1970 Wiring Issue

[Stephen B.]

The 25 amp fuse that protects the heater fan motor, back-up lights and turn signals keeps blowing. It only blows when I start the car and back up. It's a 4 speed car. I've checked the wiring at the transmission. It look fine. Any ideas?

Thanks

[John H.]

Re: 1970 Wiring Issue

The 25 amp fuse that protects the heater fan motor, back-up lights and turn signals keeps blowing. It only blows when I start the car and back up. It's a 4 speed car. I've checked the wiring at the transmission. It look fine. Any ideas?

Thanks

Stephen -

I'd start looking for a chafed/pinched wire in the backup light circuit that's become a dead short; the pink wire to the switch is 12V, and is hot any time the key is on - the light green wire runs from the switch to the backup lights, and is only hot when you're in reverse (switch closed). Your problem will be in the light green circuit.

[Thomas H.]

Re: 1970 Wiring Issue

Stephen,

Tagging onto John's post, you might want to pull the backup lamps out, then put it in reverse and see if the fuse blows. If not, put one bulb in at a time to help isolate where to start looking. If it does blow with the bulbs out, then as John said, you probably have a short somewhere in the light green wire run. The problem could very well be in the lamp socket. Take a good look at them, you might want to remove them for a closer look.

You mentioned the heater fuse blowing. If my memory serves me correctly, the backup fuse and the heater fuse are two seperate fuses. Do they both blow?

Tom

[Stephen B.]

Re: 1970 Wiring Issue

Thanks guys. I'll try your suggestions and give feedback.

In 1970 the blower motor, turn signal and back-up lights were fed from the same 25 amp fuse.

[Don L.]

Re: 1970 Wiring Issue

Stephen, if you want to isolate the reverse lights to see if the problem is there, you may also be able to disconnect the lights via the connector on the LH firewall. Look behind the LH cyl head for a green/pink-paired, 2-wire harness. There's an in-line connector that when pulled apart should disconnect the reverse lighting system. Look at the attached photo of mine. If you look hard between the p/brake booster and the vertical ignition shield (just above the vac check valve), you will see the 2-wire harness I'm describing. Hopefully the '70 has this as well, and hopefully this will help you determine if the issue is in the lighting portion of the circuit.

Good luck and let us know...

[Stephen B.]

Re: 1970 Wiring Issue

I unplugged the back-up light switchfrom the harness as you suggested. The fuse stopped blowing. I check the Ohms on the switch. When the back-up light switch is on (the 4-speed in reverse), the ohms are zero as expected. When the switch is off (the 4-speed is in 1st through 4th gear), the ohms reading is 125. That's probably enough resistance to blow the 25 amp fuse. Now the question is what's the problem. The wiring or the switch on the transmission?

Thanks for everyone's help.

[Stephen B.]

Re: 1970 Wiring Issue

I also pulled the back-up lamp bulbs for starters. The fuse kept blowing so I proceeded to unplugging the back-up switch as discussed in the abovementioned reply.

[Edward J.]

Re: 1970 Wiring Issue

Steve, The switch is not generally the problem as far a short circut is concerned, As John H. stated the green wire is grounded some where, Take a look at the wires as it leaves the switch and follow up bell housing to the firewall to bulk head connector,then I think the wires follow under the drivers sill plate the exit the rear wall to the lamps.

I would also say if you have done any work to the car that may tampered with harnesses or driving screws somewhere that may have peirced one of the harness.

[William C.]

Re: 1970 Wiring Issue

The switch is a straight "on-off" function. If you disconnect the switch from the wiring you can check it for a short and if it's ok, you are likely good there. Then you have to find a short in the wiring between the 12V feed and the ground wire.

[Jim T.]

Re: 1970 Wiring Issue

I unplugged the back-up light switchfrom the harness as you suggested. The fuse stopped blowing. I check the Ohms on the switch. When the back-up light switch is on (the 4-speed in reverse), the ohms are zero as expected. When the switch is off (the 4-speed is in 1st through 4th gear), the ohms reading is 125. That's probably enough resistance to blow the 25 amp fuse. Now the question is what's the problem. The wiring or the switch on the transmission?

Thanks for everyone's help.

Very long time ago, 1973 I replaced the clutch, pressure place, and throw-out bearing in my 1968. Not paying enough attention some wiring got pinched between the engine block and installing the bell housing. Nothing happended because I discovered it before I installed the transmission.