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si2yz

speedometer not showing speed

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Hi all

 

I need help identifying a component on this PCB. It's a zener diode (ZD4 next to the connector on the left) that I think is used to regulate voltage to the speed sensor. It has gone short circuit. Does anybody know the zener voltage?

 

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I think speed sensors are all 5 volts on older models... what're ya working on?

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It's a 2000 Honda Fortrax 350. I'm in the UK, don't know what the US equivalent is.

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Yea unless someone else took theirs off to measure, youd pretty much have to pick up another to measure it yourself.  Would be hard to find those specs online IMO, but i could be wrong.

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@AKATV would be your best bet, since he rebuilds meters all the time.  

 

Here in the states it would be a 350 Rancher.

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12 hours ago, si2yz said:

It's a 2000 Honda Fortrax 350. I'm in the UK, don't know what the US equivalent is.

 

Same ATV over here... your shorted zener replacement should be 5.0 volts to drive the speed sensor. Have fun!

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On 9/26/2022 at 10:41 PM, retro said:

 

Same ATV over here... your shorted zener replacement should be 5.0 volts to drive the speed sensor. Have fun!

Thanks retro. Do you know what type of output the sensor has? 

 

I've been looking at the 2000 - 2003 Trx350 Rancher service manual and it says to check the loom connector for battery voltage. Don't know if that's the best manual to look at.

 

The speedometer puts ~5 volts on the signal wire, is that the voltage your talking about?

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I messed up @si2yz, the display meter is a dumb client on an early 350 Rancher, it supplies nothing. I was thinking that #ZD4 zener diode on your PCB was handling the supply voltage for the speed sensor. That's wrong though, cause the meter just reads the signal voltage pulses from the speed sensor, processes (counts) that signal, then displays the speed.

 

The speed sensor output is pulsed 0 to +5.0 volts. The ECM and the display meter count those +5.0 volt pulses that the sensor produces. So in order for #ZD4 zener diode to sense the pulsed 5 volt signal voltage that is produced by the speed sensor, the zener voltage would have to be less than 5.0 volts for the circuit to function reliably. So my best guess is that the zener voltage of ZD4 is probably between 4.65-4.80 volts.

 

@AKATV , do you by chance have a junk PCB lying around with ZD4 on it, that you can desolder and measure to find the zener voltage?

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Here are the speed sensor tests in case you need them. You can omit the test adapter if you don't have one.... just pierce the Pink/Green wire while the connector is plugged in to measure the 5 volt pulse with your multimeter. Reseal the pierced wire with silicone or liquid tape after the test.

 

speed-sensor-meter.png

 

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Here is a video explaining how to find the zener voltage of a zener diode.

 

 

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13 hours ago, retro said:

I messed up @si2yz, the display meter is a dumb client on an early 350 Rancher, it supplies nothing. I was thinking that #ZD4 zener diode on your PCB was handling the supply voltage for the speed sensor. That's wrong though, cause the meter just reads the signal voltage pulses from the speed sensor, processes (counts) that signal, then displays the speed.

 

The speed sensor output is pulsed 0 to +5.0 volts. The ECM and the display meter count those +5.0 volt pulses that the sensor produces. So in order for #ZD4 zener diode to sense the pulsed 5 volt signal voltage that is produced by the speed sensor, the zener voltage would have to be less than 5.0 volts for the circuit to function reliably. So my best guess is that the zener voltage of ZD4 is probably between 4.65-4.80 volts.

 

@AKATV , do you by chance have a junk PCB lying around with ZD4 on it, that you can desolder and measure to find the zener voltage?

Sorry I am late to the party on this one-

retro-your explanation is spot on on for the speed sensor signal

I do have a junk board Zener I could measure but unfortunately, I am without a variable power supply at the moment

I did this years back when i was trying to find parts locally and got 4.63-4.78 ( notebook) across a few that I had

These are relatively easy to source off of scrap circuit boards and very cheap to purchase online as well

I see these zeners fail when speed sensor itself fails or improper testing/shorting out the harness as well as water damages

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Thanks @AKATV!!! So my zener voltage guess was spot on, now that's a rare event! :)

 

@si2yz , let us know how your ZD4 zener diode replacement goes. It's great that you are willing to repair your PCB yourself.

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24 minutes ago, retro said:

Thanks @AKATV!!! So my zener voltage guess was spot on, now that's a rare event! :)

 

@si2yz , let us know how your ZD4 zener diode replacement goes. It's great that you are willing to repair your PCB yourself.

Your very welcome!

PS - I always liked the test adapter diagram on page 20-10- its one of the many confusing jewels to be found in the manual......

If you havent zoomed in on it, take a look.....???

 

@si2yz let us know how it goes with your repair

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Thanks retro and AKATV, that will be very helpful. I'll let you know how I get on

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