Tips for electrical troubleshooting

I know that finding a short shouldn't be that difficult, but I really have no idea what I'm doing.

The last few days I've been trying to get my ls swap started and I've been having a bunch of issues. When I first wired up my ecu I didn't have any issues. I could connect with my laptop, control the fuel pump etc.

The ECU needs a 12V constant, which goes to the battery, and a 12v ignition which is on during accessory and cranking.

Here's how I wired up the car. I ran the battery cable to a power post. From the post I ran a cable to the starter. I also have the 12v constant from the ecu on the post, and the power source for a 12v switched Relay. I have a the relay powering a bus bar, which I have the ecu 12v Switched attatched too, and the wire that powers my relay board for the fuel pump/fan.

Now before I was getting the relay signal from the cluster lights in the fuse box. I could crank the engine and no fuses would blow, but I would lose power during cranking, so I had to find a different wire.

A little sidenote here. The whole time I was looking for a wire, when cranking the starter was slow, and acted like it was drawing a lot of amps from the system as all the lights would dim, or that the battery was low. However, while cranking at one point something happened and the starter actually started spinning fast. So now it acts like it has a charged battery and no starter issues.

So I found a wire that stays on during acc and cranking. But now the issue is whenever i turn the key on, I blow the 5 amp fuse going to the ecu. So I have a short somewhere in the system. Whats the best way to find it? I have a Snap on multimeter that I had to buy for school, so now i just need to learn how to use it. Thanks!

I'm not the best at this sort of thing, but if you've got a short, you've got something going to ground that shouldn't be. You might try setting your multimeter to check resistance (Ohms, or the Omega symbol) and then put one lead to ground and probe the wires in the harness. You'll probably get a reading on most of the wires, but you're looking for something that shouldn't be a ground but that shows very low resistance. That should narrow it down to a particular circuit.

I think.

Also, is it at all possible that you're not shorted out but need more than 5 amps? You could use your multimeter set to DC Amps along with a separate fuse holder to see how many amps are drawn on that circuit without the ECU. Or you could try a 15 amp fuse just to see if it blows that as well.

So, just to be clear, you're only powering relays with the acc switch, right? Not directly powering any devices?

Also, for initial startup & testing, perhaps you can run an isolated acc power to the ECU, and leave all the aux stuff disabled until you get the car started and idling?

Correct. The relays only receive power in with the car turned to acc.

In reply to wae:

The harness that's blowing the 5 amp fuse is the plug and play harness that came with the ECU. So I'm pretty sure it only needs less than 5 amp.

Start by checking all of your connections. Loose connections will cause higher current draw due to resistance, and can lead to blown fuses. Once your connections are good, start chasing circuits. Use the Ohms setting on your multimeter, which will allow you to check continuity. Basically, you're looking for a circuit that should be open (zero continuity), but isn't. You've got to do it one step at a time, between every connection in the wire. That way, when you find it, you'll have it narrowed down to the smallest area possible.

I'm hesitant to jump into e-diagnosing something like this. You'd be surprised how many problems there can be in just describing a problem accurately or what never gets said due to assumptions that turn out to be false.. but here's hoping we figure it out anyway!

Ok.. 5amp fuse to ECU is on the switched or constant 12v wire? Relay signal from cluster lights is to which relay? The one powering the bus bar or the one powering fuelpump/fans?

Starter spinning slow could be from a variety of things. Excessive resistance in the ground path wouldn't have caused all your other lights to dim due to battery voltage going down (or appearing discharged/'weak') because total current flow in the circuit would have been lower, not higher. Same with a bad connection across the solenoid or at the 'power post'. In general, when an electric motor spins slower its current draw goes up, so it could have been something mechanical making the engine hard to turn, like a failing bearing in the belt drive or something like that. The only way a bad connection or high resistance could have caused all your other lights to dim would be if ALL of it is grounding through ONE wire/connection whose voltage drop goes up dramatically with the starter current, making the voltage drop across all the other lights/etc go down only while the engine is cranking. Either way, that issue is conspicuously 'resolved' for now, but see if there's one single wire going to your battery negative that literally everything grounds through. Voltage drops proportional to all resistance in the circuit, so if something at the very end of the circuit starts dropping 3 volts it's going to make the rest of the circuit act like it's running on 9 volts.

As far as using a meter to find a short to ground or excessively low resistance, a little math goes a long way. Ohm's law says if you're blowing a 5amp fuse on a 12v circuit you're looking for resistance lower than ~2.4ohms that's causing the problem. If the circuit branches into multiple paths/connectors downstream of the 5amp fuse, one of the simplest things you can do is unplug one thing at a time while watching resistance to ground and see if one of the connectors/branches causes resistance to go up dramatically. If you tell me what all is downstream of the 5 amp fuse i might be able to give more suggestions.

Wouldn't suggest you try reading ohms with power applied either. You'll find out how good the fuse in the multi-meter is.

Yep, turn everything off. Draw circuit on paper. Like:

A +12v B fuse C relay D IGN switch E ECU F ground

Think of where there should be high resistance (ex: switch in off position) and where there should be low (ex: wire). Set meter to ohms. Measure AB, BC, CD, etc. If that all checks out then your components seem to be doing what they should. Now go A ground, b ground, C ground, etc. That will narrow in on your short.

But I bet the effort of drawing out your circuits will be enough to make you find the error.

In reply to Vigo:

The 5 amp fuse appears to be for both of the 12v constant and 12v switched power. Both wires are coming out of that fuse.

The the cluster light signal goes to a single relay which then powers the bus bar. From the bus bar I ran a 10 gauge wire to a fuse panel, which then gives power for the fuel pump and fans.

I have a ground cable going from the battery to the passenger head, and then from the drives side block to the fender.

Thanks for the other tips on using the multimeter.

I should also mention the starters back to cranking slow etc, so I need to do some troubleshooting.

I saw this recently. Thought it was a brilliant way to keep cables secure and bundled.

For the starting issue, google "voltage drop.". Using an ohm meter on a high amperage circuit is the very definition of wasted time.

IF the only wire hooked to your battery negative goes to the engine then by extension EVERYTHING on your car is grounding through the same wire that the starter grounds through. Therefore, if that wire has a voltage drop while passing a large current (cranking), then it would simultaneously make everything else on the car act like it was operating on low voltage. Set the meter to measure volts and measure from anywhere on the engine that you can get a good stab on, to the negative battery post (not terminal!). See how much voltage you measure while the engine is cranking (might want to disable the ignition so the engine doesnt actually start, and probably need a helper to turn the key). That's a voltage drop. Should be ~0.6v or less if everything in between is working well.

I prefer a test light for a lot of electrical trouble shooting, rather than a meter.
A volt meter does not load the circuit and will display "phantom" voltages.

My tip is, to not stab yourself with the pointy end!

Tip #2, someone who needs tips like this should not proceed without a manual and especially a wiring diagram.

So i've narrowed the short down to the plug and play harness. When hooked up I'm getting continuity from a power wire going to the crank sensor and ground. This only happens when the ecu is plugged in.

What would be the next step to narrow down where the short is in the harness using the multi meter?

When hooked up I'm getting continuity from a power wire going to the crank sensor and ground. This only happens when the ecu is plugged in.

Usually crank sensors would have a 5v reference voltage going to them on their 'power' wire. I've accidentally grounded this circuit on several cars and never popped an ECU or fuse, but i've seen a case of a grounded VREF wire to a cam/crank sensor actually killing the ecu to the point that it wouldn't communicate with a scan tool, so who knows. It would be surprising to me if the part of the ECU that steps voltage down to 5v could even flow enough current to pop a 5a fuse before that actual small part melted on the ECU. Ohms law says that 12v X 5a is 60w and 60w / 5v means you'd need to be flowing 12a through the little box that provides 5v to pull those 5a on the 12v circuit. I just think it's highly unlikely. Usually the 5v reference voltage circuit is shared with every other sensor that uses a 5v ref like the map and tps. Maybe try verifying that those wires see the same continuity to ground that you're seeing on the CPS vref wire.

In reply to Vigo:

I hadn't checked other sensors with a reference wire, but I had checked a couple of injector plugs. Their 12v wire was getting continuity also. I can check the other sensors over lunch and see what happens.

I don't mean to belabor the fuse issue, but it looks like you said a single 5A fuse protects both your constant 12V and keyed 12V. I'd pull the factory wiring diagram and confirm that value. It's hard for me to grasp that your entire EFI system runs on that small of an amperage value, though I admit I've never done an LS swap.

For what it's worth, I'm doing a swap right now with a much simpler EFI system (GM TBI). GM spec'd a 10A fuse each for my constant 12V and keyed 12V.

In reply to Cousin_Eddie:

From my understanding, the 5 amp fuse is only for the power going into the ecu. I can get some pictures of the Actual fuse box, which might be more helpful than me trying to write it out.

I actually gave efi source a call yesterday, and that's what he said, and to pull the crank sensor and see if it still blows, as it has 12 volts going through it.

The Five amp fuse is the one with two red wires and one white with red wire. The two 12v wires that power everything go to the relay.

With one probe hooked to the ground wires, and the other probing the 12v power wires, I found there's continuity at the battery wire going out of the relay. There's also continuity at all 12v wires at the injector plugs, and sensor plugs. If I unplug the ecu, there is no continuity except the ground wires.

That'a a Painless Wiring CirKit Boss

Seen here....

The fuse wouldn't be for both switched and continuous power because then they'd both be the same.

As I see it, disconnect the two red and one white wires. Turn everything on. Plug each one in and see which one blows the fuse. Then you've isolated the curcuit and you just need to find the ground. Or if the component it is connected to is not working properly and grounding out, or drawing too much juice.

The fuse only blows when both the 12v constant and 12v ign have power.

If anyone's curious to know what happened, I sent the ecu in to get looked at. Apparently the positive and negative got switched around at some point and blew all the cap's in the 5v circuit.