Ignition power issues

[Itali83]

Hey guys, first I’ve got to b!tch about the GM factory wiring diagrams. I’m a 20 year ASE master technician and I’ve never seen such a Sh!tty wiring schematic setup in my whole career. GM’s “better” idea of not showing each wire from start to finish is an absolute disaster. I’m trying to diagnose a simple ignition circuit and I’ve got literally 4 pages of diagram, 3 pages of notes with wire numbers and start and end positions and splice numbers etc and I still can’t follow the G.D. Power flow. Ok, rant over, now I need help.

My Dad’s 87 Jimmy. 350tbi. Turn key on, everything comes on as it should. Cel, seatbelt light, volt meter, key chime etc. turn key to start and just a quick click and only the IGNITION powered things go dead. Some lights work, headlights/park lights work. But no CEL, seatbelt light/chime, no volt meter. It’s like you shut the key off that’s it. I load tested battery and it’s good. Didn’t have time to dive in but that’s where I’m at.

Basically, I’m looking for someone who actually knows if ignition power goes through ignition switch first, then to fuse box, or other way around. Which wire etc. I need to understand the ignition system power flow so I can easily pinpoint where I need to look.
Thanks in advance.

Ben

 

[WP29P4A]

Bad grounds do exactly what you are describing. Clean and tighten the connections from the battery to the engine, and make sure there is a ground from negative battery terminal to the fender or radiator support. Verify the ground strap from back of engine to firewall. You can use a set of jumper cables to set up a temporary ground to test with, if you want to do a quick test. Clip one end of the cable to the negative battery post and the other end to the engine, did anything change when you turn the key?

It's the most common solution to the problem you are describing and the cheapest and easiest thing to check for.

 

[SirRobyn0]

I second Mike's thoughts, but will also add, you Ben may already know this with your experience, but GM side terminal battery terminals are notorious for hiding corrosion where you can't see it easy and not making proper contact. Given the single click, then loss of power I'd be looking at terminals, starter connections, and grounds. I have quite a bit of old GM experience at the shop and to be frank I have no idea if the ignition power hits the fuse box or the switch first. I've never needed to find out, but you could check to see if there is power at the fuse with the key off. If there is then it hits the fuse first. If it only gets power with the key on then switch has to be first. If I was a betting man I'd say key first. Good luck.

 

[Matt69olds]

The old school electrical diagrams in the manuals leave much to be desired.

Power gone in thru the firewall bulkhead connector, goes to the ignition switch where it is “divided” by function. It then goes to the fuse box to power individual fuse buss bars.

One hood thing about GM wiring is they have kept the same color coding pretty much since the early 60s.

Assuming you don’t have a poor connection or similar problem, check for power on the orange wire from the ignition switch as you cycle the key. You may have a defective ignition switch.

 

[Itali83]

Found the problem. It was actually a poor connection at the positive terminal of the starter. Not the start terminal, but the battery terminal. I started poking around looking at things and found where I did and didn’t have power. I tested for power at the starter and when touching the actual stud, I had no power. But when touching the factory crimp terminal on the battery cable, had plenty of power. So I took the connection apart, wire brushed and truck started like new. It wasn’t corroded much at all. None of the green corrosion or anything like that. Just a light black coating on the terminal that was enough to break contact.

I’m really frustrated with the factory wiring diagrams because I had some time and really tried to make sense of the diagram and I could, sort of, but they really made thing much more difficult for people like me who need a clear picture of how wiring runs and not just a suggestion of where a wire starts and ends.

Anyway, just wanted to let you know I fixed the problem and how the system is split between battery power and ignition power.

Ben

 

[wlwarnke]

I hate the power feed on these trucks. Too many cables going down to the starter. When I added a 2nd battery, I rewired the entire power/charging setup. Now I only have 2 wires at the starter. Battery and Crank. Alternator output goes to the battery, alternator sense goes to bulkhead. Much cleaner and less cables to corrode or melt down at the starter.

 

[wlwarnke]

I also replaced the stock junction with a terminal block.

 

[Itali83]

I’ve totally updated my suburban with extra grounds and power blocks under the hood and in the dash to basically bypass and supplement the factory firewall bulkhead and undersized charging/starting wiring.

Ben

 

[Originalthor]

Gm ignition switch is also not a very good design and can cause all sorts of weird problems. Also if you add any kind of a msd coil or super coil put it on a relay. Cause the ignition switch is the power to the coil.

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[WP29P4A]

I also replaced the stock junction with a terminal block.

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I used this type of power distribution blocks, they seem to be good quality and work well. Client wanted to reduce the amount of cables tied to the battery.

 

[AuroraGirl]

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