Speedo jumping around while driving - JeepForum.com
 
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post #1 of 8 Old 01-15-2019, 09:41 PM Thread Starter
JaimeZX
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Speedo jumping around while driving

Just started the other day. There are a couple of threads on this, but none seem to list the part number for the speed sensor in the transfer case, and I can't find it in the RockAuto catalog.

Help? Thanks!


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post #2 of 8 Old 01-15-2019, 09:57 PM
ZeeJay1997
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If you have no other symptoms than the speedo jumping, it's probably not the sensor. I could be wrong because IDK that much about <96.


I've seen several cases where the IP was at fault.


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post #3 of 8 Old 01-16-2019, 07:06 AM
Some_Anchovies
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Quote:
Originally Posted by JaimeZX View Post
Just started the other day. There are a couple of threads on this, but none seem to list the part number for the speed sensor in the transfer case, and I can't find it in the RockAuto catalog.

Help? Thanks!
If that's all your looking for on Rock Auto look under Electrical - Speed Sensor

95' ZJ 4.0L, 5-Speed Swap, NP231, 275k
Ex-Master Mechanic-turned-Software Developer
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post #4 of 8 Old 01-16-2019, 11:33 AM Thread Starter
JaimeZX
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Quote:
Originally Posted by ZeeJay1997 View Post
If you have no other symptoms than the speedo jumping, it's probably not the sensor. I could be wrong because IDK that much about <96.
I've seen several cases where the IP was at fault.
Yes... the speedo is the only symptom. Cruise control is fine, which DID make me wonder, but different cars work differently. In my E39 the speedo input is from one sensor but another one controls the cruise, and another one feeds the odometer... etc. There is one youtube video of a guy with identical speedo symptoms to mine who fixed it with the sensor.

Quote:
Originally Posted by Some_Anchovies View Post
If that's all your looking for on Rock Auto look under Electrical - Speed Sensor
Amazing! I DID look at that but all I saw were the transmission output shaft speed sensors, so I thought I was looking in the wrong place. Now a closer look shows both parts are in the same list. Thanks!

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post #5 of 8 Old 01-16-2019, 03:28 PM
zjosh93
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FWIW there is a known issue with the circuit board that drives the speedometer. Just google it. There are videos on YouTube showing which one it is. Not a difficult job to do. On my 93 the speedo jumps if you are over 50-70 MPH. I installed new MOPAR sensors, same issue. Some people have had a bad alternator cause the speedo issues. Mine went bad and I replaced it but no luck on the speedo. I have a junkyard circuit board but I haven't swapped it yet.

Also note that the speedometer sensor comes with a new plug and pigtail. IMS there was a running change in the plug so the early ZJs will need to wire in the new plug. It's an easy swap though.
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post #6 of 8 Old 01-18-2019, 02:08 AM
Some_Anchovies
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Typically what goes bad on both the tach and speedo circuit boards are the capacitors. capacitors only last so long before they reach the end of their usable life (The electrolytic ones). Replacing them fixes their issues right up.

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post #7 of 8 Old 01-19-2019, 01:45 PM
1996maroonzjf
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I had the same issue, speedo jumping around. Swapped in a spare I had from a donor transfer case I had and that one worked for a few months until it started to do the same thing over 50 mph when it was really cold out. I lived with it for a while until the speedo started pegging out over 120mph and the engine cut out a few times. I verified with my scanner and that was also showing a jumping mph.

Picked up a new Mopar sensor from the dealer and reluctantly spliced in the new connector as zjosh mentions above. Haven’t had an issue since. Using a scanner with a vehicle speed readout is a good way to pinpoint if it’s a problem with the instrument cluster or the sensor itself.

-Tim
(2) '96 4.0s
#1-'10-'18
#2- HP30/Ford 8.8 - 4.56, OME HDs, 5100's, IRO Adj Arms/Track Bars
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post #8 of 8 Old 02-02-2019, 09:39 AM Thread Starter
JaimeZX
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Quote:
Originally Posted by 1996maroonzjf View Post
I had the same issue, speedo jumping around. Swapped in a spare I had from a donor transfer case I had and that one worked for a few months until it started to do the same thing over 50 mph when it was really cold out. I lived with it for a while until the speedo started pegging out over 120mph and the engine cut out a few times. I verified with my scanner and that was also showing a jumping mph.

Picked up a new Mopar sensor from the dealer and reluctantly spliced in the new connector as zjosh mentions above. Havenít had an issue since. Using a scanner with a vehicle speed readout is a good way to pinpoint if itís a problem with the instrument cluster or the sensor itself.
Thanks, Tim. You do have a '96 though. My '95 doesn't have OBD II.

The symptoms are definitely worse when it's cold. After the vehicle is warmed up it improves, which does make me think it's electrical. I'll replace the sensor and if that doesn't help I'll move on to the cluster.

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