MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Wiring Harness Checks

Hi,

Have just fitted a harness into an ongoing rebuild (no engine fitted/1973 harness). I think I have most things in place correctly.

What would be a good (proper) sequence of checks to follow in making sure that all is well with the harness. I have been using a test light and extra battery to do some basic checks; however, as a bit of an electrical 'dumbo' I need to double & triple check.

Are there any specific tips, pitfalls to avoid?

Thanks in advance.

Iwan
IA Jones

Iwan - If you don't have one, get a tickler charger and attache it to the battery cables (disconnect the cables from the battery). You can now turn on the lights,step on the brake to see if the brake lights come on, turn on the ignition, etc., one at a time. You won't be able to start the car using the starter, but you might want to try starting the engine using the starting handle. Take each circuit and check that everything - running lights, head lights, tail lights, brake lights, ignition, etc. If there is a short anywhere in the harness, you can troubleshoot as you go and there is not enough current flow from the tickler charger to cause any damage to the harness it there happens to be a short circuit. After you have checked everything and corrected any problems you can graduate to hooking up the battery and be sure that everything will work. Cheers - Dave
DW DuBois

Or connecting a high wattage (headlight) bulb in place of the earth strap does much the same thing.
Michael Beswick

Good advice from DW, as an alternative you could connect a large wattage bulb, such a headlamp or spotlight bulb, between one of the battery leads and the battery, that way if you have a short all it will do is light the bulb and not fry your new harness, but it should allow you to test most things. Not sure about the starting handle though.

I was typing while Michael posted.
R.A Davis

Hey, Brilliant, just what I needed. Thanks all of you.

You see, I don't know why the high wattage bulb would save the harness? I'll take it from you guys that it will. No, don't go there ...... I still wouldn't understand :)

IA Jones

IA,

The high wattage bulb saves the harness because the electrical energy is expended in the form of light and heat in the bulb instead of heating the conductor in the wiring harness until it melts. Either method is a safe way to test wiring. Some trickle chargers have a circuit breaker that will trip if excess current is demanded from the charger. They are usually self resetting when the short circuit is removed.

GO TO IT!!!

JIM HASKINS
J. M. Haskins

The large wattage bulb saves the harness because the maximum current with it in series with the battery cannot be higher than the normal consumption of the bulb, i.e. 5 amps in the case of a 60w headlamp bulb.

I always use that method when connecting a new harness or reconnecting a harness that has been worked on. Because it limits the maximum current to 5 amps it affects the various circuits on the car in different ways. Low-current devices like an interior lamp will appear to work normally with the large wattage bulb not showing anything, side lights will be a bit dim with the bulb glowing dimly, headlamps will be at less than half brightness with the bulb at near full brightness, and so on. But it allows you to confirm that almost all of the switched circuits are working correctly and not shorted to earth, as well as all the supply wiring. Only the starter circuit will do nothing, with the bulb glowing at full brightness, so you have to hope that one is OK.
paulh4

This thread was discussed between 01/08/2017 and 05/08/2017

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.