Speedometer diode identification
07-03-15, 02:05 PM
#1
Speedometer diode identification
My gauge cluster is out of the car due to the common blank odometer screen and intermittent tach & speedo issues. One of the problems was a blown diode in the "ZD5" location on the speedometer board. The images below show the location on my odometer board.
The diode is damaged enough that any identifying markings are not legible. Does anyone know the specs of this diode? Or does anyone have a working gauge cluster out of the car? I'd be interested to know if there were any identifying markings on the diode.
I found this thread with a discussion about the same problem. User "mr2dude9161" posted some diode part numbers, but he wasn't able to fix the speedo issues. I think this might be because the part numbers he used are normal switch type diodes and the originals are zener diodes. I just don't know the specs for this one.
https://www.rx7club.com/3rd-generati...-my-fd-950521/
For reference, here are some other related threads with helpful pictures of the speedometer board:
https://www.rx7club.com/3rd-generati...istor-1027726/
https://www.rx7club.com/3rd-generati...ometer-599220/
https://www.rx7club.com/3rd-generati...arched-719726/
https://www.rx7club.com/3rd-generati...oblems-651138/
The diode is damaged enough that any identifying markings are not legible. Does anyone know the specs of this diode? Or does anyone have a working gauge cluster out of the car? I'd be interested to know if there were any identifying markings on the diode.
I found this thread with a discussion about the same problem. User "mr2dude9161" posted some diode part numbers, but he wasn't able to fix the speedo issues. I think this might be because the part numbers he used are normal switch type diodes and the originals are zener diodes. I just don't know the specs for this one.
https://www.rx7club.com/3rd-generati...-my-fd-950521/
For reference, here are some other related threads with helpful pictures of the speedometer board:
https://www.rx7club.com/3rd-generati...istor-1027726/
https://www.rx7club.com/3rd-generati...ometer-599220/
https://www.rx7club.com/3rd-generati...arched-719726/
https://www.rx7club.com/3rd-generati...oblems-651138/
07-05-15, 12:55 PM
#2
Recovering Miataholic
I'll make a wild guess that all those zeners are rated at the same voltage.
If the anodes of the zeners are all connected to chassis ground, they are shunt regulators and could be tested as follows... first verify that they are connected that way, then:
You could try applying 12V to the input power terminal of the board and measuring all the "good" zeners with a high-impedance voltmeter. Positive lead of meter on the cathode of each zener and negative to chassis ground. If a zener has gone bad, it is likely that the resistor between the zener cathode and the +12 volt source was either shorted to the +12 volt source or else the resistor itself had shorted. Normally one would run about 20 - 30 mA through the zener, but if that resistor is shorted, power dissipation in the zener would rise to several watts... enough to burn it out.
If the anodes of the zeners are all connected to chassis ground, they are shunt regulators and could be tested as follows... first verify that they are connected that way, then:
You could try applying 12V to the input power terminal of the board and measuring all the "good" zeners with a high-impedance voltmeter. Positive lead of meter on the cathode of each zener and negative to chassis ground. If a zener has gone bad, it is likely that the resistor between the zener cathode and the +12 volt source was either shorted to the +12 volt source or else the resistor itself had shorted. Normally one would run about 20 - 30 mA through the zener, but if that resistor is shorted, power dissipation in the zener would rise to several watts... enough to burn it out.
Last edited by wstrohm; 07-05-15 at 12:57 PM.
