The Stovebolt Site Forums The Shop Area Driveline At the point of adding in my original spedo.. to nv4500

Hi All,
Did anyone every find a way to adapt the 1938 mechanical spedo to a 94/95 nv4500 trans(electronic spedo module)? Is there a way? Or give up and go GPS?

You might search around for drivers that look like yours, but are mechanical, many have both mech and elect. On the same output. NP made that trans, so look at other NP trans. Speedo drives, 435, 540 series, Chrysler used NP trans. a lot.

Ed

Ed has given you good advice but I use gps on nearly everything now. Tire size or gear ratio has no affect on speedo accuracy and there are just about no dead spots anymore in reception. You do have the same problem as my 37 Buick with your instrument cluster being oblong and and gps units are round. They do offer cheap stick on units for speed only.

It isn't cheap, but you can buy converters off the shelf.

One example: https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=886/mode=prod/prd886.htm

That is an interesting play on words. Has anyone adapted a 1938 speedometer to work with a 1994 NV4500 transmission?
Adapting a 1938 Chevrolet truck speedometer to go in a 1994 GMC or Dodge truck. Nah! That would be weird, put hey, anything goes in the world today.

From what little I know about the NV4500:
The transmission has a mechanical driven gear that drives a mechanical-electronic module that generates a 5 volt square wave signal for an electronic speedometer.

So can we modify a cable driven 1938 speedometer that is basically a clock spring, magnets and a movement to operate on a 5 volt square signal? Nope.

What if:

Transmission side. Ditch the electronic module. Use the driven gear and R&D a mating gear (gear ratio TBD) to cable drive connector. The problem is we don't know if the MOPAR driven gear is strong enough to operate a cable that goes all the way to the dash with many bends along the way. That may be too much for a plastic gear. Probably not a good idea.

Dash side: Buy a speedometer that is NV4500 signal compatible, physically alter it to the size of a 1938 speedometer and slap a 1938 speedometer face decal on it. Not a good idea either because that's a lot of work and it may not be compatible if the owner wants to keep a 6 volt battery system.

I thought all the truck speedometers in 1938 were round. Weren't they? If yours is round, you might be able to adapt a GPS unit to yours. I'm doing that with the AD right now. To avoid having to chop up an original AD speedometer case, I'm making a new one just for the GPS unit. I've already done this, but when I did it before I didn't use a GPS unit. It will work and it will fit in the AD and if you don't want turn signal indicators in it, there will only be a connection for + (to ignition switch), one to ground and one for the high beam indicator (which will now be a small red led bulb) plus the GPS signal sensor (which I think may be able to hide under the dash). The lights will use the exact same setup as before and will snap into the same type holes as before.

The little 1" x 3/4" window you see is for the odometer and trip odometer. Sorry, but absolutely none of the GPS units I've found come with analog (wheel type) odometers. I've been searching for those for 2 or 3 years and if they're out there I guess I missed them.

The '38 speedometer uses standard cable and standard ends, the cable is not terrible long and has a pretty straight shot from trans to speedometer. The cable ends are square drive, and the screw hold down ends seem to be the standard sized that fit just about everything. Someone with a lathe could turn you an adaptor if nothing is available. All speedometer driven gears are plastic. If you know the tooth count of the drive gear in the transmission, the gear ratio of the axle, and tire diameter, you can figure what speedometer driven gear will get you close. Due to the age of the speedometer, it's linear accuracy is likely off. Mine gets farther off the faster you go, so I geared it to be correct at 60mph, anything slow or faster is slightly off.

You might be able to modify your NV4500, replace the tail shaft and housing for one with connection for mechanical speedo. I think there were a few early years with that set up. I will be using my '47 speedo with an '88 5LM60 (early NV3500). There are off the shelf convertor boxes that take the digital signal to run a motor that connects to the speedo cable. I decided to build my own. Not a simple solution, but not impossible. Here is a link to a video I made of the test run, I hooked it up in a '90 truck along side the stock speedo.

If you look at the comments about the dash mount HUD and GPS units:

*) The car battery failed. Installed new battery same thing. $300 dollars later, I figured out my unit didn't power off when I turned off the ignition.
*) Takes a long time (5 to 15 minutes) for the GPS to start and display speed.
*) Displayed speed is (3 to 5 MPH) off.
*) Most often commented: Installed the unit and worked good the first day then failed the next. Cheap cable for +12V connection.
*) The unit has an adhesive mount that is super sticky. You have one chance to sit it in place then you can't move it again.

Makes me think that the most often purchased cell phone (PMD) or mapper with cradle (with built-in GPS) would be a better option of the add-on devices.

Yes mine is round too. Thanks for the ideas guys. Looking into this!