Where to connect AFR sensor?

[rich006]

What's the easiest/best place to hook up my new AFR sensor? I suppose it should be powered when the ignition is in RUN, and of course it needs a ground as well. The sensor is right next to the starter, so I'd prefer to wire it in somewhere on the firewall if that makes sense.

 

[67Dart273]

Doesn't it come with destructions? Just what is this thing? Brand/ model/ link?

 

[rich006]

The instructions just say connect it to 12V and ground, and it needs to be on a circuit with a 10A fuse. I'm just wondering where on a 1974 Dart would be the best place to tie into a circuit that has 12 V when the engine is running. The wires pass through the firewall just outboard of the brake master cylinder, but there's plenty of length to reach anywhere.

It's an AEM Uego Wideband with a Bosch sensor. https://www.amazon.com/gp/product/B01NAJ8XZW/?tag=fabo03-20

 

[67Dart273]

OK here is the deal. There is only ONE switched 12V "ignition run" and that comes from the ignition switch. But that line is already over-taxed and is the reason many of these girls overcharge the battery--because voltage drop in that circuit under-feeds the VR and the VR ramps up the charging voltage "because it thinks" the voltage is low

So do yourself a favor and add a relay for the under-hood ignition loads. Find the "ignition run" line coming out of the bulkhead connector. This is dark blue most years but not always. This feeds the ballast resistor, the VR IGN line, the alternator field, and smog devices some years.

Break this wire, now you have the "bulkhead" / firewall end and the engine bay harness end. Feed the bulkhead end (from the key) to the coil of a Bosch relay. Feed the relay contacts with a big fuse or breaker, say 20A from the "big stud" on the starter relay. Ground the remaining coil terminal, and connect the load terminal (NO) to the other end (engine bay harness end) of the "igntion run" wire. That point is also where you want to feed to your sensor.

Also pay attention to grounding instructions on that device. It can pick up electrical noise if not. I believe you want it grounded to engine block, "same as battery"

Bosch relay terminals:

85---coil ground
86---to switched 12V IE the cut end of the "run" wire coming out of the bulkhead/ firewall
30---to fuse and to starter relay "big stud" IE 12v constant power
87---to engine bay cut "run" wire harness side, and to your o2 equipment

74 Bulkhead connector

Terminal "N" no. 16 Dark BLue is what you want---what they call the "J2" circuit

 

[rich006]

Wow, that's a lot more detail than I expected, thanks! OK, so the idea is to use the J2 circuit as a switch while the main power comes directly from the starter terminal. I wouldn't think the skinny little wires coming from the gauge could draw enough current to reduce the circuit voltage, but the manual does say 1.2A.

 

[67Dart273]

It's not what you are adding it is that it's already "in trouble." Read the MAD article on bulkhead connector problems. Voltage drop in that path causes over-charging at the battery.

 

[rich006]

For now I connected it to the seat belt starter interlock. The male end of that connector is removed, but the female end is still dangling on the fender with a connection to J2 and another connection to what seems to be a common ground shared by the lights (I think the circuit diagram said S9). All I had to do was crimp spade connectors onto the AFR gauge wires and plug them into the empty sockets in the starter interlock connector. The sensor seems to work like that. I've ordered a Bosch relay and will do that project when it arrives.