Head Unit Wiring

[marret]

Looking for a wiring diagram or information on how SMB wired the head unit and switch to choose between van battery and house batteries.

My head unit has a bad or loose connection for the van battery. It works fine when the switch is on house batteries, but has intermittent power when on van battery.

With the switch on van battery, the head unit started acting up by cutting out and restarting when I hit a big bump. Didn’t do it every time, had to be just the right bump. Then it stopped working most of the time when on van battery. Works normally on house battery.

I will likely put in a new head unit, but I want to resolve the van battery power issue at the same time.

Thanks.

[gahamby]

Is this a manual switch?

[Scalf77]

For the start battery they generally cut into the power going to the stereo, this will the go back to where the switch is (most likely, or at least it was on mine). That wire will go to the normally connected input of a relay (87a) , the output of that relay will go back to the other part of the cut wire (input power). The switch will just activate the relay and take the radio input from SMB fuse panel, to relay output (30) to the radio.

Being that failing part is when on the starters, I would suspect the connection to the relay input (87a). The connector at the relay would be the most the expected failure point. It could of course be a damaged wire from behind the stereo to the relay.

[marret]

Originally Posted by gahamby

Is this a manual switch?

Yes, toggle switch. I think Greg answered for me. Thanks.

[marret]

Originally Posted by Scalf77

For the start battery they generally cut into the power going to the stereo, this will the go back to where the switch is (most likely, or at least it was on mine). That wire will go to the normally connected input of a relay (87a) , the output of that relay will go back to the other part of the cut wire (input power). The switch will just activate the relay and take the radio input from SMB fuse panel, to relay output (30) to the radio.

Being that failing part is when on the starters, I would suspect the connection to the relay input (87a). The connector at the relay would be the most the expected failure point. It could of course be a damaged wire from behind the stereo to the relay.

I was wondering if it might be a relay and I figured they must have rerouted the wiring as you explained.

Could you please explain 87a? Is this part of stock OEM wiring?

Or, is this a separate relay installed by SMB? If so, any idea where it might be?Like near either the switch or fuse panel, or behind the head unit (wouldn’t expect this)?

Thanks much.

[Scalf77]

It would be an added 12 volt relay, The pin out of a standard relay is 85 & 86 are the coil terminals. 87 is the Normally Open input, this will connect when 12 volt and Gnd are supplied to the coil inputs. The when what ever is on the input of 87 will be switched to 30 the output of the relay. Just like a switch.

In this case they will use a relay with a normally closed input (87a), it is just the opposite of the previous 87 input. When the coil is not energized, the input on 87a will be connected to output 30. When the coil is energized it will switch to open.

So when the switch is off, then the coil is not energized and path will be original power from the van. The SMB mod is to cut the control power that goes to the stereo. This is the one that is hot when they key is in run and accessory. This goes to 87A and then the other side of the cut line goes to 30.

This is a for a ford so, the fuses and connector will not match your chevy.


i show the coil being switched on the ground side, but it may be on the positive side.

Hope this helps

[marret]

Thanks Greg, much appreciated.

[Orv]

That number code is generic to nearly all aftermarket relays. I think it started with Bosch.

[PDXDave]

I'm having a similar yet different issue with mine after having the galley rebuilt by a local van shop.
As part of the job, they relocated all of the electrical from the SMB location to higher up on the wall to the left.

The issue is that I have no power to the radio when the house power is off. It works fine when switched to house, but not at all when powered by the truck batteries.

The water heater, water pump and radio switchover switches were moved and replaced and in the process the radio switch was changed to a three position instead of the original two position. I'm not 100% sure of what this would change, as there's three wires going into it, but I'm don't know the circuit well enough to guess what changed....

Thoughts?

[Scalf77]

Originally Posted by PDXDave

Thoughts?

Being as we are in the same city, your welcome to swing by sometime and I can help take a look. More efficient then debugging over the internet. PM me if you like.