take a day or more to really get it clean. I wouldn't use a volatile solvent (like brake or carb cleaner) until last, so it dries out faster. I wouldn't use any water-based cleaner, detergent, or surfactant.

Yes, I'd expect a longer cable to work fine.

 

Well I pulled the speedo cable out of the transmission, jacked up the rear end, and closely inspected the drive gear while turning the driveshaft. One of the teeth on it had a big flat spot. So I guess that will be getting replaced.
I ordered an E7TZ-17285-B, which looks like , but I also found C30Z-17285-C, which supposedly was used on the older T18s, and a few other transmission/transfer cases. But this one is made of steel, so I ordered it, and if it looks the same as the first (plastic) part I ordered, I will be using the steel one instead.

 

Well I went ahead and replaced the speedometer drive gear, and everything is perfectly fine again, no more wobbling or innacurate readings on the speedo. The steel gear (C30Z-17285-C) did fit perfectly, so hopefully I won't have to replace it ever again. (Even if it means the plastic gear on the cable wears out a bit faster) Just for reference if anybody else runs into the same situation, my T18 uses the same gear as a T19, NP435, and Tremec A390. Along with the BW1356 and BW4406 Transfer cases, as well as a handful of other older model transmissions and transfer cases. So maybe somebody else who wants a steel drive gear can use that part number.


New and old gears side-by-side.

You can really see the wear in this picture.

The box that the new gear came in, also came with a spacer (pn. 8MTH-17288) that I did not need.

 

From where? Got link?Good info. No pic?

 

I got the gear off of Ebay, here's a Link. I'll ad a pic of the gear spacer as well.

And just to clarify for any future readers, the part numbers I have been talking about are for a seven tooth gear, which is what my axle/tire setup (3.55 ratio 235/75R15 tires) uses. I have no idea what other axle/tire combos would use the same tooth count.

 

Just to clarify for others who may read this thread: '92-up trucks don't use any speedo gears.