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#113994 - 04/14/07 8:52 am T140 wiring problem - blown fuse and zener diode  
Joined: Apr 2007
Posts: 17
David Dickson Offline
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David Dickson  Offline
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Joined: Apr 2007
Posts: 17
Melbourne, Australia
My T140 stopped on the freeway last week - the fuse had blown. When I put in a replacement fuse it blew too, even though the ignition switch was turned off. It appears there is a short circuit.

With the positive terminal of the battery disconnected, a test light connected to the negative terminal comes on if i touch it anywhere on the frame or engine even with the ignition switch OFF.
If I disconnect the lead to the zenor diode the test light does not turn on. Does this prove the zenor diode is the source of the short circuit?

What is a good way of testing the wiring for a short?
Thanks for your help
Dave Dickson

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#113995 - 04/14/07 1:29 pm Re: T140 wiring problem - blown fuse and zener diode  
Joined: Aug 2005
Posts: 1,211
Steve in Tulsa Offline
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Steve in Tulsa  Offline
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Posts: 1,211
Tulsa, Oklahoma, USA
I have had this happen to me as well. And it happened on the freeway eek
Disconnect the zener, connect battery and try it.
When a zener blows it is a direct short to ground
HTH


Steve in Tulsa
#113996 - 04/14/07 1:31 pm Re: T140 wiring problem - blown fuse and zener diode  
Joined: Mar 2005
Posts: 9,970
John Healy Online content
John Healy  Online Content



Joined: Mar 2005
Posts: 9,970
Boston, Massachusetts
Remove our fuse. Place your test light in series with the fuse connections. Remove wires until the test light oges out. You have found your short.

Or, if lacking a test light have some one hold a small bulb to the fuse connections so it lights up. One connector goes to the base and the other to the center connection. Remove wires from things until it goes out.


#113997 - 04/14/07 2:19 pm Re: T140 wiring problem - blown fuse and zener diode  
Joined: Oct 2003
Posts: 11,110
RF Whatley Online content
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RF Whatley  Online Content
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North Georgia, USA
Dave –
Steve is correct. A bad Zener is one of the few ways you can blow a fuse without doing anything to the ignition switch.

If you suspect the Zener (which would be a good assumption), then testing is quick and easy. Simply slide the 3/8 wide connector off the Zener’s big terminal and use an Ohm meter to measure between that terminal and the base of the unit. Then reverse the probes and measure again. The Zener is nothing but a fancy diode. Diodes by their very nature conduct DC electricity in only ONE direction. If your battery-operated (ie DC) Ohmmeter gets a reading in both directions, then the unit is by definition no longer a "diode". Then by conclusion one can safely assume that along with letting the "one-way traffic cop" escape, the "Zen" has also packed its bags. In layman's terms, "Elvis has left the building". laugh

Haven't got an Ohmmeter? Not a prob. Any DC circuit can test this diode (and a host of other circuit problems too). So make a plain test lead with an alligator clip on both ends out of 24" of wire. Make a second wire exactly as the first, except this time place some sort of 12V lamp (gauge, tail or parking bulb) in the middle of the test lead. Connect one end of each lead to the bike's battery (system fuse removed). When you touch the 2 loose ends together the lamp should light. Then do as explained before, place the 2 clips... 1 to the base and 1 to the 3/8" terminal... then reverse the clips.

If the Ohmmeter does not indicate or test lamp does not illuminate either way, then the Zener has failed to an "open" state. If the meter indicates or lamp illuminates both ways then your Zener has failed to a "short circuit" state. The later would certainly blow a fuse. Again, a proper Zener should pass electricity one way, but not the other.

BTW #1
However there are other ways to blow the fuse and these should also get a good looking over. The most common by far this that your Jap replacement battery is about 1/2 taller than the original equipment. When you drink a lot of beer and put on the pounds, the bottom of the seat can touch the battery posts and also blow the fuse. This is a great reason NOT to pick up fat chicks. bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#113998 - 04/15/07 4:33 am Re: T140 wiring problem - blown fuse and zener diode  
Joined: Jun 2002
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Stuart Online content
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Stuart  Online Content
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Scotland
Hi Dave,

Quote:
Originally posted by RF Whatley:
The Zener is nothing but a fancy diode. Diodes by their very nature conduct DC electricity in only ONE direction.
Errrm, koff ... Richard is sort-of right, in that the Zener is a diode (the full name 'Zener diode' is a bit of a giveaway wink ). However, what he's forgotten is that the fancy thing about a Zener is a fully-working one doesn't conduct in *either* direction until a certain input voltage is reached - that's how it dumps electricity generated by the alternator but not required by the rest of the bike.

Thus, one has to be careful interpreting the results of the test he details:-

1. If the Zener apparently doesn't conduct in either direction, it does *not* indicate the Zener has failed, it *might* be OK. Otoh, it may be that the miniscule power of the meter's battery isn't sufficient to make it conduct the 'wrong' way but, when it's connected to the power of the bike's battery, it does.

2. If the Zener does conduct even one way, I think (gimme a break, I'm typing this at 5am) this indicates it's duff.

Quote:
Originally posted by RF Whatley:
BTW #1
However there are other ways to blow the fuse and these should also get a good looking over.
Agree. Don't get too hung-up on the Zener. At best, disconnect it (but tape over the 3/8" loom connection - it's live all the time) while you check the rest of the bike; it's even ok to run the engine for a short while at low revs. with the Zener disconnected. Only if everything else's ok but reconnecting the Zener blows the fuse immediately can you be sure it's the Zener.

Quote:
Originally posted by RF Whatley:
If you run with your headlamp OFF, then the Zener has double the work to do.
2) always ride day and night with your headlamp full ON... especially on the highway.
I've *never* ridden with a headlight on in good daylight (GB doesn't have a lights-on law) - as a friend of mine said many years ago, "Gives the b@stards something to aim at". In 30 years of Triumph ownership, I've never had a Zener blow (though I accept it isn't unknown wink ) and my T160's run the high-power version of the three-phase RM24 alternator.

Moreover, genuine Lucas Zeners are rated substantially higher than even the original maximum output of the usual RM21 single-phase alternator.

One thing that shouldn't be forgotten is a hot Zener is a dead Zener - Zeners work by turning electricity into heat, and the only way they can dissipate that heat is through the sink they're bolted to. However, the oxidisation, both rust and aluminium, that inevitably builds up between an untouched Zener and its heatsink, is not a good conductor of heat.

Hth.

Regards,

#113999 - 04/15/07 4:36 am Re: T140 wiring problem - blown fuse and zener diode  
Joined: Jun 2002
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Stuart Online content
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Stuart  Online Content
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Scotland
Hi Dave,

Quote:
Originally posted by RF Whatley:
The Zener is nothing but a fancy diode. Diodes by their very nature conduct DC electricity in only ONE direction.
Errrm, koff ... Richard is sort-of right, in that the Zener is a diode (the full name 'Zener diode' is a bit of a giveaway wink ). However, what he appears to have forgotten is that the 'fancy' thing about a Zener is a fully-working one doesn't conduct in *either* direction until a certain input voltage is reached - that's how it dumps electricity generated by the alternator but not required by the rest of the bike.

Thus, one has to be careful interpreting the results of the test he details:-

1. If the Zener apparently doesn't conduct in either direction, it does *not* indicate necessarily that the Zener has failed; it's actually more likely to be OK. However, it *might* be that the miniscule power of the meter's battery isn't sufficient to make it conduct the 'wrong' way but, when it's connected to the power of the bike's battery, it does.

2. If the Zener does conduct the aforementioned miniscule power of the meter's battery even one way, I think (gimme a break, I'm typing this at 5am) this indicates it's duff.

Quote:
Originally posted by RF Whatley:
BTW #1
However there are other ways to blow the fuse and these should also get a good looking over.
Agree. Don't get too hung-up on the Zener. At best, disconnect it (but tape over the 3/8" loom connection - it's live all the time) while you check the rest of the bike; it's even ok to run the engine for a short while at low revs. with the Zener disconnected. Only if everything else's ok but reconnecting the Zener blows the fuse immediately can you be sure it's the Zener.

Quote:
Originally posted by RF Whatley:
If you run with your headlamp OFF, then the Zener has double the work to do.
2) always ride day and night with your headlamp full ON... especially on the highway.
I've *never* ridden with a headlight on in good daylight (GB doesn't have a lights-on law) - as a friend of mine said many years ago, "Gives the b@stards something to aim at". In 30 years of Triumph ownership, I've never had a Zener blow (though I accept it isn't unknown wink ) and my T160's run the high-power version of the three-phase RM24 alternator.

Moreover, genuine Lucas Zeners are rated substantially higher than even the original maximum output of the usual RM21 single-phase alternator.

One thing that shouldn't be forgotten is a hot Zener is a dead Zener - Zeners work by turning electricity into heat, and the only way they can dissipate that heat is through the sink they're bolted to. However, the oxidisation, both rust and aluminium, that inevitably builds up between an untouched Zener and its heatsink, is not a good conductor of heat.

Hth.

Regards,

#114000 - 04/15/07 5:03 pm Re: T140 wiring problem - blown fuse and zener diode  
Joined: Jan 2007
Posts: 21
cgmze Offline
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cgmze  Offline
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Saskatoon
I tried this a minite ago and something seemed to go wrong??

You can test a zener with a ohm meter it will look just like a rectifer until you reach the zener voltage (reverse bias). This page will reveal all.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/zener.html
Gary


1971 T120R matching numbers
#114001 - 04/15/07 6:28 pm Re: T140 wiring problem - blown fuse and zener diode  
Joined: Oct 2003
Posts: 11,110
RF Whatley Online content
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RF Whatley  Online Content
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Joined: Oct 2003
Posts: 11,110
North Georgia, USA
Quote:
Originally posted by Stuart:
...Richard is sort-of right, in that the Zener is a diode. However, what he appears to have forgotten is that the 'fancy' thing about a Zener is a fully-working one doesn't conduct in *either* direction until a certain input voltage is reached
David -
Stuart is quite right; I am in error. My test and results are quite valid, but you must be testing with a voltage higher than ~15V to get correct results. So any tester (such as a handheld meter) that uses a lower voltage would not give you the full truth. Suggest using a simple 12V light bulb and battery charger (these normally operate in the 14 to 20V range).


Quote:
Originally posted by RF Whatley:
If you run with your headlamp OFF, then the Zener has double the work to do. Always ride day and night with your headlamp full ON... especially on the highway.
then...

Quote:
Originally posted by Stuart:
One thing that shouldn't be forgotten is a hot Zener is a dead Zener - Zeners work by turning electricity into heat, and the only way they can dissipate that heat is through the sink they're bolted to. However, the oxidization, both rust and aluminum, that inevitably builds up between an untouched Zener and its heatsink, is not a good conductor of heat.
Important here to realize that Stuart has veered away from strict fact. While I fully agree that the object is to keep the Zener as cool as possible, it must be remembered that there are 2 ways to achieve this goal....

1) Make it easy for the diode to EFFICIENTLY rid itself of heat buildup, as he has outlined so beautifully above, and...

2) Burn up the excess power by turning ON the lights so that the Zener doesn't have to work so hard to begin with. In other words PREVENTION.

Of course, the prudent owner knowing the replacement price of a Lucas Zener will take both these steps. bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#114002 - 04/19/07 9:51 am Re: T140 wiring problem - blown fuse and zener diode  
Joined: Apr 2007
Posts: 17
David Dickson Offline
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David Dickson  Offline
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Joined: Apr 2007
Posts: 17
Melbourne, Australia
The zener diode will be replaced. This machine (a 1978 T140V) has had a Sudco Mikuni kit fitted by a previous owner — the standard alloy air cleaner box has been thrown away and the zener diode was fitted to the slotted tag that comes out of the vertical section of the frame backbone about level with the right hand carb. Will this be adequate as a heat sink?
PS Thanks for the tip about riding with the lights on all the time to reduce the load on the zener diode.

#114003 - 04/19/07 11:05 am Re: T140 wiring problem - blown fuse and zener diode  
Joined: Oct 2003
Posts: 11,110
RF Whatley Online content
BritBike Forum member
RF Whatley  Online Content
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Posts: 11,110
North Georgia, USA
David -
In a word, "No".

I believe the workshop manual suggests 8 sq in of surface area minimum for the Zener. As was said before, the Zener converts excess electrical energy into heat and you MUST get rid of the heat or the unit itself will have a very short life.

There are several options for you...

- Buy a used finned heat sink from a 1968-70 BSA or Triumph

- Use a flat aluminum plate wrapped around the battery box, such as used on 1966 and 67 models

- Convert an existing steel plate (such as the coil mounting plate on an OIF) to a thicker aluminum plate and mount the Zener there

- Fit a Podtronics or Tynpanium which have their own built-in heat sink. These can be easily and elegantly mounted to the bottom of the battery box


The sink does not have to be finned, in the direct air flow, or even in front of the engine for that matter. (Consider the stock placement on the inner breather box, if you have any doubts.) But it should be aluminum, have a flat prepared seating area for the Zener, and have a ground wire attached to it.

Hope this helps! bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#605995 - 06/24/15 12:53 pm Re: T140 wiring problem - blown fuse and zener diode [Re: RF Whatley]  
Joined: Jan 2015
Posts: 93
steve-d Online happy
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steve-d  Online Happy
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Pennsyltucky, USA
I apparently blew my Zener. Bike totally stopped working and blown fuse. I had recently replaced the aluminum boat anchor air box for K&N filters and retro side panels. As the Zener was attached to the air box, I fitted an aluminum plate as a heat sink and mounted the plate to the tab on the frame oil tube.

The Zener did not, however, have a separate ground wire going from the Zener's mounting nut to the battery. The Zener was just bolted to the air box. Possibly these details changed over time at the factory. Or possibly it should be incorporated whenever observing there isn't a grounding wire. A belt and suspenders approach. Steve


73 T140V
#606054 - 06/25/15 12:48 am Re: T140 wiring problem - blown fuse and zener diode [Re: steve-d]  
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Posts: 9,301
Stuart Online content
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Stuart  Online Content
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Scotland
Hi Steve,

Originally Posted By steve-d
The Zener did not, however, have a separate ground wire going from the Zener's mounting nut to the battery. The Zener was just bolted to the air box. Possibly these details changed over time at the factory.

Lucas appears to have spent at least the 1960's gradually persuading BSA to pay for more and more return wires in the harness; certainly the 'dry frame' Zener mounting has a Red return cable attached to the heatsink mounting bolt. So the oif lacking it would appear to be a retrograde step, particularly for such an important circuit; are you sure there wasn't a return wire attached elsewhere on the original air box, that served as the return from more than one component attached to it?

Originally Posted By steve-d
Or possibly it should be incorporated whenever observing there isn't a grounding wire.

Ime, definitely; if you're replacing the blown Zener with another, I'd attach a dedicated return wire from the mounting.

Originally Posted By steve-d
A belt and suspenders approach.

Not really. More a good-quality belt vs. a scrotty old piece of string.

Hth.

Regards,

#606062 - 06/25/15 3:07 am Re: T140 wiring problem - blown fuse and zener diode [Re: steve-d]  
Joined: Mar 2006
Posts: 3,469
L.A.B. Online content
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L.A.B.  Online Content
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Posts: 3,469
Norfolk, UK
Originally Posted By steve-d
The Zener did not, however, have a separate ground wire going from the Zener's mounting nut to the battery.


My '78 model T140V certainly has the Zener ground wire.


Moderated by  John Healy 


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