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Intermittent short

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Michael Oritt

Michael Oritt

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Hi All--

Several years back I installed a separate fuse panel and one circuit controls my brake and turn signals. Now there appears to be an intermittent short somewhere in the circuit that blows a fuse after a few days of use.

I become aware of the situation when the flasher light in the dash fails to come on, but I have not been able to pin down whether the blow is caused by my applying the brakes or putting on the flashers.

The last fuse that blew was 20 amps. I currently have a 30 amp installed and it has not blown yet, but I have the feeling this condition is being caused by an intermittent short to ground, not a circuit overload. My suspicion is that it is somewhere in the control box but I would not know how to test for an intermittently faulty condition. Any suggestions will be appreciated.
 
Mike

If you had a short, intermitant or otherwise, the 30 fuse would blow anyway?

I had a problem with the brake switch over ride relay, but it was not a short, all the terminals required cleaning, this would not blow a fuse.

Your best bet is to trace and check all the wires.

A good start would be to check the wires under the car, they can get damaged along the chassis rail, unless you have re-routed them, then look into the boot - sorry trunk, and examine the harness there to the indicators / brake lights they are the easy bits then move through to the front.

Are the bulb holders new or originals, you may have a break down there.

Sorry i can not help further.

Bob
 
HI Michael,
1] can you tell me what the total load in amps for the defective circuit is.
2] Is the field of the brake turn signal relay mounted under the bonnet powered from this defective line also.--Keoke
 
Hi Michael,
One thing you might try is putting the brake circuit on another fuse. If you have a short upstream of the flasher relay, that might narrow down the search between the brake or turn circuit. If both circuits blow, that would make me think the fault is in the common wiring of the two. I.E. the relay back to the indicator lights. The photos you sent me years ago of your fuse block were on a computer which is long dead, and I don't remember what you changed exactly. Was it 2 6-circuit blocks?
 
Make a jumper to put in the line before the control box with a fuse holder for inexpensive glass fuses. If you use a lower amperage fuse than what you have in your fuse block it may blow before you need to keep replacing thos blade type fuses if that is what you have installed. I would then go to poking wires and pressing on everything in the system to try and duplicate the problem. Often you can find the fault and zero in on exactly where you need to prod to make it happen.
 
If you had a short, intermitant or otherwise, the 30 fuse would blow anyway?

Not neccessarily :
You see all fuses require that a specified amout of current must flow for a specified time before the fuse will blow.

Cosequently, if the fault current was lower than required or the time the fault persisted at that current level was less than required the fuse would not blow.--Fwiw--Keoke
 
Hi Michael, I'd start with the easy stuff first. Check for a shorted light socket, wire abrasions, etc. No luck? Remove one wire at a time from your installed fuse holder and with wire in hand check with a VOM set on resistance and with one probe on ground check which wire may be grounded. Of course without key on.

Good Luck, let us know what you find.
 
I did not think there was going to be an easy solution but rather a lot of trial and error. Perhaps I'll just replace that fuse with a 16D nail and see which wires burn up--that'll certainly tell me where the problem was!

I'll come back when I figure it out but thanks for all the good suggestions.
 
I did not think there was going to be an easy solution

Well to do it analitically we need a little more data as I indicated earlier.

However, if the 30 amp fuse does not blow then you can go back to a lower valued "SLOW BLOW" fuse such as the 20 amp one or it may acconodate a 15.

Let me Know how the 30 amper does OK---Keoke
 
Hi Mike,

Is you license plate light working and have you added a fuse to that circuit? If not, I would check the 3-Red wire connector in the area of the flasher under the bonnet that brings power to your brake and taillights for excess resistance.

It is not uncommon for water to short the unfused license plate light and melt the insulation in the rear harness. Depending upon the situation of the wires, and adding harness flex and vibration from normal Healey operation, an intermittent short can occur.

Should this not be the case, I would start by disconnecting the red wires from this connector to eliminate the tail and license plate lights and place the lowest appropriate fuse in the holder. Since it is an intermittent, I would drive the car in a very safe protected area (as you have no rear lights) to induce the issue should it not appear immediately. If no problems were encountered, I would then perform the same with the white/purple and white/brown connections to eliminate rear directional and stop lighting circuits.

Following the above, and since power gets to the control box from 2 main sources (Flasher and Brake Switch), I would then eliminate each circuit by installing a temporary fuse by attaching bullets on an in-line fuse holder. This action would provide some direction in isolating the issue, however since intermittent, it may take time and the temporary fuses will provide some protection for the circuits during the period.

Good luck and all the best,
Ray (64BJ8P1)
 
Keoke said:
I did not think there was going to be an easy solution

Well to do it analitically we need a little more data as I indicated earlier.

However, if the 30 amp fuse does not blow then you can go back to a lower valued "SLOW BLOW" fuse such as the 20 amp one or it may acconodate a 15.

Let me Know how the 30 amper does OK---Keoke
Click to expand...
Why not put a 30 amp "resettable circuit breaker" that semi-trucks use instaead of a slow blow fuse? They snap in the same way the originals do.
Patrick
 
Well Patrick, the characteristics of the two devices are not the same and it may just be the case that there is not a short in the circuit at all. Further, as a final solution we would like to stay with the standard type components we have in the car "Fuses" wich are readily available.--Fwiw--Keoke
 
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