Got Power? (Well, the electric kind...)

So as I’ve mentioned a few times, the previous owner of my truck (well, one of the previous owners...) popped out the original G63 and dropped in a 4G52 and a Weber. Now, there are a lot of things they didn’t do right, but a truck that runs and drives for $500 was a steal and I needed something to tinker with.
One of the things I’ve seen noted a lot of times in my reading here on the forum is “where to tap into the harness’ whether for the electric fuel pump or some other gizmo that needs switched (ignition) power. Regular old “battery” power is easy enough to do by adding another fuse block under the hood, but for those of you with a Weber and hence “limited” emissions worries (and thus little need for many of the sensors and widgets the old Mikuni needed), I thought I’d try to summarize some of my electrical systems findings from spending most of last Saturday tidying up under the hood.

First of all, on this 86 model, there’s a diagram on the inside of the hood showing where most of the various vacuum-based sensors and switches are... on mine, it was on the drivers’ side - sort of “above’ the battery.

You’ll find these sensors and switches scattered all over the fenders and the firewall and their presence may also have something to do with the options on your truck - A/C, automatic, cruise, etc... Since my truck is a stripper, no options were hurt in this process.

I could simply make them go away, along with the wiring that went to them (all were unhooked in my truck, confirming that - for my purposes, they weren’t necessary. (YMMV - “Check five times, cut once”). Any of the wires and plugs that were cut were also capped off and wrapped back into new vinyl sheathing on the harness.

As I went through this, I took the time to check all of them for current with the engine off, the ACC switch on, power while the starter was cranking, and with the engine running.

What I found, for my ‘86, was that there are at least six (roughly 12 ga.) black wires with a white trace that all carried switched (ignition) power, as well as the “second” starter wire (black with red tracer). Four of these went to one “gang” Packard-style connector which I kept live after removing the other “return” wires and their actual female connecters from the plug) The other two I simply cut and capped off and taped back into new vinyl wrap for the harness.

Now, before you go willy-nilly wiring up amps and driving lights and Lord knows what, understand that all of these run through the existing fuse box, so while I used the “second” starter wire on my truck for the fuel pump (this wire is unused on the 4G52 starter but present in my harness for the original engine) you obviously should research what goes where. In my case, I also used one of the wires in the Packard connector to repower the back up lights (now LEDs) as the lead for them had been taken out by the PO and simply seems to have disappeared and another for the two small bulbs in the (aftermarket) volt and oil pressure gauges. Yes, I could have tapped into the dash light circuit, but my own confidence in the headlight harness and its ability to handle any additional load, plus having to root around under the dash was offset by my belief that these now unused switched power circuits could easily handle two small bulbs which will soon be replaced by LEDs.

While I was at it,I was able to remove all of the (now rotten) vacuum lines and their various (broken) connectors - about three miles of these were in use for the sensors I mentioned earlier.

The overall effect of this, as well as cleaning all the grounds I could now easily find, was to lose quite a few nicked and fatigued wires (some of which did have power) and to make for a considerably cleaner underhoood experience. (I’ve already reported in an earlier post regarding the vigorous scrubbing I gave everything under the hood with “Bathroom” CLR and the hose).

I hope this helps anyone looking to find a small switched power source in their present harness and to not be afraid to lighten up the unused (or non-useful) pieces of their harness after a Weber install or even an engine swap. I strongly recommend you take the time to determine what you are cutting out (before it’s laying on the driveway) as well as (If you choose to use the existing powered wires) what additional load you are putting on the electrical system. Personally, I based my decisions on what I’ve learned here as well as studying the harness diagrams and my own multimeter testing. Don’t take my word as the Gospel - do your homework and make smart decisions.

But, the overall effect is a much cleaner looking engine bay with less clutter and fewer stray wires to cause more mischief within an already old harness.