1970 Cougar Base 351C - No turn signal when headlights are on

First time posting here. Have been trying to troubleshoot this for a while. My 1970 Base model cougar has a problem with the turn signal. Basically with headlights off, flashes fine. If headlights are on, intermittently will start functioning. The turn signal light just stays on solid and only starts to flash when I turn headlights off. Is this a not enough voltage problem? As far as I know all the turn signal components are original. I don’t want to just swap turn signal parts like switch and sequential unit but if I do need to I can. Just looking for some direction.



Is the engine running when you discovered this? Your system has more power when t he engine s running.

Have you verified all the bulbs are good? Including the marker lights. They are pat of the 1970 turn signal system.

Use the trouble shouting guide on this site to help you. Cougars Unlimited LLC Home Page

Agree with Badcatt - sounds like you might not have your engine running. In that case, without the alternator charging, headlights can pull the battery voltage down enough so that the flasher unit never heats up enough to open and flash. Same thing can happen with a bulb out - also noted by Badcatt.

Sounds like it could be a bad ground at the socket for the bulb. When the ground is bad, the parking light filament will try to find ground through the turn signal filament and cause the indicator to light solid.

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Thanks y’all yeah forgot to mention this is with engine running. I did have the issue before where I had to swap tail light bulbs. That issue turn signal did not flash with headlights on or off. Thats a good idea on the marker lights haven’t checked those. Will check and let you know.

Clean grounds are so important with the electronic flasher modules. You need a clean ground between the bulb and the socket, socket and the tail light housing, tail light housing and rear tail light panel of the car. There is also a small black wire on the driver side of the trunk latch receiver bracket that is the ground for the module itself. And lastly, don’t forget to make sure there is a good ground between the battery and the car body. Oh, and is the alternator putting out enough to keep the battery voltage up?

Someone once told me that Vintage Ford alternators did not charge at idle speeds. Is that not true? It would explain his perceived problem.

True for the old DC generators, but AC alternators do still charge at idle.

Except 1 wire alt that need extra rpm to excite the internal regulator…never liked them for that reason…something about reving a freshly started cold engine that seems wrong

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