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Discussion Starter · #1 ·
My 74 CJ5 with stock speedometer has suddenly started reading about 20% low and simultaneously, the odometer is reading about 20% low (indicates 0.8 miles when traversing 1.0 actual miles). Previously, the speedometer and odometer were fairly close to correct. As I see it, there are only three possibilities, the gear in the transfer case, the cable, the speedometer/odometer itself. Any suggestions about how I should go about troubleshooting and isolating the defective component? I don't hear any new noises since the problem appeared.

 
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Discussion Starter · #2 ·
Did you just put on new, bigger tires or change your ring/pinion gears? Either of these changes will change your speedo readings.

Did it go together nice and easy? You did something wrong.Will have to do it again./wwwthreads_images/icons/crazy.gif
DaveC
 
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Discussion Starter · #3 ·
How does changing your R&P change the speedo. I have bigger tires and I am about to change the R&P. I have my speedo set for the 33" tires that I run but I never heard anything about R&P changing things.
THANX

Lee R. Byrd
 

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Mine started doing the same thing and then the speedo cable snapped in two....
You may as well start with the cable since its the easiest to check.

'79 CJ-7
 
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Discussion Starter · #6 ·
Your speedo AND OD will always be off if the driven gear(one on the end of the cable) is not matched to your overall gearing. Tire size, R&P ratios and trans final drive ratio ALL determine your overall gearing. Since all trans except for overdrive units have a 1 to 1 top gear we can leave them out of this. If your speedo says your do 60mph and time it for 1 minute, the OD will say 1 mile. Now if the speedo says 60 mph and your actual speed is 75mph you are actually traveling 1 1/4 miles per minute. The cable will not produce a lower reading. When your cable gets old and crusty (not lubed anymore) your speedo will bounce. This is because the cable can't spin freely.It gets bound up and then releases the energy or breaks.. It momentarily give a slower reading then immediately give a higher reading.
The only way to correct your speedo and Od are to get the correct drive gear for your final drive ratio. I just did this for my truck(running 35s and 4.10 gearing).
I got in touch with United Speedometer Service www.speedometershop.com (800-877-4798). You will need to give them the following info.
Tire size (not whats stamped on the tire but it's actual height)
R&P ratio
# of teeth on the drive gear in your transfer case(take out the cable and look it with a flash light).

From this info the can calculate the exact number of teeth your driven gear should have to have an accurate speedo and OD.

Did it go together nice and easy? You did something wrong.Will have to do it again./wwwthreads_images/icons/crazy.gif
DaveC
 
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Discussion Starter · #7 ·
Thanks for the suggestions. Let me clarify that I haven't changed anything. Odometer and speedometer were both okay and reasonably accurate until a couple of weeks ago. Odometer/speedometer assembly is about five years old. However the cable is the original. Think I'll check out the cable over the weekend. That's the easiest first step.

Thanks for the help.

Tim Kirby

 
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Discussion Starter · #8 ·
Before you replace the cable (which you'll probably have to do anyhow). Undo it from the cluster, put a drop or 2 of oil in, re-attach, and test it. If the problem is not as pronounced, it's definitely the cable.

 
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Discussion Starter · #9 ·
If you disconnect either end of the cable, try pulling it out. It should not come out. The cable may be broken and the two ends are just grinding on each other.

Did it go together nice and easy? You did something wrong.Will have to do it again./wwwthreads_images/icons/crazy.gif
DaveC
 

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check the square end of the cable that goes into the T. case Mine was worn round just in one little section needless to say it did not work too well either.Use graphite to lubricate it or the new cable.

Learn to let go of what does not serve you ,but forces you to serve it
 

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The dana 20 transfer case uses a plastic, yes plastic, gear in the rear of the transfer case. Rule out the broken cable and the other issues first. If you have to check this gear you can disconnect the rear driveshaft and then remove the rear housing on the transfer case(about 6-7 screws) you should be able to pull the rear assembly off. To get to the plastic gear you will have to remove the rear yoke and press the shaft through the front to seperate the bearing. The plastic gear actually is just pressed on the shaft. I had the same problem once and the speedo kept decreasing until eventually it would not work at all. Then it would only work until the transfer would heat up. The gear was actually sliding down the shaft and would partialy lose contact with the speedo gear shaft. I had to slide the gear back up and secure it.
For those of us that have changed running gears or tire sizes I wonder if someone makes other ratio gears?
Good Luck
Pigpen

 
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