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Erratic speedometer problem on my 1998 GT

 
red98vert
New User
Posts: 4
Joined: 01/14
Posted: 01/25/14 01:23 PM

For the past 3 years, I have been having trouble with my speedo registering correctly.  When in park it usually registers 20mph.  When moving, it reads 10 to 20 mph higher than the actual speed.  If speed goes above 70, the speedo sticks there and doesn't lower until I get back below 30mph.  The odometer and trip odometer seem to work fine - I know that can be a problem on Mustangs.  Not sure if it is the speedo cable, the instrument cluster, or something else.  Any help would be greatly appreciated!  

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wayne712222
User
Posts: 103
Joined: 10/13
Posted: 01/25/14 03:54 PM

first thing.. DO you have a scan tool that displays live data stream...
with a friend in the passenger seat.. to handle the scan tool... take a test drive and see if the Vehicle speed sensor data to the PCM  is actually ROAD SPEED.. not the incorrect data the speedo is displaying.


i am looking at the 98 mustang wiring diagram..

the VSS is grounded thru a PINK and Orange striped wire to G203.. somehere under the dash.. or at the ends of the dash.. the pink and orange wire also goes to PIN 33 on the PCM

the PCM is grounded thru G100 under the hood.. and also to G203 under the dash.

the VSS output goes thru a grey and black striped wire to 3 devices.. the PCM on pin 58, the speed control amp pin 3 and the instrument cluster..

this is why i want you to check the PCM with a scan tool while the car is in motion..  if the VSS is properly grounded and sending the proper signal to the PCM.. the PCM will show correct speed..

if the instrument cluster is improperly grounded..  the data from the PCM will be correct but the speedo in the cluster possibly incorrect.


i will have to DIG out another book to find G203... its could be up under the lip of the dash board.. or over on one end behind the kick panel...

use the scan tool first.. see if the PCM speed is accurate..

post what you get..  

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red98vert
New User
Posts: 4
Joined: 01/14
Posted: 01/26/14 12:25 PM

sounds like the 98 wiring diagram is the right one to use.  i do not have a handheld scan tool, but perhaps i should purchase one?  also, is that speedo a mechanical drive or electric?  is it a major undertaking to find those wires mentioned?  thanks for your quick reply  

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wayne712222
User
Posts: 103
Joined: 10/13
Posted: 01/26/14 02:00 PM

for a scan tool.. look in the print issue.. or your AAA magazine.. HF has coupons for 20 or 25% off. there are several different versions.. i just got this one and have been happy with it..  this one is just over 100 bucks. and if the green check mark is on.. the car will pass a smog test..  a neat feature.  the green check mark also lets you know that the engine controls are not in default mode that is backing down on the performance and fuel economy..  it does a bunch of other amazing stuff for the price.

Harborfreight 60694 Zps5acdb9d2



96 98 Mustang38vssdiagram Zpsdb5dadf9


this shows the VSS- or ground connection into the pcm connector.

96 98 Mustang38engine Zps95d3522f

slide the bar below to the right and scroll back up to view the right side of the diagrams above  

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wayne712222
User
Posts: 103
Joined: 10/13
Posted: 01/26/14 02:05 PM

you will also need this 2007 info.  it should be the very close..


2007Mustangdashg201 G202 Zpsfd2cd132

2007Mustang G203 G205 Zps1ea00b18  

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wayne712222
User
Posts: 103
Joined: 10/13
Posted: 01/26/14 02:35 PM

there are a bunch of ground connections in the engine bay... please also take those apart and wire brush both sides and reattach them.

Fordcoresupportgrounds Zpsc9737ea7

if you find stuff like this.. you need to fix it..

Fordbatterytobodygroundwire Zps50c65ba5

there is also a braided ground strap from the back of your engine to the firewall..  

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red98vert
New User
Posts: 4
Joined: 01/14
Posted: 01/26/14 05:42 PM

Thanks for all the info.  Do you recommend that I check all of these ground connections first and then run the scan you originally mentioned?  

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red98vert
New User
Posts: 4
Joined: 01/14
Posted: 01/26/14 05:45 PM

Do you recommend that i check all of these ground connections before i run the test with the scan tool?  seems like that might solve the problem by itself?  thanks for all the info!  

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wayne712222
User
Posts: 103
Joined: 10/13
Posted: 01/26/14 06:26 PM

if you disconnect the battery or the ground connections..  you may loose any stored codes.. increasing your diagnostic time.. waiting for the codes to reset..


run the scanner to read the active codes...  write the code numbers down...


get into the live data stream..

Obd2scandatachart Zps65411833

Obd2scandatachart2 Zps6e93e7de

http://manuals.harborfreight.com/manuals/60000-60999/60694.pdf


here is the PDF version of the manual for the 60694..  

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wayne712222
User
Posts: 103
Joined: 10/13
Posted: 01/26/14 06:34 PM

you will also want this..

its the 1998 how the OBD2 system works..

https://www.motorcraftservice.com/vdirs/diagnostics/pdf/obdsm986.pdf


it will open your eyes to what you are seeing on the scan tool...  

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