BN1 - Electrical gremlin...

[matt78]

Hello.

I'm back with a new question regarding electrics on my 100/4.
Everything seems to work properly, but the 35A fuse blows when driving, without any obvious cause - switching anything on, for example...
Therefore, in order to find the culprit and after a wiring diagram check, I decided to add a slightly smaller fuse on every 4 green wires protected by the 35A one. Unfortunately, when I look at the car, there are... FIVE green wires connected to the 35A !
Does anybody know where the fifth one is coming from ?
Additionnally, except plugging / unplugging each wire in turn, is there an easy way to identify their function ?

Thank you in advance for your advices.

Matt

 

[Keoke]

HI Matt:
I think the wire you are calling the 5th wire is the one that goes to the fuel gauge?
Maybe try the wire going to the wiper motor first. Probably one at a time is your best bet.

 

[Duane_Rhynard]

Hello.

I'm back with a new question regarding electrics on my 100/4.
Everything seems to work properly, but the 35A fuse blows when driving, without any obvious cause - switching anything on, for example...
Therefore, in order to find the culprit and after a wiring diagram check, I decided to add a slightly smaller fuse on every 4 green wires protected by the 35A one. Unfortunately, when I look at the car, there are... FIVE green wires connected to the 35A !
Does anybody know where the fifth one is coming from ?
Additionnally, except plugging / unplugging each wire in turn, is there an easy way to identify their function ?

Thank you in advance for your advices.

Matt

Matt,

According to my BN1/2 wiring diagram, the diagram shows the following connections:

A4 Connection (Green Wires) After 35A Fuse
1: Flasher
2: Trafficator and Stop Switch
3. Fuel Gauge / Wiper Motor / Heater Fan Motor
4. Gearbox switch / Overdrive Relays

A3 (White Wires) Before 35A Fuse
1. Coil SW Terminal
2. Petrol Pump

Thanks,
Duane

 

[vette]

Since you have 5 wires, maybe your car was rewired to later spec at one point. Here's what the 100/6s and 3000s show in their manual.
1 wipers
2 fuel gauge & tach combined
3 flasher
4 stop lamp switch
5 heater switch

Since you may be interested in rewiring to include a multi fuse fuse box I'll show you what I did. Attached is a pic of one of two fuse boxes in my Healey. This box is fed from the ignition switch and is supplying all the green wires that were on the 35 amp fuse. I call it the "ignition - on" fuse box. There is a second fuse box on the passengers side that feeds the devices that were originally supplied by the 50 amp fuse and I call that box the "always-on" fuse box. It supplies things such as the horns, Lucas driving lights, radio/sterio (if there was one) and two power plugs for I-Pad and Cell Phone and GPS. If you notice a coil wire marked in my 1st pic on the top of the fuse box, that is not fused as it is just the point where i picked up power from the ignition switch to the coil.
Ideally you would want to keep as much load off of the ignition switch as possible because that has proven to be kind of a weak link in the system given its age. But I made the decision that the things on this "ignition-on " fuse box are the things that I want to go off with the ignition switch. Note that your head lights are not fused. They are supplied by a brown/blue wire directly from the voltage regulator location and it is the same wire that goes on to feed your ignition switch.
Here are the devices that I fed from my "ignition - on" fuse box:
1 fuel pump
2 O.D.
3 heater motor
4 turn signals
5 wipers
6 fuel gauge & tach
7 brake lights

I don't have the fuse sizes I used with me now but generally I have mostly 3 or 5 amp fuses in use.
I believe I am using a 10 amp fuse on the O.D.

Hope this can help.

 

[matt78]

Thank you for your messages.

@ Duane : that's exactly what I thought, having also checked by myself the wiring diagram. You can understand how puzzled I was when I saw the 5 green wires under the bonnet...
@ Vette : yes, this is an interesting hypothesis that I didn't really thought about !

But I'm still in a fix, trying to identify where each wire goes... for example, on my car:
1) only ONE wire supplies the power to the indicators AND allow them to blink : according to the original BN1 wiring diagram, there should be another wire (one for switching the indicators, one for the flasher). Could this be a clue of a different wiring, as described by Vette ?
2) I managed to identify the wire supplying power to the wiper motor AND the fuel gauge, therefore at least this one would have been kept unchanged - original wiring, as described by Duane, and my own shop manual...

By the end of this week, I should be able to fit all the fuses (15 A ?), and try to label the three wires still unidentified.
Should you have any additionnal advice, please let me know !

 

[vette]

Matt, I don't think I would use 15amp fuses in all the circuits. Remember they were all under a 35 amp fuse before. I don't have my specifications with me but I know the largest fuse I have is a 10 amp and it is on the O.D.
I do remember that I had my wiper motor on a 5 amp fuse and it worked for a summer then one day blew. I only had a 7 amp available so that is what i stuck in it. I think my fuel pumps are on a 5 amp fuse. Heater blower motor is on a 5 amp, I think. Brake lights are on a 3 amp.
Attached is the schematic for an early 3000. I hope it is readable. Your turn signal lights may be wired as in this schematic. The green wire from the fuse goes to the flasher 'B' terminal and at this terminal power is extended by another wire that goes to the trafficator center terminal. When the trafficator is pushed one side or the other it flips the relay mounted on your left front inner fender apron. (although I am not familiar with a BN1, that location is where the relay would be on a BJ7).
On early 100/6s the trafficator is powered directly from the 35 amp fuse. and the flasher gets its power directly from the 35 amp fuse. If you have five wires at the 35 amp fuse I will guess that your flasher circuit is wired with the trafficator and the flasher both sources with separate wires from the 35 amp fuse.

 

[Michael Oritt]

Matt--

Short of using a test meter with really long leads to check continuity for each green wire you can get a small DC battery-powered electrical testing device that, when clipped onto one end of a wire, generates a tone that is detectable via a separate sensor which only needs to be held against the jacket of that wire.

When wires are bundled together it is necessary to separate them by perhaps 1/2" so that the sensor can read the correct one and if your greens are run alongside each other you may have to detach the terminal ends.

Good luck--sometimes it is easier to just rip all the old stuff out and start over....

 

[matt78]

At last, I fitted a 6 way - fuse holder with 15 A fuses (no smaller ones in my stock, and taking into account that formerly the 35 A fuse was doing its job, I guess that 15 A is on the safe side, even if I'll lower the value as soon as I've solved my current problem).

Thank you for this information (didn't know this kind of device). The harness is not the original one, and seems to be recent.

At the moment, there are 2 wires still to be identified (one of them should be the overdrive, but no idea regarding the other one...): interestingly, when I switch on the ignition with the key, the fuse fitted on one of them blows. Conclusion : it is grounded, and I suppose that it's the overdrive circuit, because when driving the car, OD doesn't engage no more.
So I'm going to focus on the OD wiring, and especially on its feeding side (before the main circuit), because the fuse blows with the OD switched OFF) : any additional idea, or well known issue to check regarding this one (I already replaced the solenoid following all the advices found on this forum; and it worked well) ?

 

[Michael Oritt]

Matt--

Though I swapped my OD transmission out a number of years back I seem to recall that the recommendation is to use a slow-blow fuse for the OD. There are two relays: An initial one to draw in the valve that draws, I believe, 19 amps for a very brief period, and a secondary circuit that hold it in which draws, again from challenged memory 3 or so amps.

All that said, if it is a dead short it will blow even a 35 amp fuse--that is, till the wire melts in which case the source of the smoke should help you find the problem....

 

[matt78]

You are right, I already read that a slow-blow fuse was useful (mine isn't, and I don't know where to source it from...).
I think that my current issue isn't linked to this point, because :
- the fuse blows when switching the ignition, BEFORE trying to activate the OD,
- 2 weeks ago, before fitting all the 15 A fuses, the main 35 A aldready blown... w/o using the OD, of course.
So it is urgent to find where the short is !

Matt

 

[Keoke]

Matt!! what you have done is make the wiring the fuse which will protect the fuses while it silently destroys part of the harness..?

 

[matt78]

Matt!! what you have done is make the wiring the fuse which will protect the fuses while it silently destroys part of the harness..?

I'm sorry but I didn't catch what you are talking about
The new wiring is as follows : + 12 V through white wires --> 35 A fuse --> 5 x 15 A fuses, one on each green wire. If there's any issue on one of these, the 15 A blows first (that's what happened).
What's wrong with this, except that the 35 A is useless now ?

 

[Keoke]

When a fuse exceeds the value of the load and it's the short circuit current . Heat is pulled from the fuse into the wire which is cooler and continues to do so until the wire's insulation melts or the wire melts.
Because ,as the wire heats up it's resistance increases decreasing the current through the fuse consequently the
fuse will not blow.

Each of those green wires require an appropriately sized fuse..

All inductive circuits-Coils- like the OD solenoid or motors require Slow blow fuses to handle the Initial inrush curent.

Hope this helps

 

[Michael Oritt]

When used as a verb the word "fuse" means, approximately, to join or blend together.
In this case the wire's green insulation and copper strands will be "fused" together into one blackened glob.

 

[Keoke]

In this case the wire's green insulation and copper strands will be "fused" together into one blackened glob.

Yes Unquestionably so------:lol:

 

[matt78]

When a fuse exceeds the value of the load and it's the short circuit current . Heat is pulled from the fuse into the wire which is cooler and continues to do so until the wire's insulation melts or the wire melts.
Because ,as the wire heats up it's resistance increases decreasing the current through the fuse consequently the
fuse will not blow.

Each of those green wires require an appropriately sized fuse..

All inductive circuits-Coils- like the OD solenoid or motors require Slow blow fuses to handle the Initial inrush curent.

Hope this helps

OK Keoke, thank you for this addendum.
I well understand, but I don't know where I could source this kind of slow-blow fuse (AH Spares, for example, only sells 35 and 50 A fuses, with no special mention). Do you have any recommendation ? And what are the required values ? Those described by Vette ?

Beside this point, I need to find where the short is, because the fuse blows with the OD switch OFF...

 

[Keoke]

Do you have any recommendation ? And what are the required values ?
MATT:If possible
Go to a regular auto parts stockist and perhaps you can get US fuse types.

The proper value of each fuse is determined by selecting the value of the load in Watts =P at the end of each green wire from the shop manual.
Then solve for Current P=EI where E is 12volts, once you have I multiply it by two and that is the approximate QUICK BLOW fuse size for that wire. do this for each wire.

For the OD just Purchase a "SLOW BLOW" Fuse with a rating of 10 Amp's.

Hope this Helps you out a bit.

 

[matt78]

Hi.

Still trying to find the culprit...
Good thing is that I managed to find what the unknown 5th green wire was (it feeds the heater motor), and that the centrifugal switch isn't involved (since when it's disconnected, the fuse still randomly blows...).
I also tried to examine and shake every wire, in order to identify the source of the short, but without any success...
So, before thinking of replacing the OD wiring harness, I wonder about the likelihood of the throttle switch and/or the 2 relays being faulty and randomly grounded : according to your experience, won't one of these be prone to such malfunction ?
Thank you again for your replies and advices.

Matt

 

[blueskies]

Check the throttle switch. If you have an extra one, swap that in and see if that makes a difference. I once had a faulty throttle switch burn up the overdrive harness.

 

[matt78]

Check the throttle switch. If you have an extra one, swap that in and see if that makes a difference. I once had a faulty throttle switch burn up the overdrive harness.

Yes, this could be the solution (can't test it, I don't have any spare).
This afternoon, I replaced the OD harness and the 2 relays, and got the same result : OD activation ok in the shed, so new wiring is good, but fuse blown as soon as the car is on the road...
I ordered a throttle switch. I'll make a specific new wiring, just in case...
Should the problem persist, I won't have any additional idea !
Keep you posted.

Matt