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Reverse Light in Automobile Repaired

By on October 6, 2017
relay specification









My automobile was presenting difficulties with the reverse light from the beginning when my father bought it. The complaint was the reverse light worked rarely.

In the gearbox, which is located down the automobile there is a switch and its function is to send 12 volts to both light bulbs (2×21 watts) in the rear to indicate the reverse is selected.

Reverse light in automobile repair

The culprit of this situation was the switch located in the gearbox. When it broke it was replaced and so on. It happened many times. But one day I took a decision, not to buy more switches for three reasons:

  • It’s very hard to get in the market.
  • Every time you buy it the price is more expensive.
  • I was not going to buy it all my life.

I had to find out why it was breaking constantly. So I took a radical decision: to connect an intermediate relay among the gearbox and the rear light bulbs (2x21watts).

In this case I got a relay from an unused UPS (Uninterruptable Power Supply) as shown in the photo below:

relay spec

As you can see this relay could support some Amps per contact, and it has two poles with two contacts open normally, which were connected in parallel to improve the current (Amps) rate.

The other step was to calculate the Amps consumption by the Ohm’s Law.

P= U*I

looking for I:

I= P/U


P= 2 (light bulbs) * 21 watts = 42 watts.

Voltage = 12 volts (Battery)

I= 42w/12v= 3.5 Amps

Conclusion: I do not know how the manufacturer did not realize of this malfunction. This relay coils support the 3.5 Amps with no difficulty. This relay, already connected was hidden down the dashboard and it has been working for about 20 years without any damage. Mission accomplished.

This article was prepared for you by Humberto Rodriguez, one of our ‘Master Authors’ from Cuba.

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  1. Yogesh Panchal

    October 6, 2017 at 2:37 pm

    Good job!

  2. Albert van Bemmelen

    October 6, 2017 at 2:58 pm

    I understand that the gearbox switch was not functioning Humberto. And using a relay is fine to switch on the in-reverse lights. But since no relay switches on its own you still need to switch that relay?

  3. Waleed Rishmawi

    October 6, 2017 at 7:56 pm

    good job on the repair. but you are using my line..mission accomplished. lol

  4. Parasuraman

    October 8, 2017 at 2:51 pm

    Pretty good! Some ingenuity applied! I suggest you send the link of this article to the automobile manufacturer.

  5. Robert Calk

    October 8, 2017 at 4:25 pm

    Good job, Humberto. Did you wire a toggle switch to control the relay?


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