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Please help me to understand how to use a zener diode

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FAVahalik

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I am attempting to understand how a zener diode would work in the following application:

This is for a Suzuki Hayabusa motorcycle. It has a speed restriction that will not allow the bike to exceed 10,200 rpm in 6th gear. Gears 1-5 redline at 10,800 rpm. This limits the bike to a speed of 186 mph. Without the restriction, the top speed would be 192-194 mph.

There is a wire that runs from the gear position sensing unit (GPS) that passes the following voltages to the ECU:

Gear 1 - 1.8 v
Gear 2 - 2.26 v
Gear 3 - 2.99 v
Gear 4 - 3.68 v
Gear 5 - 4.38 v
Gear 6 - 4.7 v

An author on another forum suggests placing a zener diode of 4.3 volts @ 250 mw from the wire that runs to the ECU to ground. If I understand correctly, the diode will let voltages up to but not exceeding 4.3 volts through to the ECU. The voltage over 4.3 volts will be sent to ground so the ECU sees gears 1-5 normally. If I understand correctly, when the GPS sends 4.7 volts to the ECU, only 4.3 volts will be received by the ECU and the rest will be sent to ground. Thus, the ECU will see gear 6 as gear 5, thus removing the speed limitation. It is important that the ECU see gears 1-5 at the correct voltages, because different ignition maps are also involved in the selection. Gears 5 and 6 are identical except for the rpm limitation.

My question(s) are: Am I understanding how the zener diode will work? Since I am unsure of how much current is involved, is this dangerous in the sense that the .4 volts being sent to ground might constitute a short, or will the diode not pass that much current? Since the bike uses a battery similar to a car battery, a direct short would be very bad. I suspect the current is already limited, since the voltages being passed are way below the 12V battery circuit. Do I understand correctly that if the diode were connected inline that it would pass only .4 volts (when the voltage rises to 4.7 volts)?

I would appreciate any help someone would care to give. I am very much a novice when it comes to electronics, so please forgive questions that might seem obvious to you guys.
 
well first off, I feel a little uneasy helping someone get their bike to go almost 200MPH... just promise not to go getting put in jail or killed...

the diode would be connected with the cathode to the output wire, and the anode to ground... since when it breaks down at 4.3v, it will allow current to flow from the cathode to the anode (reverse, aka zener breakdown)

chances are, the voltage coming out of the gearbox would be rather weak, from a small sensor or something, and as a result it wouldn't be capable of much output current, so it shouldn't damage the zener, and hopefully not the sensor either. If it isn't so weak, you may have to put a resistor in series with the output, before the zener diode.

If you want to be cautious, you can just start with larger values of series resistor (100 ohms or so should be reasonable) and see what the voltage output is then. as long as the ECU isn't drawing any significant current from that line (which I wouldn't imagine it would) then it should be pretty much stock voltages for gears 1-5 and then be clamped to around 4.3v when the zener kicks in.
 
Personally I think it sounds rather bizarre (not to mention pointless?), limiting it to 186 mph?.

Are you sure it's not just that the engine won't reach top revs in top gear?, as is common on many cars and bikes.
 
Nigel Now that is a funny reply i nearly choked on my tea. However i agree
 
Thanks for the replies guys. The 186 mph restriction was a gentlemen's agreement between Kawasaki and Suzuki in 2002 to limit the top speed to the speed limit of the Autobahn.

Speeds like these are usually run at places like the salt flats or in my case The Texas Mile which is a 2 mile abandoned but kept up runway. There are more expensive de-limiter mods that involve pulling the GPS and removing a resistor from the 6th gear sensor and jumping it to the 5th gear sensor. This diode mod would be very easy, since the wire is readily accessible by propping up the gas tank.

I just wanted someone to double check my intepretation of this particular fix. Thanks again.
 
FAVahalik said:
Thanks for the replies guys. The 186 mph restriction was a gentlemen's agreement between Kawasaki and Suzuki in 2002 to limit the top speed to the speed limit of the Autobahn.

I was under the impression that there are various sections of the Autobahn that don't have any speed restrictions?, most parts have sensible limits - but I doubt there's any with a 186mph limit?.
 
Hi,

This particular hack is well known:

https://turbododge.com/forums/printthread.php?t=14937&pp=40

https://www.homemadeturbo.com/forum...18f4f1e3f53bbe79638ade9&topic=28198.msg290316

Although i doubt the reasons for the limiting are quite as given.
It is not considered to give proper engine management above the
zener diode capping point.

Your informant was quite correct, it is an acceptable hack and seems to
work quite well.
As to your understanding of the zener diode action,
you could consider that the increase above the zener voltage,
is kind of 'bled' off to ground.

Best of luck, and mind how you go.
John :)
 
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