Best 12V switch source for accessory

[70spider]

Hello,
Well I am trying to dial in my spider (carb) and decided to install a wideband O2 sensor and need an ignition switched 12v source. My spider is a '70. According to the website the Bosch 4.9 uego sensor has a 3 amp draw. I thought about running a relay and a toggle switch but I would probably forget to turn it off, tehe. Thanks for any help.

[spider2081]

I think if you have a test light to confirm you could check that fuses 9 and 10 have power only when the ignition switch is on. Fuse 10 has an unused terminal on the fused side. You could use a female push terminal and a #16 wire to pick up switched power from fuse 10's unused terminal.

[70spider]

Ok I verified that #10 is a key on 12v source. It looks like the power source is wired to the voltage regulator and the accessories that run of the double blue/black wire go to the engine fan relay and the to the charge indicator light. So my next question is how many amps does the fan relay draw? The fuse is a 10 amp and the O2 sensor draws 3 amps. If I run a 4 or 5 amp fuse between the fuse box and the sensor I should be ok? I'll be running the ground back to the battery.

[70spider]

So, I was doing some research on how many amps the fan might draw and it appears that they draw anywhere from 15 to 30 amps. With this in mind I decided to use a relay for the O2 sensor. Thanks for the help I think it should work out.

[spider2081]

I have never actually owned a 70 Spider but I think that fuse 10 would have been an 8 amp fuse only because most all the fuses Fiat used are 8 amps except the fuse for the horn and the coolant fan motor. The diagram I'm looking at shows fuse 10 only protecting the orange wire to the voltage regulator. The wire to the charging light is from the unfused end of fuse 10. My diagram does not show fuse 10 powering the coolant fan. It shows "hot all the time" fuse 1 powering the fan through a relay. Are you sure your cars coolant fan is powered through fuse 10??
To answer you question, by the wire diagram I'm referencing, Yes I think you will be fine with a 5 amp in line fuse using #16 wire to power your O2 sensor from the unused (spare) fused terminal on fuse 10

[70spider]

I got my pdf diagram from Mirafiori in it one of the light blue/black wires coming off the fuse box goes to the engine fan relay. I haven't physically checked it yet. As far as the #10 having a 10 amp fuse my bad it is an 8 amp. Here is a link the diagram I have been using: https://www.mirafiori.com/faq/content/6 ... MPLETE.pdf

[spider2081]

Yes that diagram differs from the one I have. The one I have shows the fuse panel has internal jumpers connecting the inputs of fuses 1&2, 3&4, 5&6, 7&8, jumped together. I am assuming the input side of fuses 9&10 are also jumped together as fuse 9 is switched power from the ignition switch. Fuse 10 on my diagram has no power input shown going to it. It does show an unprotected output feeding power to the battery warning light in the tachometer. It does not have the light blue/black wire shown in the link you posted. In the link you posted the light blue/black wire powers the fan relay's coil so the relay can be switched on or off by the coolant temperature switch. The fan's power is switched through the relays contacts and that power originates at fuse 1.
It is important to realize wire diagrams are for reference only and need to be confirmed with the cars actual wiring. Sort of like a blue print used for construction. After the building is completed there is a second drawing titled "as built"
The internal jumpers in a Fiat fuse panel are visible by looking into a fuse panel on the fuse side of the panel. Behind the fuses one can see how the fuses are connected together.
I have seen these same fuse panels used in VW and Volvo cars of that time. The same fuse jumpers are in their fuse panels.
If you remove the fuse panel mounting screws and drop it down you can check to see if fuse 10 has an empty terminal on its output side, The way the panels are designed the input side of a fuse has one terminal the output side of the fuses have 2 terminals.

[70spider]

I have confirmed that the diagram I have been using is correct in my car, at least for that circuit. I plan on using the open terminal on the output side for the O2 sensor. I checked the voltage with key out, no volts, and with the key in the on position, 12v. So to add extra protection I ran a 4 pole bosch style relay. The power for the relay is drawn from the alternator and protected with a 10 amp fuse, the O2 sensor gauge is then protected with a 5 amp fuse. I guess we will see, if all else I can just use a toggle switch with a big sign "Turn Me Off". Thanks for the help.

[spider2081]

I think its easiest to access the unused (spare) terminal on fuse 10 by removing the fuse panels 2 mounting screws and dropping it down a bit. While doing that I would be interested if you see if fuse 9 and 10 are jumped together inside the fuse panel. I would like to correct my wire diagram if the jumper is there.
I like relays. I have some friends who call me " One more relay, Dave" . I think your approach is great and very safe.

[70spider]

I would be interested if you see if fuse 9 and 10 are jumped together inside the fuse panel

I am not sure I couldn't tell, difficult to see with everything connected.
Well all went well as far as the electrics are concerned, the the O2 sensor gauge worked perfectly and nothing went wrong when the radiator fan kicked on. Now on to bigger fish, dropping the oil pan and checking for a spun bearing