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I want to install fog lamp on my Maruti Suzuki alto 2009 model, but it did not came with any provisions for such modifications so I have to do it from scratch, the problem I am facing is that I cannot find any relay from authentic manufacturer, all the relays in my local shop are cheap and not from any reputed brands and costs around 50-100rs (around 1-2$), but in one shop I have found a genuine Bosch relay for around 200rs (around 4$), the problem is that this relay says it is an horn relay, after researching in internet and also in local car mechanic shop about differences between horn relay and light relay some says that as horns are not meant for longer use they might wear out prematurely if used with fog lamps. So my question is should I buy this bosch horn relay and put it with fog lamps and not worry about it or should I get try to find proper relays. Also please inform if any other informations are required

Lamps are 2X35W. added image of the relay

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  • Please post lamps specification (total power consumed, eg. 2x30W) and relay symbol - if relay is powerful enough to continously provide sufficient current, it would be okay. My guess is - it won't be.
    – Mark
    Aug 4, 2017 at 10:27
  • thanks @Mark ,not quite sure about the symbol, will update it once I see it again in around 5 hours.
    – Nilabja
    Aug 4, 2017 at 10:34
  • Please provide the details of the relay - it should have on it the current rating etc.
    – Solar Mike
    Aug 4, 2017 at 12:31
  • @SolarMike I will update it once I reach my home in around 3 hours
    – Nilabja
    Aug 4, 2017 at 12:32
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    @Mark - While not a bad suggestion to the uninitiated, if it is a question anywhere near being a car question, EE.SE will send it here. I would not ask that question there and believe we are fully capable of answering it here.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 4, 2017 at 13:58

2 Answers 2

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A lamp relay is for continuous energizing of the relay coil. A horn relay may or may not be.

However, 2x35W is a low enough current where you would not need a relay, if your switch is rated for 10A or so.

One reason for using a relay is to reduce losses due to the (small) resistance of a longer electrical run.

If you provide the Bosch number, I could look it up. In general, my experience with Bosch relays is that they tend to not have interim duty relays.

Here is an example of a Bosch relay I use for fog lights:

0332019157 Bosch - Multi Purpose Relay; 40 Amp; With 4 Terminal Connector and Bracket $8

and here is a lower cost alternative: 141951253B Wehrle - Fog Light Relay; Multi-Use Relay with 4 Prong Connector; 12V/40A $3.46, but comes without a socket and bracket.

You may not find these locally, but I wanted to give you some concrete examples, and prices in the US, with US$.

Addendum #1 You included a photo of the proposed relay. The literature I have on that relay is limited, as it appears to be marketed in India and other locations, not in the US. It also appears to have limited application to horns, and I would therefore assume that the relay coil is not designed for 100% duty cycle. You might look for a different part.

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  • Get an H4 fog lamp assembly and use this wiring kit along with a proper relay. amazon.in/Thimson-H4-Headlamp-Wiring-Kit/dp/…
    – Lawyer Aidroos
    Aug 4, 2017 at 17:51
  • That is an EXPENSIVE wiring harness!!!
    – mongo
    Aug 4, 2017 at 18:34
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    But is expensive always the best? and will H4's fit the op's lamps? i always found those pre-made harnesses to often have poor crimps, cheap terminals and always too much length in the wrong place. I mostly made the wiring to match the vehicle and load specifically. Even down to two relays and two fuses - one per side etc
    – Solar Mike
    Aug 5, 2017 at 21:44
  • thanks for suggestion @NumairAidroos but mine fog lamps are not h4, and they came with wiring harness, my issues is not wiring harness but as everyone pointed out that I don't need a relay for these but still I wanted to use it because in my country price difference between 1.5mm2 and 2.5mm2 (need it because wiring harness is not long enough) wire is more than the cost of relay
    – Nilabja
    Aug 7, 2017 at 3:48
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Your total current draw is around 6 Amps for the lights. (2 x 35W = 70W/12V = 5.8Amps)

You could just control them directly with a rocker switch rated at 10Amp (or higher) and fused accordingly (7.5A - 10A). However, if you want to use a relay for the sake of keeping things clean and organized, then the horn relay you show will more than meet your needs.

A horn relay has no extra special magic in it that couldn't be found in any other relay that is rated for the same specs...12V/30A. It just happens to have a nice little Horn label on it so we can assume it will have a low current coil. The coil of the relay ( the part you attach the switch to #85-#86) is going to be rated for a low current like 12V/150mA max. This way you can have a small switch with small gauge wires, control a huge amount of current if needed.

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  • thanks, I will accept this if everything works out as you said
    – Nilabja
    Aug 7, 2017 at 3:54

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