truck cab lights as turn signals

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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=135660
Printed Date: May 14, 2025 at 11:12 AM

Topic: truck cab lights as turn signals

Posted By: agood1
Subject: truck cab lights as turn signals
Date Posted: December 30, 2013 at 9:55 AM

I'm thinking about making the 2 outside cab lights on my truck blink with the turn signals. I got it figured out as long as the cab lights are turned on. Basically, i will have the trigger from the OEM turn signals open the circuit on the cab lights which was turn off the cab light. This will cause the cab light to blink opposite of the OEM lights, but I'm ok with that. Then problem comes in when I don't have the cab lights turned on.

I don't need a flasher in the circuit, since the signal will come from the OEM lights.

Replies:

Posted By: davep.
Date Posted: December 30, 2013 at 10:49 PM

I know how to do it. Use the following circuit for each side. You will need two relays, and two sets of capacitors / resistors. Be sure to use the front turn signals for input, as the rear lights are also brake lights.

When the running lights are on, the cab lights will illuminate normally. When a turn signal is commanded, the turn input will close the relay coil (86), and connect the cab light (30) to the turn signal input (87). The capacitor will hold the relay closed, and the cab light will blink with the turn signal. About 3 seconds after the last turn signal pulse is received, the capacitor will discharge, and the relay will return the cab light to the running lights position (87a). When running lights are off, the cab lights will flash in sync with the turn signals.

Jumper 86 to 87. (Ignore the "battery 12 volt").
30 = cab light
85 = ground
87a = running lights

posted_image

Posted By: agood1
Date Posted: December 31, 2013 at 7:48 AM

ok, I think I understand. Couple questions
Is there a component across terminals 86 and 85? The diagram shows what looks like a resistor, and the same symbol is on the "+12 line from turn signals".

I assume you mean ignore the red line marked "fused 12v(+)?

The cab lights are LED, will that make any difference in the cap or resistor values?

Thanks!

Posted By: davep.
Date Posted: December 31, 2013 at 11:29 AM

Is there a component across terminals 86 and 85? The diagram shows what looks like a resistor, and the same symbol is on the "+12 line from turn signals".

Those are diodes. I'm not sure why the author included the one across 85-86. The one in the "input from turn signals" must be there, or the cap will discharge into the turn signal bulb filaments.

I assume you mean ignore the red line marked "fused 12v(+)?

Yes.

The cab lights are LED, will that make any difference in the cap or resistor values?

I don't think so. The cap value is determined by the relay coil resistance, and the time duration desired. I've used this circuit, and the time-on is about 3 seconds when used with a 'typical' 30/40A cube relay.

Thanks!

You're Welcome.

Posted By: agood1
Date Posted: December 31, 2013 at 11:45 AM

ok, are you sure about a 10k capacitor? Thats pretty big and not carried at the local radio shack.

Posted By: agood1
Date Posted: January 02, 2014 at 9:42 AM

Worked perfect. Thank you very much for the assistance.

Posted By: davep.
Date Posted: January 06, 2014 at 9:54 PM

agood1 wrote:

Worked perfect. Thank you very much for the assistance.

It's so cool to see feedback regarding the final outcome. I'm glad you figured out your project, and are pleased with the results.

DaveP