Speedo wrong, but is that all?
 

So as my trek into Jeep repair continues, I've got a new issue. The
speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
repair work done to it... after which, the speedo started reading
incorrectly. I haven't learned anything about the speedometer system on a
Jeep yet, though I have seen that it's electronic, so I'm hoping the manual
(when it arrives) will give me details on recalibrating it, but here's my
concern... it dawned on me that perhaps the rear end was swapped out as a
part of the repairs (entirely possible), and that the new one that was put
in, for some reason, had a different gear ratio and that's causing the
massively incorrect speedo readings. Now if this is the case, that'd be
just fine unless the gear ratio in the front differential did not match,
correct? It seems to me that if the front and rear differential gear ratios
do not match up, engaging 4WD would quickly break something if you were to
drive very far on anything but the loosest of gravel.

I don't have a way to ask the repair shop if the rear differential was
changed out in any way, and the normal trick of rotating the wheels while
the vehicle is elevated and counting the rotations of the driveshaft doesn't
seem to be working as the differential is a bit too loose for that and it
just spins the other wheel in the opposite direction (turning the driveshaft
and counting wheel rotations isn't working either). Is there any sort of
trick to figuring out from casting marks or ratio tags on the differential
housings what the internals might be? Or if it's the same assembly that
came with the vehicle originally? Judging by the amount of yuck on the
housing I'm guessing it's the original, but then that doesn't explain the
huge speedo discrepancy. Maybe someone out there knows how the speedo
system well enough to explain how this could happen. Thanks in advance for
any suggestions!

Bryan

Re: Speedo wrong, but is that all?
 

"The Hurdy Gurdy Man" <bryan@linux.webicommerce.com> wrote in message
news:blgodk$diq$1@nnrp.atgi.net...
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly.

Tire size same? Did the transmission or transfer case get changed?
There's a plastic gear that is changeable that drives the speed
sensor... perhaps that gear is wrong now? You don't have a
cable drive speedo, do you? Later speedos are basically a
meter movement; the computer puts out volts and the speedo
displays mph.
__
Steve
..

Re: Speedo wrong, but is that all?
 

"The Hurdy Gurdy Man" <bryan@linux.webicommerce.com> wrote in message
news:blgodk$diq$1@nnrp.atgi.net...
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly.

Tire size same? Did the transmission or transfer case get changed?
There's a plastic gear that is changeable that drives the speed
sensor... perhaps that gear is wrong now? You don't have a
cable drive speedo, do you? Later speedos are basically a
meter movement; the computer puts out volts and the speedo
displays mph.
__
Steve
..

Re: Speedo wrong, but is that all?
 

You need to only jack up one wheel to spin and get the ratio.

I chalk mark the tire and the u-joint so I can count it easier. With
one wheel in the air only, the wheel needs two rotations to give you the
ratio.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

The Hurdy Gurdy Man wrote:
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly. I haven't learned anything about the speedometer system on a
> Jeep yet, though I have seen that it's electronic, so I'm hoping the manual
> (when it arrives) will give me details on recalibrating it, but here's my
> concern... it dawned on me that perhaps the rear end was swapped out as a
> part of the repairs (entirely possible), and that the new one that was put
> in, for some reason, had a different gear ratio and that's causing the
> massively incorrect speedo readings. Now if this is the case, that'd be
> just fine unless the gear ratio in the front differential did not match,
> correct? It seems to me that if the front and rear differential gear ratios
> do not match up, engaging 4WD would quickly break something if you were to
> drive very far on anything but the loosest of gravel.
>
> I don't have a way to ask the repair shop if the rear differential was
> changed out in any way, and the normal trick of rotating the wheels while
> the vehicle is elevated and counting the rotations of the driveshaft doesn't
> seem to be working as the differential is a bit too loose for that and it
> just spins the other wheel in the opposite direction (turning the driveshaft
> and counting wheel rotations isn't working either). Is there any sort of
> trick to figuring out from casting marks or ratio tags on the differential
> housings what the internals might be? Or if it's the same assembly that
> came with the vehicle originally? Judging by the amount of yuck on the
> housing I'm guessing it's the original, but then that doesn't explain the
> huge speedo discrepancy. Maybe someone out there knows how the speedo
> system well enough to explain how this could happen. Thanks in advance for
> any suggestions!
>
> Bryan

Re: Speedo wrong, but is that all?
 

You need to only jack up one wheel to spin and get the ratio.

I chalk mark the tire and the u-joint so I can count it easier. With
one wheel in the air only, the wheel needs two rotations to give you the
ratio.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

The Hurdy Gurdy Man wrote:
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly. I haven't learned anything about the speedometer system on a
> Jeep yet, though I have seen that it's electronic, so I'm hoping the manual
> (when it arrives) will give me details on recalibrating it, but here's my
> concern... it dawned on me that perhaps the rear end was swapped out as a
> part of the repairs (entirely possible), and that the new one that was put
> in, for some reason, had a different gear ratio and that's causing the
> massively incorrect speedo readings. Now if this is the case, that'd be
> just fine unless the gear ratio in the front differential did not match,
> correct? It seems to me that if the front and rear differential gear ratios
> do not match up, engaging 4WD would quickly break something if you were to
> drive very far on anything but the loosest of gravel.
>
> I don't have a way to ask the repair shop if the rear differential was
> changed out in any way, and the normal trick of rotating the wheels while
> the vehicle is elevated and counting the rotations of the driveshaft doesn't
> seem to be working as the differential is a bit too loose for that and it
> just spins the other wheel in the opposite direction (turning the driveshaft
> and counting wheel rotations isn't working either). Is there any sort of
> trick to figuring out from casting marks or ratio tags on the differential
> housings what the internals might be? Or if it's the same assembly that
> came with the vehicle originally? Judging by the amount of yuck on the
> housing I'm guessing it's the original, but then that doesn't explain the
> huge speedo discrepancy. Maybe someone out there knows how the speedo
> system well enough to explain how this could happen. Thanks in advance for
> any suggestions!
>
> Bryan

Re: Speedo wrong, but is that all?
 

Your speedometer will be cable driven:
http://go.jeep-xj.info/HowtoSpeedoGears.htm
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

The Hurdy Gurdy Man wrote:
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly. I haven't learned anything about the speedometer system on a
> Jeep yet, though I have seen that it's electronic, so I'm hoping the manual
> (when it arrives) will give me details on recalibrating it, but here's my
> concern... it dawned on me that perhaps the rear end was swapped out as a
> part of the repairs (entirely possible), and that the new one that was put
> in, for some reason, had a different gear ratio and that's causing the
> massively incorrect speedo readings. Now if this is the case, that'd be
> just fine unless the gear ratio in the front differential did not match,
> correct? It seems to me that if the front and rear differential gear ratios
> do not match up, engaging 4WD would quickly break something if you were to
> drive very far on anything but the loosest of gravel.
>
> I don't have a way to ask the repair shop if the rear differential was
> changed out in any way, and the normal trick of rotating the wheels while
> the vehicle is elevated and counting the rotations of the driveshaft doesn't
> seem to be working as the differential is a bit too loose for that and it
> just spins the other wheel in the opposite direction (turning the driveshaft
> and counting wheel rotations isn't working either). Is there any sort of
> trick to figuring out from casting marks or ratio tags on the differential
> housings what the internals might be? Or if it's the same assembly that
> came with the vehicle originally? Judging by the amount of yuck on the
> housing I'm guessing it's the original, but then that doesn't explain the
> huge speedo discrepancy. Maybe someone out there knows how the speedo
> system well enough to explain how this could happen. Thanks in advance for
> any suggestions!
>
> Bryan

Re: Speedo wrong, but is that all?
 

Your speedometer will be cable driven:
http://go.jeep-xj.info/HowtoSpeedoGears.htm
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

The Hurdy Gurdy Man wrote:
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly. I haven't learned anything about the speedometer system on a
> Jeep yet, though I have seen that it's electronic, so I'm hoping the manual
> (when it arrives) will give me details on recalibrating it, but here's my
> concern... it dawned on me that perhaps the rear end was swapped out as a
> part of the repairs (entirely possible), and that the new one that was put
> in, for some reason, had a different gear ratio and that's causing the
> massively incorrect speedo readings. Now if this is the case, that'd be
> just fine unless the gear ratio in the front differential did not match,
> correct? It seems to me that if the front and rear differential gear ratios
> do not match up, engaging 4WD would quickly break something if you were to
> drive very far on anything but the loosest of gravel.
>
> I don't have a way to ask the repair shop if the rear differential was
> changed out in any way, and the normal trick of rotating the wheels while
> the vehicle is elevated and counting the rotations of the driveshaft doesn't
> seem to be working as the differential is a bit too loose for that and it
> just spins the other wheel in the opposite direction (turning the driveshaft
> and counting wheel rotations isn't working either). Is there any sort of
> trick to figuring out from casting marks or ratio tags on the differential
> housings what the internals might be? Or if it's the same assembly that
> came with the vehicle originally? Judging by the amount of yuck on the
> housing I'm guessing it's the original, but then that doesn't explain the
> huge speedo discrepancy. Maybe someone out there knows how the speedo
> system well enough to explain how this could happen. Thanks in advance for
> any suggestions!
>
> Bryan

Re: Speedo wrong, but is that all?
 

"The Hurdy Gurdy Man" <bryan@linux.webicommerce.com> wrote in message
news:blgodk$diq$1@nnrp.atgi.net...
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly. I haven't learned anything about the speedometer system on a
> Jeep yet, though I have seen that it's electronic, so I'm hoping the
manual
> (when it arrives) will give me details on recalibrating it, but here's my
> concern... it dawned on me that perhaps the rear end was swapped out as a
> part of the repairs (entirely possible), and that the new one that was put
> in, for some reason, had a different gear ratio and that's causing the
> massively incorrect speedo readings. Now if this is the case, that'd be
> just fine unless the gear ratio in the front differential did not match,
> correct? It seems to me that if the front and rear differential gear
ratios
> do not match up, engaging 4WD would quickly break something if you were to
> drive very far on anything but the loosest of gravel.
>


I think you have hit on a very real possibility. If the rear end was swapped
aftger a gear change, then the front and rear will now be geared
differently, and this could be a big problem for the tcase.


Raise one of the rear tires and spin it a full revolution, the drive shaft
will spin twice the number of turns as the ratio. Or, turn the raised tire
one-half revolution, and watch the driveshaft. Don't forget to set the trans
to N.

I don't think you can use the casting marks because we suspect the gears are
not the same anyway. The most common ratios you will find are 3.07, 3.73,
4.10. These are the factory ratios, and the 4.10 is for the 4 banger motor.
You should be able to see the obvious difference in 3, 3.75, and 4 turns of
the driveshaft.

The gears could have been swapped out, but the speedo was never corrected,
and this would explain what you are noticing, and the front and the rear are
likely to have the same gear sets.

You need to determine if the same gears are in the front and th erear before
you worry about anything else.



> I don't have a way to ask the repair shop if the rear differential was
> changed out in any way, and the normal trick of rotating the wheels while
> the vehicle is elevated and counting the rotations of the driveshaft
doesn't
> seem to be working as the differential is a bit too loose for that and it
> just spins the other wheel in the opposite direction (turning the
driveshaft
> and counting wheel rotations isn't working either). Is there any sort of
> trick to figuring out from casting marks or ratio tags on the differential
> housings what the internals might be? Or if it's the same assembly that
> came with the vehicle originally? Judging by the amount of yuck on the
> housing I'm guessing it's the original, but then that doesn't explain the
> huge speedo discrepancy. Maybe someone out there knows how the speedo
> system well enough to explain how this could happen. Thanks in advance
for
> any suggestions!
>
> Bryan
>

Re: Speedo wrong, but is that all?
 

"The Hurdy Gurdy Man" <bryan@linux.webicommerce.com> wrote in message
news:blgodk$diq$1@nnrp.atgi.net...
>
> So as my trek into Jeep repair continues, I've got a new issue. The
> speedometer is wrong by like 10 or 15 MPH or so. Now, this Jeep (a '94
> Wrangler YJ) was in a pretty bad accident a while back, and had a lot of
> repair work done to it... after which, the speedo started reading
> incorrectly. I haven't learned anything about the speedometer system on a
> Jeep yet, though I have seen that it's electronic, so I'm hoping the
manual
> (when it arrives) will give me details on recalibrating it, but here's my
> concern... it dawned on me that perhaps the rear end was swapped out as a
> part of the repairs (entirely possible), and that the new one that was put
> in, for some reason, had a different gear ratio and that's causing the
> massively incorrect speedo readings. Now if this is the case, that'd be
> just fine unless the gear ratio in the front differential did not match,
> correct? It seems to me that if the front and rear differential gear
ratios
> do not match up, engaging 4WD would quickly break something if you were to
> drive very far on anything but the loosest of gravel.
>


I think you have hit on a very real possibility. If the rear end was swapped
aftger a gear change, then the front and rear will now be geared
differently, and this could be a big problem for the tcase.


Raise one of the rear tires and spin it a full revolution, the drive shaft
will spin twice the number of turns as the ratio. Or, turn the raised tire
one-half revolution, and watch the driveshaft. Don't forget to set the trans
to N.

I don't think you can use the casting marks because we suspect the gears are
not the same anyway. The most common ratios you will find are 3.07, 3.73,
4.10. These are the factory ratios, and the 4.10 is for the 4 banger motor.
You should be able to see the obvious difference in 3, 3.75, and 4 turns of
the driveshaft.

The gears could have been swapped out, but the speedo was never corrected,
and this would explain what you are noticing, and the front and the rear are
likely to have the same gear sets.

You need to determine if the same gears are in the front and th erear before
you worry about anything else.



> I don't have a way to ask the repair shop if the rear differential was
> changed out in any way, and the normal trick of rotating the wheels while
> the vehicle is elevated and counting the rotations of the driveshaft
doesn't
> seem to be working as the differential is a bit too loose for that and it
> just spins the other wheel in the opposite direction (turning the
driveshaft
> and counting wheel rotations isn't working either). Is there any sort of
> trick to figuring out from casting marks or ratio tags on the differential
> housings what the internals might be? Or if it's the same assembly that
> came with the vehicle originally? Judging by the amount of yuck on the
> housing I'm guessing it's the original, but then that doesn't explain the
> huge speedo discrepancy. Maybe someone out there knows how the speedo
> system well enough to explain how this could happen. Thanks in advance
for
> any suggestions!
>
> Bryan
>

Re: Speedo wrong, but is that all?
 

I have what is clearly the world's worst news feed being that none of the
replies to my original post have shown up yet, but I can see on Google that
there have been quite a few. Thanks to all that have replied! Here's what
I've done and discovered so far, but it's raised a few more questions as a
result (isn't that always the way?).

I put one rear tire up in the air, shifted into neutral, marked a spot on
the tire, spun it twice and counted the number of turns of the driveshaft.
This gave me about 3.08 ~ 3.10 turns for two tire rotations... accounting
for any slop in the process, I'm guessing this means I have 3.07 gears on
the rear. It's a 1994 with a 4.0L engine and a five speed manual
transmission, which I'm assuming is reasonable. So then I tried this same
procedure with the front, but... nothing. With one tire on the ground, one
in the air, and the various gear selectors moved into all kinds of different
positions (in gear, neutral, 2WD, 4WD, etc.) I couldn't get the driveshaft
to move while I turned the wheel. I don't see any sort of locking mechanism
on the wheel hubs, and the only tube running into the front differential
that seems like it migh control something on the inside seems to only be a
vent tube that runs up by the radiator, so I'm not sure what I'm missing
here... sometimes you can turn the driveshaft by hand and get a very slight
bit of movement in the wheel, but definitely nothing solid and absolutely
not the other way around. Is there some different trick I should be trying
here? I'm a stranger to the ways of 4WD, and my shop manual is still four
days out, so I'm at a loss for what to try next. It seems to me the lgoic
behind the procedure is sound, but then I could very well be overlooking
something critical.

The speedometer is most decidedly an electronic one and not cable driven,
and presumably has been since the day it was born as the gauges have never
been changed. The tires are slightly larger than the originals, but that
was the case before the accident and the speedo offset due to that was a
fairly minor one. Certainly not the 15 or so MPH it's reading off now. I
see that quadratec.com sells a recalibration box, plus there's a bevy of
potential speedometer gear changes that can be done, so once I figure out
exactly why it's incorrect I'll make my next move. It is possible that some
portion of the drivetrain was swapped out during the repair, especially
considering how much cleaner the transmission is than various other parts of
the car... but that information is only suggestive of what happened, and not
conclusive.

I'll get under there this weekend and pop out the current speedo gear to see
what it is. I don't want to pop open the front differential cover and
actually count gear teeth, but if that's the only solution then I will.
It's probably overdue for a fluid change anyhow. There's also a temptation
to just put the thing up on four stands and put it in gear to see what
happens... might be a faster way of figuring out what's up with the front
differential!

Thanks again to everyone who has helped on this rather perplexing problem,
hopefully I'll be to the bottom of it soon.

Bryan