Electrical Troubleshooting - not my forte - Brake Lights/Turn Signals

[JeremyD]

OK - I keep blowing fuses - the one that controls turn signals and brake lights.

My 1974 has the pedal mounted mechanical switch (which I like)

So far I have rebuilt my two rear light boxes - including repairing a ground in the passenger side box (which I thought would alleviate the problem). Well, they have new seals now - and they look better - so at least I have that going for me. (I traced all the wiring too in the rear and it all looks good - is covered, etc.)

Just want to confirm I am on the right track.

I think I isolated the problem to the turn signals - While removing my seat - I decided to test my brake lights by keeping the switch turned on and the brake light depressed. Even with it on for 10 minutes or so - the fuse did not blow - Nor did it even get warm to the touch.

SO - I think it's possibly the turn signals. When I switched to fiberglass bumpers I did a bunch of work to the frontal area. So I am guessing I may have a short there?

I have LED tail signals (which have typically in the past flashed FAST) these do not - would that indicate I have a current draw somewhere?

I should be looking for wire to metal contact, right? not necessarily a bad or non existent ground? My car's wiring is a little bit of a hack job after 36 years - Looks like there's been a few stereos and alarms wired over the years that have just been cut off and left.

Sorry - I have a multimeter - but mostly use it to check current.

[T77911S]

a test light works well too. sometimes better because you can connect the light to 12v and look for a ground, which i you could with a MM, but the light is easier to see. but what ever works best for you.

if you did work up front, that would be the first place i would look. also make sure you have the correct type and wattage bulb in all the sockets.
in order to blow a fuse, other than too much load on the circuit (much like plugging in a bunch of heaters at home and popping a CB), power is shorting to ground. either directly from a hot wire to the body, or a hot wire to a ground wire. now, a hot wire is something that has power on it and is before any kind of load. what i mean by that is that the wire feeding power to say a light bulb is hot, but even though the negative side of the bulb has 12v on it, and it could feed another bulb, that wire could short to ground and not blow a fuse because the first light bulb is limiting the current. not to confuse you, but maybe this will help you help yourself. also, a hot wire will not blow a fuse if touching another hot wire, it has t be something that goes back to the NEG side of the BATT.

now to this LED stuff. what is the current draw of the LED lights? i have never looked into the LED's so i dont know if they need more power than a standard bulb or not. LED's on a 74??? it just hit me. sorry.

if the brakes did not blow the fuse, did the turn signal do it? it would help to find out what conditions blow it. does it blow all the time or right away, or at random?

gotta go, work calls.

[T77911S]

sorry, work.

i hope that helps. after a certain point, your kinda own your own with stuff like this. sometimes it just comes down to tracing and wiggling wires and doing a visual of as much of it as you can.

i just helped someone on a BMW that had a smilar problem, except his blew when he hit a bump. i had him wiggle all the wires to the circuit that he could. he found a bundle that finally made it blow. even after looking the wires over several times did he find a very small nic that was blowing the fuse.

i think the turn signals are the worst circuit on any car to work on. they make so many loops through the emergency flasher, the flasher rly, the light switch, and other places, such a pain to trace out.

[Bill in OKC]

It looks like you have eliminated the brake lights. Can you tell if it is the left or right turn signal the same way? If both left and right signals cause the fuse to blow, maybe look into the switch on the steering column. LEDs usually draw less current than a regular bulb. I think it is the reduced current that causes them to flash faster, it doesn't heat up the relay (bi-metallic strip I think) like a regular bulb. On bikes, they sometimes replace the standard flasher relay with an electronic one that is not dependent on current for the flash rate.

[JeremyD]

Yeah that's why I put the LEDs in there to see if they would flash faster. They draw a lot less current. How to test the turn signals. Stalk turn signal switch is fairly new.

How to test turn signals?

[Bill in OKC]

Just like the brakes, turn on the left one and see if the fuse heats up, then the right one. It could help you narrow down where the problem is. If both sides cause the fuse to get hot or blow then it is something common to both sides like the turn signal switch or something common in that path. edit: or a wiring issue that happened to both sides at the same time which would be rare.

[JeremyD]

Thanks - that makes sense

[scarceller]

You really need a Amp meter, preferably a clamp on style that simply clamps around a wire and reads dc amps. You can get these cheap like at harbor freight for about $10.00.

With this style meter you can read actual current in Amps and easily figure out what is triggering the high current condition. Simply activate the left then right blinkers and read the current draw. No guessing required.

Are you certain you have the correct stock bulbs in place? Someone could have put bulbs in with much higher current requirments.

[Bill in OKC]

If nothing makes the fuse blow or get hot, then it gets harder. An intermittent short somewhere, still leaving the brake light wiring as a possibility.

[JeremyD]

Maybe I'll pick up an amp meter. I'm taking off tomorrow so I'll have a little time in the morning where I plan on removing my front fender and checking out a few things. I'll report back my findings.

I've done a tone of things Porsche related in the past - but for some reason, electrical always stumps me a little.

[JeremyD]

Well, I thought I was making progress - even found some discoloration in the passenger side headlight bucket where some arching had been taking place - and obviously overheating. Probably explains why one of my buddies said my passenger side headlight looked different from my driver side



So after redoing all the connections - heat resistant connectors - etc - I was pretty proud of myself - and went for a drive.

Unfortunately - still blowing a fuse -

Works without the engine running. I can use the brakes - had the front headlights on - and turn indicators - but when I start the engine - the fuse blows - and just to the brake lights/turn signals. very frustrating!

[T77911S]

that does look like it was hot. resistance can also cause something to get hot. i pulled most of the major wires out of my fuse block ( one at a time), cleaned the wire, the screw and the fuse contact points to reduce resistance.

you had a key statement on your last post. "when i start the engine" pehaps a PO wired something into that circuit. with the key "on" and enging off, does the fuse blow?

intermittent problems- you cant fix it if it aint broke. IE, you have to figure out what conditions make it blow. otherwise you just have to inspect ALL the wiring on that circuit. it sucks, i know.
look for places where the wires can move, or are bent around something, check all connectors. grab the wire bundle and move it around.

[JeremyD]

Thanks T77911S - Originally I thought it was just my turn signals - but yesterday - I had my lights on, my turn signals on - even put my brights on - and tested - all without blowing the fuse. When I started my car and put just the brakes on - the fuse blew.

The alternator has been recently rebuilt. I have a gauge that shows volts - they top out at 14 volts when charging. Should the circuit be that different once the car is running?

So far I have:

Inspected, cleaned all the taillights - including fixing a broken ground that had pulled away from the clam shell back.
Cleaned all bulb recesses
Inspected all rear wiring harnesses (look good)
Replaced an engine ground that was coming loose.
Cleaned fuse block
replaced the electrical tape connectors for both headlights
Cleaned both turn signal bulb housings
traced wiring on front connectors wiring harness
Cleaned front wiring harness ground.
The Alternator has been replaced (rebuilt) - fairly recently

Stumped at this point

[ossiblue]

Originally Posted by JeremyD

Thanks T77911S - Originally I thought it was just my turn signals - but yesterday - I had my lights on, my turn signals on - even put my brights on - and tested - all without blowing the fuse. When I started my car and put just the brakes on - the fuse blew.

Can you clarify these just a bit?

I had my lights on, my turn signals on - even put my brights on - and tested - all without blowing the fuse. Is the "test" you mention to apply the brakes and see if the fuse blows?

Can we assume when you ran the above test that the ignition was in the "on" position as the turn signals won't function without the ignition on?

When you preformed the second part of your post, " When I started my car and put just the brakes on - the fuse blew" Did you still have all the other devices on (headlights, turn signals) or had you turned them off and just applied the brakes?

Can you post a picture of your fuse box showing the all wires that are connected to the terminal where the fuse blows, or at least describe all the wires connected to the load side of the terminal?

[JeremyD]

Originally Posted by ossiblue

Can you clarify these just a bit?

I had my lights on, my turn signals on - even put my brights on - and tested - all without blowing the fuse. Is the "test" you mention to apply the brakes and see if the fuse blows?

Can we assume when you ran the above test that the ignition was in the "on" position as the turn signals won't function without the ignition on?

Yes = ignition was in the "on" position - fuel pump was running (it runs when the ignition is turned to the on position with the 74's) and you can hear the CDI box humm...

Originally Posted by ossiblue

When you preformed the second part of your post, " When I started my car and put just the brakes on - the fuse blew" Did you still have all the other devices on (headlights, turn signals) or had you turned them off and just applied the brakes?

Turned off all other lights/turn signals. etc when I started the car - it idled for a few minutes (high idle with the hand throttle up, ~2500 rpms

Brakes lights came on - could see the reflection in my tool box. Then I let them off - then as I put them back on - the fuse went.

Originally Posted by ossiblue

Can you post a picture of your fuse box showing the all wires that are connected to the terminal where the fuse blows, or at least describe all the wires connected to the load side of the terminal?

I'll take a picture when I get home and post. Thanks for your help.

[ossiblue]

Thanks, I think this info will help. So as far as you can tell, the brake light fuse blows when no other electrical lighting load is activated but only when the engine is running.

Couple of other questions:

How long have you had the car?
Did you always have a fuse blowing problem? If not, when did it start?

You did mention some evidence of old alarm/stereo wiring. You may wish to trace any abandoned wires to see if they tap into the brake circuit--I'm thinking alarm with flashing brake lights, for example. Otherwise, I'm stumped.

[JeremyD]

Had the car since March of last year. It had turbo look body. Ran it for a few months then took it off the road. installed IROC bumpers/oil cooler, front vent, etc.

Replaced/rebuilt alternator (light was going on)

Just started - right before my last DE in November - was able to run the DE in November without using turn signals

[T77911S]

sorry, i just need to be clear.

with the engine on i understand the brakes will blow the fuse. (nothing else on)

with the engine off, and the key on, do the brakes blow the fuse. again, sorry if you have to repeat, i road the slooooooow bus.

i dont see why the engine being on/off would make any differance and just want to be able to take away that distractor if it does not matter.

alright, if this does not work, you wont like my next post.
unplug both LED lights. if the wires are shorting to ground, the LEDs will not make a difference. then try each one by itself. you could also put a MM in place of the fuse to measure current in all conditions, no leds, lft, rt, both. well, you dont need the MM with no led's, you just want to see if the fuse blows, but i am curious of the current of the LED's.

[JeremyD]

Nope = when the engine is off and the key is on the on position - nothing happens.

Then the engine is running - apply the brakes and poof - the fuse blows - doesn't make sense, does it?

I relocated the battery to the smugglers box - but was able to directly take the leads that where on the battery there and restring to the new location. The only thing that I had to change was the ground which I placed on the bulkhead (after cleaning off all the paint - made sure there was a good contact, etc...)

[T77911S]

is it fuse 8 that is blowing?
i think there is a crossed wire that has to do with the charging system for it to only blow with the engine running. like the alt is trying to charge thru the brake circuit or something like that.
the alternator is the only thing that comes into play once the motor fires up. you mght check the ignition switch, maybe the emergency switch. i will try to dig thru the 73 schematic i have, maybe someone can suggest where the 2 could come to someplace that would allow a wrong connection.