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turn signal doesn't work when lights are on

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Old 07-01-2016, 07:00 PM
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turn signal doesn't work when lights are on

my turn signals work when lights are off, but when i turn my lights on, the left turn signal doesn't work, my right one does but not left, any clues why?
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Old 07-01-2016, 07:19 PM
FordMan007 FordMan007 is offline
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Not a pro...

My best guess is that you have a ground somewhere. Did the lights and signals use to work together? Have you taken any fuses or connections/wires apart? Or did it just happen? If it just happened the. 90% sure it is a ground somewhere, you have to do a little bit of testing to figure out though. If you took it apart then double check all the wires. Make sure when you are testing that you have the lights and the signals on. Hope this helps
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Old 07-01-2016, 09:01 PM
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Agree with fm007. When the running lights are off the turn signals will ground through the running light circuit at the rear lights. Bulbs are both running and turn signal. When running lights are on that ground path is no longer available. Check for poor ground on the left rear light.
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Old 07-01-2016, 10:56 PM
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what does it mean if i can take the grounds off completely and the lights still do the same thing.
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Old 07-01-2016, 11:40 PM
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it means your grounds aren't grounding. When you have a good ground, current will flow through the light and into the chassis. The fact that there is no difference with them disconnected means you are not getting current flow to ground.

Use sandpaper or similar to clean up the contact area so that you get good conduction through the connections. (Both in the socket where the bulb bottom sits, and where the ground connects to frame.)
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Old 07-01-2016, 11:59 PM
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ok man thanks, will try my best to clean it up.
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Old 07-02-2016, 12:09 AM
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could i use a voltmeter and test the ohms of resistance and look for a good ground that way?
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Old 07-02-2016, 01:14 AM
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Nope. It only takes just a fraction of an ohm to bugger things up completely, and these circuits draw way too much current for an ohms test to be of use. Rust and corrosion is the enemy of these old (and new) Trucks.

You CAN use a voltmeter though, while the circuit is under load. Called a "voltage drop" test. This is one of the easiest, most effective skills to learn. Wish I knew about it when I was 17.

http://www.aa1car.com/library/voltage_drop_testing.htm

You can bet that the ignition and accessories in your truck aren't getting a full measure either, by the symptoms you describe. That's how it works. As a general rule, replace ALL the grounding cables once they reach 20+ years or so, grind down to shiny bright metal at the block, firewall, and frame.

The headlights and turn signals use the frame as part of the return path. This is the likely culprit (or part of it). Oftentimes after engine or transmission work, someone will forget a ground strap or leave it loose, etc. Very common problem. This can cause other problems too. During engine starting maybe 250 amps are drawn in an instant, if the normal engine ground path is not there an alternative route will be used, through things not designed to carry current.

Brake lines. Fuel lines. Throttle cables, wheel bearings, gears, etc. These will even get red hot, not fun. Good grounds are important for Happy motoring!
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