The Stovebolt Site Forums The Shop Area Engine Shop 2 speed rear axle speedometer adapter

I have an issue with the two speed rear axle adapter on my firetruck. I don't have the short square drive that fits in between the speedometer head and the adapter head. However, that's not really the issue. I wonder what other than appearances, does the adapater make on the speed shown on the speedometer? I've got the 20" tires and all other things are as originial but how much difference does the speedometer show if I'm in a "high" position or the "low" on the axle? What it shows on the speedometer(I'm in high position)always seems to be either too high or too slow compaired to the traffic around me. And that is the reason for my asking the question. I've seen it go to 60 MPH and other times, it doesn't turn at slow speed.

I know it can't be much of an actual speed difference but somebody on the stovebolt must know the answer. In advance, Thanks.

Bill

the adapter has two gears in it, and it is shifted by a small wire link going to the vacuum switch that changes two speed rear. The percent difference in the ratios in the adapter are identical to the difference in the rear axle ratios.

Could be the cable needs to be lubed, or the speedometer needs calibration or servicing, and or the wrong speedometer drive gear in in the transmission considering your ring and pinion and tires.

Grigg

Thanks. I have the wire connected to the adapter and when I switch the lever, it clearly changes speeds in the axle. I've lubed the cable and am sure I have the correct adapter(I used the shop maunal to match it up), and I have the original transmission but I'm not sure how you calibrate a speedometer. I am considering just removing the adapter and running with just the cable. How much off will my speedometer display be?

The "adapter" is really a speedometer speed correction device to account for the two speed rear axle. It must change gears at the same time the rear end changes gears.

Don't remove the "adapter" unless you know for sure it is the problem. So far it sounds like the "adapter" is fine, and the speedometer or cable is the problem.

Grigg

ok thanks. I'll keep it in there and try to find a way to fix it.

Update from today: As I drove the firetruck this afternoon, I shifted from high to low on the two speed rear axle lever and the speedometer started working! I guess it needs a little work...

Is the adapter fully shifting when it should, or is it getting stuck in neutral?

Grigg

It works in the "low" position but doesn't indicate any speed at all in "high" position. Is there a neutral?

Are we talking about just the speed correction adapter on the back of the spedo?
Forget about the two speed vacuum switch for now.
Remove the link rod and just switch the adapter while driving, and see if it works in both positions, high and low, "neutral" will be in between the two.
I am guessing that the link rod is not the right length, or something is out of adjustment, you should know for sure soon enough. If you do shift the adapter fully and it does not work in one position then there is probably something wrong with the adapter.

Grigg

Thanks. Yes it is the adapter behind the speedometer. It sounds like a plan. I have to work today so I'll try it on Friday when I get down to the truck.