Lcarter114

We just bought a 2008 3500 Express Extended passenger van. In order to make it more comfortable for my wife to drive and for safety we decided to add a backup camera and side view cameras. I installed a new stero and wired the backup camera. I am using a video switcher to control which camera gets displayed to the stero. I have run 12v to 3 relays and was planning on turning on the relays using the turn signals and backup signal. For the backup camera I used a fuse tap of the backup fuse. For the turn signals I tried coming off the signal wire from the switch on the column. The wire from the switch is normally closed which is not a problem(not perferable but will work) The problem is I only get 11 volts and it will not switch the relay. Does anyone have any suggestions for how to make the cameras work off the turn signal?

mountainmanjoe

It's actually normally open, but it's not active-high, which is lucky for you that the relay wasn't energized. The signal switch circuit (dark green-left tan-right) is grounded (goes to 0V) when activated. DO NOT hook up a relay to this low impedance circuit without protection, or you will fry your vehicle computer and have a $200 problem on your hands.

To do this properly, you'll need to build a circuit with several components. If you want, I can sketch you a quick schematic. If you want to keep it simple, use a manual switch.

Lcarter114

That would be great if you could send me the schematic. I guess the next question is how will I know if I damaged the computer. I used a fused circuit but tried to hook the relay up temporarily to see if it would work. I still have the van apart but there doesn't seem to be any issues.

mountainmanjoe

I don't mean over current protection.
The coils inside relays generate huge voltage kickback spikes that can fry digital electronics such as the computer. The spikes need to be suppressed.
If your turn signals still work then you're probably OK.

Lcarter114

Thanks for the schematic that is great information. I checked everything on the van and it is good no problems. Whew! I have another question just for future reference. In the fuse box there are several empty spots that have voltage when I check it with my meter, however there are no clips to hold the fuse or allow the fuse to get voltage. Does anyone make inserts that can be put into these spots to make them usable, or does it have to be done from the backside of the fuse box?

mountainmanjoe

I'm not aware of any inserts. I only know about piggy-back fuse taps.

I just realised you said TWO side cameras. So instead of tying the two transistors together like I drew, you would keep them separate, each with their own relay for each camera.

dberladyn

That seems too complicated. I would not do that. In fact, I think GM even recommends against it. By adding what you have, you are changing the resistance of the BCM Input. I am too lazy to draw it out, but I believe what you are suppose to do is feed the relay positive power all the time and simply switch the negative lead. In regards to the BCM, you are to add another relay for it's signal as well so that it gets a pure negative ground without chance of feedback, or a change of resistance values.

I am sure your idea works and it is cool in it's own right, but for the average Joe stumbling upon the circuit, what GM suggests through what I wrote out is probably the easiest to make sense of.

mountainmanjoe

There's good reason I drew it that way. I think it's a simplest way to safely add a relay. You never mix digital and analog signals without a device like a transistor.

No. I specifically chose MOSFET transistors because they draw virtually no current. Adding them to the BCM inputs should not affect it.

Adding noisy devices like relays directly into a digital circuit is a terrible idea. That is why I isolated it with the MOSFETS.

That's why I drew a separate ground node for the relay.

mountainmanjoe

Now that I think about it, I don't think you need a relay. Relays are for high power devices. This is just to activate the video switcher, yes? Maybe you should describe how it works or post the model of it.

Lcarter114

I was using relay because I was using the same signal for the camera and the video switcher. The video switcher is a Nav-Tv svs-6. The instructions for the video switcher said to run accessory power to the camera and a positive signal to the switcher. I didn't want the cameras on all the time so I decided to use a relay to supply power to the camera.and video switcher and use the one signal to acti8the relay.