the tail lite and brake/signal bulb on pass rear of my 74 is noticabily dimmer than other side. same bulb but different filaments. i cleaned the grond with no improvent. i noticed with ground unhooked, the bulb was still lit. i even cut the ground wire and its still lit. where is it getting its ground from?

 

Check the socket itself. It's possible that the wire that runs to the tailight has a bare spot on it where it goes into it. Or corrosion in the socket might be it. It sounds like you have a bad ground from the tailight wire somewhere in the harness. I'd start with the socket and work my way back to the light switch making sure all connections and grounds are clean and tight. It might take awhile.

 

you are probably grounding through the brake signal light filiment hook the ground back up see if it changes also check the sidemarker light

 

This may sound simplistic, but have you tried changing the bulb itself? It could be that one of the filaments inside the bulb has broken and is touching the other. Have a buddy watch while you step on the brake pedal and see if there's any change. Make sure you have the tailights on when you do it.

 

It takes an 1157 and the offset Pins on the side have to go into the correct slot in the socket or you will get bad things happen.

 

Need to know a few things...

Which taillamps do you have?

1973 and Early 1974s:

They use a seperate taillight lens and metal lamp housing.

The lights ground through the metal lamp housing.


Late 1974s:

Use a combination plastic lamp and lens housing with seperable plastic lamp sockets.

Used 1974.5-1979

 

Here is the Serial Number change pertaining to wiring of the rear lamps:

1973/74 F100/350 Taillamp Bodies.

1973/74 Before Serial # T00,001

1974 After Serial # T00,001

 

its a late 74 with plastic housing. first thing a tried was switching bulbs. today i'll try a different socket first and report back. thanks for the ideas so far.

 

this morning, i replaced the socket. yes, i am using an 1157 bulb. bulb still comes on without ground connected to body but i noticed the side marker light also comes on when its ground is touched to ground wire for tail/brake light(not yet connected to body ground). maybe this is normal. i was going to take ground screw out for left side but it wouldn't budge. it works ok. anyway, main task was to get pass side as bright as drivers side and it is. so problem was is socket. all it ok now.

 

If a bulb can't get ground through its normal place....because everything runs through the T/S switch, it will find ground through another source.

Check for corrosion everywhere around that bulb and socket.

 

Yes this is normal, the lights are finding ground through the TS switch with the ground disconnected.

Glad you got it fixed.

Here are some more suggestions to look for to possibly make your lights even brighter

You can clean up the grounds wire eyelits and where they attach to the body with a little sand paper and some dielectric tune up grease to help prevent further corosion. Use new screws that are not rusted as well etc... Use white lithium grease in the lamp sockets to help prevent corosion.

There is also a ground strap from the engine to the firewall of the truck. Make sure it is clean and tight. Also make sure the ground cable from the battery to the engine, and/or frame is also clean and tight.

hope this helps...