TR4/4A TR4A Electrical

[KVH]

I've got a basic question. With a car off, and the ignition switch off, touching a continuity tester leads, one to the negative and one to the positive poles of a bulb socket--should there be continuity? I'd think yes, since the ignition only has to do with power and charge, not continuity, right? I'm dumb on electic.

 

[piman]

Hello KVH,
yes, that is correct but up to a point. If you are checking a single bulb it will tell you if it and the holder is good. Try that on a light unit it can give misleading results as you can read other bulbs connected in the same circuit so does not necessarily prove the one you are testing.

Alec

 

[KVH]

Does that mean that if I check one of two tail lights the continuity signal could be the other one, even if I pull that bulb out? I'm checking the license plate bulb holder sockets to be sure I've wired them right.

By the way, Moss sent me the wrong bulbs for the license plate. They sent me two small bulbs and the ones that fit the socket are much larger. Do you happen to know whether those are common bulbs these days.

Off the topic: How may TR3s and 4s do you suppose are still running the streets of England? I'm impressed by the resiliency of those cars--and the shops that make the parts!

 

[piman]

Hello KVH,
you are on the right lines, but the tail lights are also connected to the front side lights and the number plate lights so you would need to remove them all. It is easier to disconnect either the feed or the ground from the lamp unit you are checking (I would suggest the feed and check between a clean point on the body and the wire into the lamp unit. This checks that the light unit had a good connection to earth)
The bulbs are common in the UK, I can't comment about your part of the world.

Alec

 

[vettedog72]

My bet is that you have a short. I think you are reading a SHORT if you read near "0" ohms on your socket (assuming a good bulb is out of the socket and the light switch is off).

 

[Geo Hahn]

[ QUOTE ]
My bet is that you have a short. I think you are reading a SHORT if you read near "0" ohms on your socket (assuming a good bulb is out of the socket and the light switch is off).

[/ QUOTE ]

Picture this: One current path is the ground. The other is the 'hot' wire which also goes to other bulbs on the same switched line (other plate bulb, tail lamps, etc) passes thru the filaments of those bulbs to the ground side of those sockets.

What you say would be true only if all bulbs on that switched circuit were removed.

 

[vettedog72]

I am corrected! I was thinking about a direct circuit; should have looked at the diagram.