Yet another speedo gear question - Page 2 - JeepForum.com

Reply
 
LinkBack Thread Tools
post #16 of 26 Old 06-19-2017, 04:07 PM
2jhanna
Web Wheeler
2002 TJ Wrangler 
 
Join Date: Jun 2007
Location: Denton County, TX
Posts: 7,932
Since the speedo gear it what it should be, and since the odometer is as close as it can get.

I would look into a possible problem with the speedometer itself, maybe a way to recalibrate or replace it.

Is the speedometer off a fixed amount (regardless of speed) or a fixed percentage?


jhanna
denton county, tx
2jhanna is offline  
Sponsored Links
Advertisement
 
post #17 of 26 Old 06-19-2017, 04:30 PM
biffgnar
Moderator
 
biffgnar's Avatar
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Westchester, NY
Posts: 22,266
Quote:
Originally Posted by Jerry Bransford View Post
I'm not sure what you were thinking but the TJ's speedometer can of course be recalibrated to different tire size and axle ratio combinations by replacing its snap-in speedometer gear. Only the Rubicon doesn't have that option.

http://www.4x4xplor.com/speedo.html

It of course isn't always going to give a precision mph indication due to minor tire diameter differences but the right speedometer gear gives a mph indication that is good enough to work with and to remove any possible excuse for a speeding citation.
He was talking about flashing the PCM.

Romney in 2020!
biffgnar is online now  
post #18 of 26 Old 06-19-2017, 04:31 PM
biffgnar
Moderator
 
biffgnar's Avatar
2006 TJ Wrangler 
 
Join Date: May 2008
Location: Westchester, NY
Posts: 22,266
Quote:
Originally Posted by 2jhanna View Post
Is the speedometer off a fixed amount (regardless of speed)
How would this be possible? Doesn't it have to be a percentage?

Romney in 2020!
biffgnar is online now  
 
post #19 of 26 Old 06-19-2017, 04:55 PM
2jhanna
Web Wheeler
2002 TJ Wrangler 
 
Join Date: Jun 2007
Location: Denton County, TX
Posts: 7,932
Quote:
Originally Posted by biffgnar View Post
How would this be possible? Doesn't it have to be a percentage?
What if the needle itself wasn't "zeroed" on the speedometer's spindle.

Like if you pulled the needle off, moved it over to the 10mph mark and pressed it back on. The speedo would always be plus 10.

I guess the same could apply if the needle was at a negative position, just resting on the stop, then the speedo would always be the off the same minus amount.

jhanna
denton county, tx
2jhanna is offline  
post #20 of 26 Old 06-20-2017, 06:38 AM Thread Starter
secretnarwhal
Registered User
2000 TJ Wrangler 
 
Join Date: Sep 2014
Location: Detroit
Posts: 33
It's off by a constant percentage, and I haven't noticed the needle "jump" before like other speedometers that I've seen fail, so I am fairly confident that the gauge itself is working properly

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
secretnarwhal is offline  
post #21 of 26 Old 06-20-2017, 07:52 AM
rouxbicon
Registered User
2003 TJ Wrangler 
 
Join Date: Oct 2005
Location: Baton Rouge, La
Posts: 672
If you have or can get your hands on an OBD2 reader, they can usually generate a digital speed reading directly from the PCM. You can then compare this to the GPS. I'm guessing this would be close (within 1.5%) based on the OP's odometer comparison.

The PCM is supposed to position the speedometer needle appropriately based on the digital number, but it sounds like the OPs may not be doing so?

Isn't there an instrument panel test that, among other things, steps the speedo and tach needles through their ranges? Similar to the key on, key off DTC check...

'03 red Rubi
rouxbicon is offline  
post #22 of 26 Old 06-20-2017, 09:16 AM Thread Starter
secretnarwhal
Registered User
2000 TJ Wrangler 
 
Join Date: Sep 2014
Location: Detroit
Posts: 33
Good call. I can get my hands on a reader pretty easily, so I'll check it out.

The instrument panel test is done by holding down the odometer trip button, then turning the key from off to on, continuing to hold down the button (I asked google). With the button pushed, the dash then illuminates each dummy light one by one, then checks each gauge. I'll do this during my lunch break.

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
secretnarwhal is offline  
post #23 of 26 Old 06-20-2017, 09:23 AM Thread Starter
secretnarwhal
Registered User
2000 TJ Wrangler 
 
Join Date: Sep 2014
Location: Detroit
Posts: 33
For anyone else who is curious, it looks like the speedo is supposed to jog straight to 20 mph, sit for a second, move straight to 55, sit, then 75, sit, then return to 0 mph during the test

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
secretnarwhal is offline  
post #24 of 26 Old 06-20-2017, 09:36 AM
2jhanna
Web Wheeler
2002 TJ Wrangler 
 
Join Date: Jun 2007
Location: Denton County, TX
Posts: 7,932
This may sound redundant, but if you're on the open road, interstate, etc. and you set your cruise control to 60 mph, what does your GPS read.

jhanna
denton county, tx
2jhanna is offline  
post #25 of 26 Old 06-20-2017, 10:16 AM Thread Starter
secretnarwhal
Registered User
2000 TJ Wrangler 
 
Join Date: Sep 2014
Location: Detroit
Posts: 33
if only I had cruise control...

If the speedo reads 60, the GPS will say 57ish mph

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
secretnarwhal is offline  
post #26 of 26 Old 06-20-2017, 11:24 AM Thread Starter
secretnarwhal
Registered User
2000 TJ Wrangler 
 
Join Date: Sep 2014
Location: Detroit
Posts: 33
Ok the test is initiated the same way I described before, but the video I watched was for a later model TJ. My dash tested things in a different order. I might upload a video of it to youtube later.

Surprisingly, during the instrument panel test my speedometer was reading about 2 mph fast at every point. (20, 55, 80, 100, 80, 55, then 20 mph). I guess I'll figure out how to pop off the needle and slap it back on pointing 2 mph slower. What might complicate things is the stop that the needle rests on.

What I'm thinking now is that I've got both a constant value error (from the needle being positioned incorrectly or the PCM sending a signal that's a little high) and a constant percentage error (just normal variations in tire diameter and the factory setup). I believe it's reading 2 mph too fast + a smaller percentage error than my previously estimated 8%. At 74 mph the speedo reads 80 (8% error), and at 22, the speedo reads 25 (13% error). If you factor out the speedo reading 2mph fast, meaning at 74 it would read 78 and at 22 it would read 23, there would be roughly a constant 5% error.

Now That I've completely overthought a minor issue and put entirely too much effort into it, I think I'll have it fixed fairly soon.

'00 TJ, 4.0, 5-speed, 114k
'91 YJ, 2.5L 5-speed, 116k *sold*
secretnarwhal is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.



Email Address:
OR

Log-in









Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome