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Jay, Nissan Technician
Category: Nissan
Satisfied Customers: 18553
Experience:  20+yrs experience with Nissan & Infiniti Trained & Certified.Currently still working for Dealer
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How can I test my speedo head to make sure it is good? I

Customer Question

How can I test my speedo head to make sure it is good? I have what appears to be good AC voltage coming from the sensor to the instrument panel .5 to over 2 volts depending on speed. Symptom is speedo reads low most of the time. Anywhere from 10 - 40 kmh when I can going over 90 kmh. Some times it fluctuates rapidly bewteen 80- 100 kmh if say I am going 100, or 90-110 if I am going 110, etc. After a while it will eventually start working properly, and then my cruise control will work. There is no check engine light which would tend to indicate the sensor. This is on an 95 4x4 pathfinder SE M/T VG30E engine.
I would like to be able to A) test the speedo meter itself, as I have the circuit board off the back of the meter, as well as B) test with the circuit board on and using the inputs at the panel connectors, or where the screws hold the meter to that panel.
Submitted: 3 months ago.
Category: Nissan
Expert:  Jay replied 3 months ago.

Hello my name is ***** ***** thank you for your question.

This speedometer system is very basic and easy to figure out. This is how easy it is for your issue.

If you speedometer did not work at all, that is when it is the trickiest. But if you remove the sensor and the gear is not stripped and the gear can produce ac current and the speedometer works at slower speeds, just not higher speeds. That is a speedometer issue. The speed sensor either works or it doesn't on these trucks. So the fact that your speedometer is working just not the right speeds, that verifies the speedometer it's self is your issue.

Do you have any more questions on this?

Customer: replied 3 months ago.
The speedometer works at any speed when it is working properly. Usually it doesn't start working I have been driving for a significant amount of time. Sometimes starts working within in about 15 mins, sometimes takes hours. Usually by then I have been driving for quite some time. To me the problem is a loose connection. If either the sensor or the speedo was bad it would never work correctly at any time and that is not the case. What I need to be able to do is test different parts of the circuitry. Either the signal is not getting to the instrument panel properly, or there is a problem from the instrument panel to the speedometer. Research on the web indicates that there are lots of problems with cracks forming where the screws attach the speedo to the panel. What I am looking for is a way to test the speedo out of the vehicle, or by injecting a known good signal at different parts of the wiring in the vehicle so I can determine where the bad connection/wiring is. should I inject a sine wave, a square wave? One that goes from + .5v to -.5v or from 0 to +.5v. Is there something I can buy somewhere for testing this, or do I need to build something myself.John
Expert:  Jay replied 3 months ago.

I will have blow some dust off the manuals to see what I have for that year for you. But I can say with 100% certain, that if the speedometer operation is erratic, it's a speedometer. If it was a speed sensor it would be all the time.

Expert:  Jay replied 3 months ago.

I will be online later this evening around 7 pm EST and I will look up that info then for you...ok?

Customer: replied 3 months ago.
Thanks Jay.What are the chances it is a bad connection. A lot of people with the same problem have replaced both sensor and speedo, and still end up with the same problem.
Expert:  Jay replied 3 months ago.

The only connection would be the 4 philip head screws that mount the speedometer assembly to the meter assembly. If they are all tight, it's a speedometer.

I have been a Nissan Tech since 1990. I dealt with these trucks plenty back in the day to say.

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