I got a rebuild M38A1 speedometer

Deadguy · Post by **Deadguy** » Fri Nov 02, 2012 6:12 pm

off Chuck on the board, and a 15 tooth speedometer drive from Pete Debella. Both are installed, both are working fine, and the speedometer is still reading way off. It reads high, like I have civy gears in my axles, but I have the correct gear in the transfer case, the correct transfer case model, and the correct speedometer. The cable I have is the correct waterproof one Pete Debella sells. At this point, I'm at a loss. Does anyone have any ideas?

Post by **wesk** » Fri Nov 02, 2012 8:17 pm

Before you drive yourself to the edge I would suggest pulling the rear diff cover and counting teeth on the ring and pinion.

What are you gaging your actual speed with?

Deadguy · Post by **Deadguy** » Fri Nov 02, 2012 10:05 pm

I'll do that as soon as I get back from Mass in a week Wes. I was thinking, if the speedometer is reading high, the gears must be the lower military gear ratio, right? If they were higher, the speedometer would be reading closer to the actual speed? The engine, transmission, and transfer case are rebuilt like new, so I'm thinking if I had a higher gear ratio as well, the vehicle would be faster than the current top speed of around 50 (60 with the overdrive).
I'm using a GPS to determine the actual speed.

Post by **wesk** » Fri Nov 02, 2012 10:14 pm

If you are positive the diff gears are 5.38's and the tranny and transfer gearing are not pickup or wagon and you have the correct drive and driven speedo gears and the drive cable is new then the only source for the high error is the gage. Send or take the gage to a speedo shop and have them Cal. Check it.

Deadguy · Post by **Deadguy** » Fri Nov 02, 2012 10:26 pm

What would I look for in the transmission/transfer case? I had Novak rebuild them, and they say they are correct. The drive gear fits the 15 tooth military speedometer drive, not sure if that makes a difference.

Post by **wesk** » Fri Nov 02, 2012 10:52 pm

It's all about ratios which means you would need to count teeth on each gear in the tranny and transfer. I'd start with the easy part and pull the speedo and send it to the shop for a calibration check.

Deadguy · Post by **Deadguy** » Sat Nov 03, 2012 12:43 am

Ok, one more question. I swapped the speedometer I had in there, with a rebuilt one from Chuck here on the board. Both are reading high. Do you think it could really be two speedometers in a row? And if the speedometer is not the issue, can it still be calibrated to give a more accurate mileage reading?

bluewatersailor · Post by **bluewatersailor** » Sat Nov 03, 2012 7:59 am

Are your tires 700x16 NDCC? If they are smaller or larger, the speedometer accuracy will be off. Just a thought. David

Post by **wesk** » Sat Nov 03, 2012 8:59 am

Just how high is the reading at 35 and 50 MPH?

A speedo or tach has a reasonable range of adjustment available.

Here you can download a copy of the Speedo service manual:
http://jeepdraw.com/images/jeepdraw/ADO ... Repair.pdf

Deadguy · Post by **Deadguy** » Sat Nov 03, 2012 11:29 am

At 35 MPH it reads about 60. At 50, the needle is way off the charts. The tires are correct, I just changed them recently as the ones that came with the vehicle were dry rotted.

Deadguy · Post by **Deadguy** » Sun Nov 04, 2012 12:13 am

That speedometer service manual is pretty daunting. I'll double check, but I am 95% certain that I have the stock 5:38 gears. With a second speedometer that is a rebuilt military unit and still reads the same, a 15 tooth speedometer driven gear, and a military speedometer cable, I think that leaves the transfer case? What should I check next? Novak rebuilt the Transfer case and transmission for me, what could they have conceivably done to alter the speedometer reading? I know when I had a CJ, with bigger tires the speedometer read wrong, and I needed a new driven gear to accommodate it, is that an option?

Post by **wesk** » Sun Nov 04, 2012 1:10 am

I wouldn't waste another ounce of energy or money on the problem until I have that gage bench checked and re-calibrated if needed.

When and by whom were those two speedos overhauled?

Deadguy · Post by **Deadguy** » Sun Nov 04, 2012 2:05 am

The first one I got off Ebay. So, who knows. The second one I got from Chuck on the forum here. They look similar, but have slight differences, and Chuck tells me the one he sold me is a rebuilt military correct one. The thing is, both read the same, but both are not reading the correct mileage.

Post by **wesk** » Sun Nov 04, 2012 4:09 am

Then take both of them to the speedo shop just in case only one is in any condition to be calibrated.