2001 HZJ78 Speedometer issue

[DetectiveFat]

Hello everyone,

I am at my wits end with my HZJ78 speedometer. After a longer trip i went to wash the underside of my car as usual, i used a rotary sprinkler that i dragged under the car a few times. After this procedure my speedometer stopped functioning. After turning on the ignition it moves maybe a millimeter up (unusual as my HZJ74 speedo only moves when the car is in motion) and then just stays there until i turn the car off. My first guess naturally was the sensor, it appeared damaged with the metal cracked and overall in bad shape. So to check i decided to swap in the speedo from my HZJ74 as it is the same part. This did not fix the issue as the speedo still didn't move. After putting the speedo back into the HZJ74 where it worked fine previously, it now also does not function, but there is no slight lift of the needle when i turn on the ignition.

My best guess would be some issue with the wiring harness in the HZJ78 which is somehow frying the speedo sensor, i have no idea how to even approach this as electrics are my weak spot.

Has anyone ever encountered this?

Best regards
Jan

 

[EngineBlockEggs]

Hi Jan. My bet would be that you messed up the speedometer cable on the underside of your truck when you washed it. I've only dealt with this on an LJ78 but that cable is pretty finicky. I would check the connectors and see if that's the problem. I'm sorry if you already checked the cable and I misunderstood your comment.

 

[JuanJ]

Hello,

As you already know, the speedometer cable of a 78 Series connects to a pickup sensor in the transmission. It may be possible that your sprinkler loosened a contact. It is also possible that a water jet damaged the sensor.

You can try cleaning the contacts and the sensor. If that does not work, you may have to replace it.





Juan

 

[DetectiveFat]

Hi Jan. My bet would be that you messed up the speedometer cable on the underside of your truck when you washed it. I've only dealt with this on an LJ78 but that cable is pretty finicky. I would check the connectors and see if that's the problem. I'm sorry if you already checked the cable and I misunderstood your comment.

Thank you for the response, i checked the wiring harness and cable and it does not appear damaged visually, the wires when the ignition is on supply some power, with 1 being 10.6V 2 being 0 and wire 3 being 12.6V. This appears on both cars, the HZJ74 where the speedo worked and the HZJ78 which i think broke somehow two sensors.

Hello,

As you already know, the speedometer cable of a 78 Series connects to a pickup sensor in the transmission. It may be possible that your sprinkler loosened a contact. It is also possible that a water jet damaged the sensor.

You can try cleaning the contacts and the sensor. If that does not work, you may have to replace it.





Juan

Thank you Juan for the response

I connected and reconnected the connectors a few times, and they appear quite clean. I have another sensor on the way but it does not explain how the HZJ78 keeps breaking the sensors, as it appears to have broken the sensor from the HZJ74.

 

[JuanJ]

Thank you for the response, i checked the wiring harness and cable and it does not appear damaged visually, the wires when the ignition is on supply some power, with 1 being 10.6V 2 being 0 and wire 3 being 12.6V. This appears on both cars, the HZJ74 where the speedo worked and the HZJ78 which i think broke somehow two sensors.



Thank you Juan for the response

I connected and reconnected the connectors a few times, and they appear quite clean. I have another sensor on the way but it does not explain how the HZJ78 keeps breaking the sensors, as it appears to have broken the sensor from the HZJ74.

Hello,

Maybe it is time to check crossmember bolts and their torque specs. Apparently, something is loose enough to rattle the sensor into breaking. Transmission bushings may need some attention as well.





Juan

 

[DetectiveFat]

I am writing this to sort of close the thread as the issue has been solved. As i thought in the beginning the only issue was the sensor in the 78. Today the replacement arrived from Amayama ( very long wait as i ordered the parts on 21/08/2026), and it works perfectly.

All my worries and extra work was caused by my impatience and lack of care as during the swap of the sensors between the two cars i misplaced the key/pin between the gear and the sensor itself causing me to believe that now both sensors were damaged. This was not the case at all.

Thank you for the responses, i most likely will be back