I have been tearing everything apart from my rear end and up to find the problem with my tail lights. All lighting works until I use the headlight switch. As soon as my head lights are turned on my tail light fuse blows. I still have brake lights and signals but no tail lights. I've pulled and cleaned the switch and replaced wiring under the dash. All my rear wiring has been cleaned and isn't corroded. Anyone have any suggestions?

Also- this may not be related but this only started occurring after I installed and HEI, is there a possibility this could have effected anything?

 

I had the same problem with my '73 wagon. The moment I turned on the headlights, the taillight fuse would blow. I put in a same-amp circuit breaker so I wouldn't keep blowing fuses as I was troubleshooting, and I even bought a "short finder."







I first did what you did, checking for shorts under the dash and at the lamp sockets themselves. No go. I ended up tracing the wiring all the way from the front of the car to the rear, pulling up carpeting, removing trim, and whatever else was required. The short finder was actually more helpful in determining where the short was not.

The problem ended up being a pinched wire in the area behind one of the taillights. I took the entire taillight housings off on both sides and discovered the problem. The car had been repainted, and I'm sure the pinch occurred when the body shop reassembled the taillight to the car.

It was quite a feeling of satisfaction, no matter how much of the car I had taken apart and had to put back together, to finally be able to turn on the headlights, have the taillights stay on, and be able to drive the car at night.

 

Ordinarilly an HEI installation will not cause lighting problems, where did you pick up power for it? The other thing I would suggest would be to pull all the tail lamps and see if the fuse still blows. What you have is direct short somewhere and you are going to have to unplug harnesses 1 by 1 to isolate it. Stick with the tailight crcuit only.

 

I'm going to bet that you do not have the optional under hood light. The underhood light is on the same fuse as the tail lights and the pigtail that the underhood light plugs into is installed on every car made. Look below the brake booster on the firewall for a single dark brown wire coming through a grommet. Do not confuse this with the tan wire that goes to the distribution block. This underhood lamp pigtail is supposed to be tied up in the harness, but it can come loose and either get pinched between the fender and the firewall or burn through on the exhaust. Either one will blow the tail light fuse.

I chased this short for four months on a 68 442 once, until I pored over the wiring diagram and traced every single wire connected to that fuse.

 

Let's clarify this a little further. The brown wire for the underhood light originates at the fuse block inside the car and is only a few inches long. An intermediate wire connects to the lead off the fuse block and runs through the firewall and terminates by the brake booster or master cylinder. The hood light then connects to that intermediate wire. If the intermediate wire is not found by the booster or master cylinder (maybe someone removed it) then also check down by the fuse block inside the car to make sure the initial section of wire isn't damaged or that the intermediate wire was not pulled back inside the car and may have been damaged, cut or pinched.

Brian

 

Yup, it's your hood light wire, for sure. You moved it when installing the HEI and now it's lying on top of something and shorting to ground.

- Eric

 

Alright I will be running through it this weekend again and i will post on what I find. Thanks for the advice.

 

if the underhood wire is okay,would it have to be a bad switch?i started there but I've heard the new one could be faulty??getin confused.can ya help me out?

 

what ended up being the problem?i'm goin thru the same thing.STUMPED!!

 

Hey you guys are awesome !! My 1970 W-30 442 had the same exact problem... blowing the fuse to the tail lights when turning on Headlights (or low beams). After removing the head light switch, I buzzed out the din plug to the switch. (Asside: what fun it was to remove that one...you actually have to press a button on the back of the headlight switch to release the **** & shaft - which pulls completely out of the switch box - and that enables you to remove the headlight switch assembly ). I found that the positive connection to the low beam circuit was shorting to ground. Well, I did a first level test shaking wires from front to back listening for a break in the Volt Meter beep (beeps when sensing a short). No luck there. So after about 1.5 hours of coming up dry, I reverted to the forums and found this wonderful post! It likely saved me many long frustrating hours of labor (and swearing) and probably a couple of hundred $$ giving up to the electrical shop. At very least my swear jar will be a bit lighter.
The problem was indeed the HOOD LIGHT ! There was a broken wire to the socket which was touching chicken wire under the hood. OMG! Thank you Colpage1 for posting your problem and Joe Padavano for the absolustly key information. You guys rock !
Cheers.