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Reverse Camera Won't Display When Engine Starts...

Posted by James on

OK, so we all know how frustrating something is when we think we've got it fitted correctly YET it still isn't working. So we thought we would blog about it! Reverse cameras: they are a brilliant accessory to your head unit and they are not expensive so it always make sense to add one to your vehicle. 

OK, so most of the time installing a reversing camera is dead simple. You wire to the reverse bulb, give some DC power to the camera itself and the RCA has a nice built in trigger wire along side it to be connected to the "reverse/back detection" wire on your head unit. Lovely Jubbly!

BUT when it's not working, questions are raised:

1) Is the camera faulty?

2) Is the head unit faulty?

We can honestly say, in the last 2 years we've not had one faulty camera returned. Some suspected as faulty, but when returned and tested they've worked perfectly well. With a head unit, the CAM-In port in on the RCA board so if that's faulty then pretty much all the other RCA connections will be too; again very unlikely. 

So it's got to be with the wiring, right? Well... sometimes. One thing that often catches customers out is when they extend any cables they are using the wrong AWG cable to do so. You want to use the same AWG cable if you ever extend. 30 AWG is the normal AWG on a reversing camera.

Now the most common fail with a reverse camera is when you're connecting the camera to a canbus European vehicle such as Vauxhall, VW, Audi, Skoda, SEAT, BMW or Mercedes. What you may find is that you test to see if the camera is working on AV-IN and it is! so... why when its plugged into CAM-IN, it's not? Very annoying. 

Sometimes, you may even find that the camera is working but as soon as you start the engine it stops displaying when you put the vehicle in reverse. A puzzler huh? Well, we are here to explain why this happens and most importantly, how to over come this.

Why does this happen?

The reverse switch on the back of the head units require a +12v connection into them, If there is not +12v then it won't switch. Basically on the canbus European cars, because they have a very integrate canbus system and the vehicles will often tell you when bulbs are out on the vehicle, the voltage on the reverse bulb will drop from +12v (on key turned to accessories) to below +12v, even sometimes as low as +9v, when the engine is started. Hence this is why the trigger for the reverse isn't working when the engine is running but when you turn the engine off and put the keys into accessories position, it may display when in reverse. The canbus system controls the voltage and ampage of the reverse bulb positive pole and because of the actions that happen when the engine is started, this is why it fails.

How do we sort this out? 

You will need a relay to basically convert the reverse bulbs voltage into a constant +12v signal. 

How do we install the relay?

Get yourselves down to Halfords, Maplins or any decent auto-electrical store. If you go to Halfords, their part number is HEF553 (item code 184005). The relay is around £7 or around £4 on trade card (trade customers only). The relay is a 4 pin relay and looks like this: 

Halfords HEF553 Relay 12V 30A 4 PIN


This relay is going to convert the reverse bulb voltage into a stable +12v voltage for our reverse switch on the head unit. We are going to need to wire the reverse camera like this:

Rear Camera DC plug (red and black) - The red wire needs to be connected to a +12v switch ignition source. The black to an earth point.

The rest, we will be using the relay.

Pin 87 - The red trigger wire on the RCA plug needs to be plugged into here and only here (the other end will be connected to the head units "reverse" or "back detection" wire)

Pin 86 - This is where your reverse bulb positive pole wire goes into (you would spur a wire off the reverse bulb positive pole and into this relay pin position)

Pin 30 - Spur a wire off the red DC power plug into here. This will now be a +12v switched feed going into the relay.

Pin 85 - Earthed to chassis.

Then job done. Give it a test and the camera should now display when the engine is running and the image should be nice and clear.

We hope this helps anyone with this issue.