Re: 1973 Ford F-600, 330 V8, 4-speed tranny, 2-speed rear end, cable-driven speedometer.

Hi All,

Normally when I shift up from low to hi range on the two-speed rear end, I notice that the speedometer needle "jumps". I have to assume then that somewhere down the line the speedometer also has some sort of 2-speed drive unit in order to register the correct MPH to match the low or hi rear axle selection.

Occasionally however, when I shift up from low range to hi range, the speedometer actually jumps down by approx. 10 MPH! In other words, say I hit 50 mph in 4th gear low and shift the axle to hi range. The speedo needle actually jumps down and reads 40 MPH (and I know I am still doing 50 MPH). Basically then, no matter how fast I am going, the speedometer is reading approx. 10 MPH slower when in the hi range, and generally will not quit doing this until I shut the truck off and then restart.

So, what gives here? What drives the speedometer? Does the speedo cable run down to the transmisson? Or does it run back to the rear axle? Is there some sort of 2-speed speedometer drive unit? And why would the thing occasionally read approx. 10 MPH slower? Any ideas or experience with this???

Thanks in advance for any answers and/or advice.

Paul.

 

Yes, there is a little gearbox in the speedometer cable that changes the ratio to compensate for the rear axle shift. It sounds like yours is sticking in 'low', so when you upshift the axle the speedometer ratio isn't changing. Follow the sppedometer cable up from the transmission and you will find it. I think on an F truck they are usually up against the firewall on the engine side, but it has been so long since I have seen one I can't be sure. It should have a wire going to the axle switch, make sure you have a good connection and ground. Stewart-Warner used to make those things.

 

x2 on Louisville joe's post. Our '58 had the two speed, and the speedo would "shift" when you shifted the axle. It started to stick at some point, and eventually gave up altogether.

This is a Chev, says vacuum shift, but a pretty clear pic of the speedo unit:

Viewing a thread - 2 speed rearend quit working on 77 Chevy C60

Page 28 here, says no service possible, "Replace a faulty unit."

http://www2.dana.com/pdf/AXSM-0029.pdf

There you go, it seems.....

 

Thanks for the info., LJ & 85xxxx. I suspected there had to be some sort of little gear box unit, but wasn't sure where it may be located. Although I have been under the hood and the chassis numerous times, I haven't as yet noticed any sort of gearbox. But then again, wasn't really looking for one on any of those times, either.

The 2-speed rear end is electric and not vacuum, and operates just fine. Although my truck is in pretty decent shape, I'll bet the speedometer cable & gearbox have never seen a drop of lubricant since new. Imagine they are getting a little dry now in that 40-year-old (almost!) truck.

Paul.

 

I've had the same problem with my trucks, the speedo reading wrong. You can clean those speedometer transmissions , the ones I took apart the grease was either hard as a rock or gone. Some of these units are bolted togather and some are rivited. I don't know of a souce of parts.

Have A Nice Day-------Hotwrench

 

On my F 600 it is located on the back of the speedometer head and has a wire from the two speed switch that ativates a solonoid to change the ratio.

 

I have pulled my instrument cluster out on a couple of occasions in the past to replace burned-out light bulbs. No gearbox on the back of my speedo head.

Last night however, while under the hood and topping off the brake fluid, I noticed the speedometer cable coming through the firewall, looping around and heading down underneath the cab on the inside of the left frame rail.

Hmmm, I think I'm starting to get a sneakin' hunch about where my speedo gearbox might be located.

Unfortunately, I may have to put the speedo gearbox on the back burner for awhile. I wasn't topping off the brake fluid just for kicks & grins. In the past few days, I've suddenly noticed some wetness on my left rear inner dual and on the driveway. Uh-oh!!!

Paul.

 

One thing about it, when you decide to find the speedometer gear box it will not be hard to find. Just start at the transmission and follow the cable to the firewall and through the firewall until you get to the speedometer. It will be there somewhere along that route.

 

Hi ms_michelle,

Yeah, I'm sure I can find it now, when I finally get around to looking for it. I'm kinda' new to these "healthy" trucks, so I'm still learning.

I suspected there was some sort of gizmo somewhere that would switch the speedometer ratio along with the 2-speed rear end, but not sure what it was or where it might be located. And (gulp!) horror of horrors, I was sure hoping it wasn't going to be something buried way down deep inside the 4-speed tranny. Yeouch!!!

Thanks!
Paul

 

When you do get into it here is a wiring diagram that should get you on the right track.

 

Thanks for the diagram, ms_michelle. That really puts things to light for me. I should be able to get right after it, next chance I get to work on it.

Paul.