• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)

TR4/4A One Last Light Problem in my Restoration

Share this page

KVH

KVH

Obi Wan
Silver
Offline
Everything is perfect except for one light. My right taillight is apparently illuminating both filaments, running lights and stop lamp. If I try to reverse the bulb it starts to look good with only the running light filament igniting, but, of course, the bulb won't fit properly that way. I have green/pink to green and red to red in the taillight wiring. I could switch them but then I'd have green to red and that doesn't sound right. I'd sure like to hear any ideas. Thanks.
 
Green/pink? Do you mean green/purple (for the brake light filament)? Also, what happens when you press the brake pedal? Same thing with both filaments? Do they get brighter (or dimmer) with the brake pedal?

I'm still thinking there's some sort of ground issue?
 
The good side, left, gets very bright when I push the brake pedal. The right just sort of flickers and gets barely brighter. There is a ground wire on the left rear but not on the right rear where I’m having trouble. I think that’s normal, however.

When I remove the fitting and holder from the taillight housing and just touch the bulb to it, the bulb illuminates properly. The extreme brightness is when I install the bulb into the housing. I wonder if there’s some kind of a short on the housing. Or maybe I just have some other wires connected in properly
 
If it’s an aftermarket socket it may be assembled incorrectly. Just try switching the wires around and see if it works. We just had to do that while we were helping finish our friends BJ8. You may be able to take the contact plate out and turn it 180.
 
It’s definitely aftermarket. Just bought it four months ago. I’ll switch those wires. And report back.
 
KVH said:
The good side, left, gets very bright when I push the brake pedal. The right just sort of flickers and gets barely brighter. There is a ground wire on the left rear but not on the right rear where I’m having trouble. I think that’s normal, however.
Click to expand...
I don't know the 4A well enough to say for sure, but it sounds like a (lack of) ground issue on the Right side. For whatever reason there's a ground wire on the LH lamp -- factory or not, it appears it's a: needed and b: doing its job.
 
+1 on lack of right socket ground. The stop and tail filaments connect at the base, which should provide a direct path to ground. If it does not, current can flow through both filaments and possibly find a path to ground through another 'cold' (low resistance) filament, like the plate lamp. This can happen in a TR6 because the construction of the removable sockets allows corrosion to block the path to ground. I can't really explain it, but I did read about "cold filament conduction" in incandescent bulbs.

Jeff
 
Last edited:
+1 bad ground. Could be inside the socket (even new ones) or not.

Also a good idea to add a ground wire, IMO.
 
Where would I add a ground? To one of the mounting bolts? Or are you saying I need to ground that socket? By the way, does this give a hint to anything: When I hold the bulb up to the filament connectors so it lights up, but after the connectors are pulled out of and away from the socket, and then touch a test lamp on the steel shank of the bulb the test lamp lights up. Why is there current on the exterior bulb housing?
 
KVH said:
Where would I add a ground? To one of the mounting bolts? Or are you saying I need to ground that socket? By the way, does this give a hint to anything: When I hold the bulb up to the filament connectors so it lights up, but after the connectors are pulled out of and away from the socket, and then touch a test lamp on the steel shank of the bulb the test lamp lights up. Why is there current on the exterior bulb housing?
Click to expand...
That is an excellent question; I don't have even a bad answer at the moment, though. :(
 
Fixed. I just had to reverse the wires. The housing apparently was manufactured with the wires crossed on the filament holder. No ground issue. I checked that first. Wish I understood this stuff and could figure things out faster. Thanks for all the help. I needed it.
 
KVH said:
Fixed. I just had to reverse the wires. The housing apparently was manufactured with the wires crossed on the filament holder. No ground issue. I checked that first. Wish I understood this stuff and could figure things out faster. Thanks for all the help. I needed it.
Click to expand...

Happy to have been able to help.
 
Glad you got it sorted.

The outer sleeve is the ground for the bulb. Internally, it has two filaments, a lower power (higher operating resistance) filament for the tail lights and a higher power (lower operating resistance) filament for the brake or turn lights. Both filaments are grounded to the outer sleeve, which in turn has to be securely grounded to the car body.

The whole "ground" thing tends to be confusing, the term itself is a holdover from when electricity was poorly understood. On a car, it refers only to the custom of using the car body (and frame) as one of the conductors for an electrical circuit. You still must have a complete circuit from one side of the power source (battery) through the load (the filament) and back to the other side of the power source.

Those two filament bulbs are also confusing; if one bulb is not grounded but power is applied to only one filament, the current flows through both filaments in series and back to ground through one of the other bulbs. Thus you can wind up with a lamp that seems to work when only a single function is tested, but then goes out when both functions are being tested.
 
Back
Top