Intermittent Speedometer

[JoshT]

Any ideas as to why my speedometer just won't work on occassion? It isn't an issue of it being inaccurate or erratic, it does nothing.

There have been a few times since I got my truck on the road that the speedometer just does nothing, next time I drive it it works fine. Both times that it happened recenty I pulled over checked out the cable connections on both ends and they were in place, seemed to be connected properly. Of course when I hop back in a drive the truck it works fine again. Works fine more often than not.

Mechanical speedometer, so pretty obviously not an electrical issue. As I mentioned it seems that the cable is connected properly. If the cable was broken I would expect it to not work at all, ever. If the cable was dry and sticking, I'd expect needle to be bouncing a lot.

Ordered a cheap GPS speedo from Amazon to get by until I fix the problem, but I do need a working speedometer as the speeding ticket I got driving the car last weekend proves.

Yes I posted this to FTE as well. Won't get to work in truck until the weekend and higher priorities to fix. So the more ideas or things to check I have the better.

[basketcase0302]

Usually gonna be the cable or the plastic gear down on the tranny end Josh. You can unhook the cable on both ends to see if it looks rounded/stripped out. DC has them for $19.95, (I'm not sure of any one else that stocks them but the DC part should be a decent one).
It could still be in the odometer itself but they are pretty dependable.

https://www.dennis-carpenter.com/en/tru ... housing-64

[JoshT]

Yeah, it's completely dead now. Guess something finally gave up. Haven't driven truck much since posting this, but it hasn't worked again since. Figuring on just replacing both the cable and driven gear if I can figure out the right parts to get.

Really I needed to replace the driven gear anyway, the speedometer was off enough that it needed a different tooth count. Going by a chart on the site, this appears to be the correct style. Can anyone confirm?

https://www.amazon.com/Speedometer-Driv ... nk20368-20

Also need to know what the correct length for the cable would be. I've read various things from around 70 all the way up to 100+ inch length. Also read that varies based on drivetrain. Anyone know what the correct length for a 68 f-100 4x4 with an FE and manual transmission would be?

[basketcase0302]

Josh,

The cable slides out easy enough, (once you have both ends loose this is easier to do unless it's seized). That's also the point you can verify / do an inspection to see if the length looks too short. NPD lists a "cut to fit" cable, (that I've never used before) but might be a much easier way than guessing at the right length?
https://www.npdlink.com/product/cable-c ... %26year%3D
That chart is referenced here a lot and I've never seen anyone dispute it.
I was sure that DC sold a "kit" with the assortment of the plastic gears, (all different colored) but did not find it.
Here's the NPD gear choices which are cheap enough to order a few, (one of a few different colors to change out) as they are real easy to change.

https://www.npdlink.com/search/products ... 0000&year=

[JoshT]

Josh,

The cable slides out easy enough, (once you have both ends loose this is easier to do unless it's seized). That's also the point you can verify / do an inspection to see if the length looks too short. NPD lists a "cut to fit" cable, (that I've never used before) but might be a much easier way than guessing at the right length?
https://www.npdlink.com/product/cable-c ... %26year%3D
That chart is referenced here a lot and I've never seen anyone dispute it.
I was sure that DC sold a "kit" with the assortment of the plastic gears, (all different colored) but did not find it.
Here's the NPD gear choices which are cheap enough to order a few, (one of a few different colors to change out) as they are real easy to change.

https://www.npdlink.com/search/products ... 0000&year=

Thanks, but I still think I'm just going to replace the whole cable. Those cable core kits cost about as much as a complete cable, and I really don't know if what's there was the right length considering multiple body swaps on the truck over the years. NumberDummy over on FTE was able to provide me with the correct cable length for my truck. Apparently it is supposed to be 70.8" long. I've seen brand new listed on the internet for 20-30 bucks.

Even NPD has them: https://www.npdlink.com/product/cable-a ... &year=1968

At $20 for the whole thing it's kinda hard to justify replacing the core in the existing cable for over $30.

I did reference that chart, but...
A) My exact factory combo wasn't listed (3.50 gears with 8.15x15 tires) so I was going closest match I could find.
B) Wasn't sure if the photo I was getting when searching that part number was correct, you know how places like to use stock photos for parts listings.

I was going to pick up a set of gears like this: https://www.ebay.com/itm/Ford-SPEEDOMET ... 3464974493

Found several more sets the other day, but they elude me at the moment. I'll find them again.

[DuckRyder]

There is a speedo gear calculator on https://www.428cobrajet.org/

Also if you google image search you’ll find the Factory Ford MPC speedometer gears for most combinations. You can probably get the OE gears from NPD for less than aftermarket.

[urchinhead]

I had the same problem when I first bought my truck. I was able to disconnect the speedometer cable on both ends, and shoot in some cable lubricant that I bought from a motorcycle shop.

That fixed the problem.

[JoshT]

Still haven't looked into this any further yet, but I might give that a try if nothing appears overly worn or broken when I look.

Since last post I was chasing an issue with the engine. It misfire at idle that only happened while braking and idle hanging when gas pedal released. Turned out to be a bad brake booster causing the misfire. Not sure about the hanging idle, but it seems to be gone with the booster port plugged. Waited months on ordering a replacement and it either being canceled or going back order. There wasn't much point in spending time out in the heat working on a minor issue with a truck I couldn't use anyway. Finally sent my old booster off for rebuild and got it back this week. Hopefully I'll have time tomorrow or Sunday to get booster installed and hopefully wrap up the running issue.

Next up is either fixing the intake oil leak and distributor change on this truck or replacing engine in other truck. Once those are handled this will (hopefully) be on the short list of things to fix.

[JoshT]

Got the booster in and brakes are working better than ever. Driven it a couple of time since being back together and speedometer is still dead.

Did notice while driving that the odometer is still rolling even though the speedometer does nothing. Makes me think the issue is actually in the speedometer head rather than in the cable or transmission.

Currently working on putting an engine in the other truck, but once that's done I'll have to pull the cluster back out and see if I can find anything obvious. Need to pull it any way, my temp gauge is physically broken (my fault) to can do both. Hopefully it's something simple. I want to get a Dakota Digital dash eventually, can't afford it right now, and would rather not replace speedo not just to do it again later.

[JoshT]

Figured I'd post an update since I finally got a round to this months later.

Sill don't know what was wrong with it. I pulled the cluster and investigated, but found nothing obviously wrong. Pulled the transmission end of the cable, checked both gears, and everything looked good. Pulled the cable core to inspect, clean, and re-lube, once again nothing wrong. Finally I put it all back together and it worked. Best I can figure is maybe the core wasn't seated on the speedometer properly? Nothing felt off when I took it apart, but who knows. It's working now, so not going to worry about it any more.

Worked out well though because I was able to replace the damaged gauge and finally got my high beam indicator working. The latter turned out to be poor contact on the bulb holder.