Fixing PO's wiring hacks while swapping to BBC...

[K5_489]

As I'm new here, and no one else presumably knows me, I'll just preface this with - I am VERY familiar with electricity, and low voltage DC, in general. I work on security systems and radio systems for a utility company for a living. I'm just 100% how a few things are wired/operated in these trucks

Truck is a 1986 K5 Silverado, originally equipped with a 305. Previous owned replaced it with a TBI 350 long block, topped with an Edelbrock 1406, and hacked up a bunch of the wiring in the process (wires/sockets cut, then extended by twisting and wrapping in tape...a few wires were "lucky" enough to get wire nuts installed, lol). I do not know if it previously has had a wiring harness change to a different year/model, but I don't think so.

I'm now in the process of putting in a Mark IV BBC 489ci motor, and fixing a bunch of the wiring in the process. I eventually found a factory service manual wiring diagram for it, and have been slowly working through the harness, as many of the wires have been sitting out in the open, so they're well faded, and many had the factory plugs cut off. I've got almost all of it done, but am slightly unsure what to do with a couple...

1st - I have a wire that the factory service manual diagram calls out as "carb fuel solenoid". It shows battery voltage only with the key on. I'm assuming this was some kind of solenoid that physically cut off fuel flow to the factory carb when the key went off? Something which the Edelbrock wouldn't have, nor the I could put some heat shrink on the end of, shove up into some loom, and have it as a handy ignition hot wire for some future accessory use?

2nd - oil pressure switch/choke heater wiring - the pink/dk blue/lt blue combo. I understand this was to connect to an oil pressure switch that also only provided power to the choke heater element when the engine was running (which makes sense...no point in heating up the coil with the key on if the engine isn't running). I also understand that both the lt blue and dk blue wires are spliced inside the plug housing. However, with the plug not connected to anything, key on, I have battery voltage on BOTH the pink and dk blue wires. Based on the wiring diagrams, I wouldn't have expected to see voltage on the dk blue wire, which would be provided when the pressure switch closes. There doesn't appear to be any modification or crossing of either of these wires all the way back to the firewall connection.

Am I missing something here, or should I start digging under the dash for wiring modifications, presumably somewhere on that dk blue wire?

 

[Matt69olds]

The fuel solenoid was in the carburetor, also called a mixture control solenoid. It was supplied power from the ECM fuse, the ECM would trigger the solenoid by controlling the ground. With the solenoid unplugged, the metering rods were pulled up out of the jets, causing the engine to run full rich.

The choke heater was wired a couple different ways depending on if the truck was equipped with gauges or idiot lights.

 

[K5_489]

Gauges in the truck now, and being a Silverado model, I would assume always was a gauge truck?

EDIT: Wait...so that solenoid would have had a two blade connector on it? The wire I pulled was by itself, and it appeared, at least from the wiring diagram that I was looking at, that the solenoid itself was grounded through the solenoid body to the carb/engine. Of course, that doesn't mean that the PO didn't pull a ground wire out of the harness before I got the truck, as the connector body on this wire was also gone, though the terminal itself is still present.

Also, I'm assuming this ECM was a separate box from the ESC box, as I have the ESC box unplugged now. I'm using that harness as an entry point through the firewall for wiring to the coil tach signal, trans temp sensor & trans converter lockup clutch (I swapped in a full manual 4L80E about 6 months ago).

 

[Matt69olds]

I’m not real familiar with the truck CCC systems, they were pretty primitive compared to similar model year car CCC. The mixture control solenoid may have been a one wire setup. I’m pretty sure the TCC was a simple vacuum and thermal switch, and timing control was very limited.

Does the pink wire lose power when you pull the ECM fuse?

 

[K5_489]

For the time being, I just put the oil pressure switch in the port above the distributor, and ran that carb solenoid wire to one side of the switch. Other side of the switch I ran a new wire to the positive side of the choke heater, and then bundled & stufffed the factory oil pressure switch harness back up to the firewall.

I did find this post from last year from a thread where someone was trying to track down the choke fuse repeatedly blowing, that explains how that choke circuit works - https://www.gmsquarebody.com/threads/choke-issues.23875/#post-454300

It's basically like the alternator/battery light - depending on condition, the dk blue wire on the circuit is connected to one side of the choke light, which can act either as a hot or a ground, and now it makes sense that I was seeing power on both the pink AND blue wires in a key on engine off state. The same behavior would be seen on the field wire on an alternator. I just approached it from the wrong angle, assuming that the choke light was grounded in the IP, and provided power from the oil pressure switch. Which I now realize would have been backwards - if that was the way it was, the choke light would be on at all times while the engine was running....it was late, I was tired, and I was freezing outside...that's my excuse, and I'm sticking to it

I never had the choke circuit completed while I was doing all this, as I hadn't yet wired in the choke heater itself at that point, so I was still confused a bit as to why I never got the choke light to turn on, but now that makes sense as well - without the choke connected, there was never a path to ground for the choke light.

Yes to the TCC lockup - originally a vacuum electric switch on the firewall, vacuum thermal switch in the intake. Right now I'm just running the TCC lockup completely manually on a toggle switch in the cab, but I do plan on running it through the vacuum electric switch & brake switch as well once I get it back on the road to provide a slight bit more of automatic operation.

I remember playing with the ESC systems back in the day on my S-series trucks, and it was a rare occasion I made it a week on a new-to-me truck before ditching it in favor of a traditional HEI setup, lol.