Technical Methodical methods for troubleshooting electrical systems

I am trying to help my dad troubleshoot his electrical gremlins.

He has a 36 pickup with an OT engine setup. After sitting for a few days, the battery is drawn down. I disconnected the battery positive lead and used a light to check for draw and it definitely has a draw.

I started pulling fuses one at a time trying to isolate the circuit, but after pulling every single fuse the draw was still there.

One thing I did note was that if I touched the battery post to the terminal it would arc one time only and then the draw would stop.

Help me figure out what is going on here.

This thing has no extras, just basics. It has an electric fuel pump (in tank) AC and lights, that's it. No stereo, no power windows, no power locks etc.

My dad won't drive it very far because he is afraid of it leaving him stranded. He's 78 and needs to be cruising instead of worrying......


Thanks,

Root

 

Has it got an alternator? Sounds like one of the diode packs has gone AWOL

 

Something is holding juice until it's "touched".... An alt can have a bad diode, a horn relay can have the fritz, a stereo will play games........Just keep digging a lil deeper. Start disconnecting hard parts, and keep checking your draw. you'll be surprised at the things that are working "normally".

 

It does have an alternator. Had not thought about that.

 

always test with the circuit in tact, opening a circuit can cause a fault to temporarily disappear. get an inductive amp meter and put it on the earth cable near the battery, then do the fuse test again. if that gives you no definite results then start disconnecting "hard" circuits.

 

He's gonna pull the alternator in the morning and take it over to a local alternator shop. That makes the most sense to my feeble brain.

 

Use a meter on 10A scale, pull the neg battery cable, put the meter in between the cable and battery. 50 milliamps or .05 amp or lower is an ok draw. Pull fuses, pull power cable from the back of the alt, ign switch, a/c clutch or relay, fuel pump or any fuses that control them......do it one at a time watching the meter.

 

Keep in mind that not every circuit may be fused or obviously fused. A bad alternator regulator could be the problem, but if it's not, check to see what may still have power; things like lighting, starting circuit, power to the ignition switch, etc. If you have any capacitors installed anywhere for 'noise', try disconnecting them

 

Root,I hope it's a problem with the alternator,my car was doing the same thing and all the sage advice said alternator,Mine checked out fine.

I now rely on a kill switch. HRP

 

I have changed the ignition switch recently, because it was acting like a bad switch, but that really didn't have any effect. It had a hesitation between switching the key to the start position and the actual starter solenoid engaging. I added grounds etc.

 

BTW an alternator can still check out ok for charging if it's drawing power, the shop needs to check the diode pack and use an oscilloscope to check the waveform.

 

I am having the same problem with my 27 roadster , no extra circuts at all , I am going to disconnect the alternator
and see if the battery goes flat after a few days .

 

If you have anything protected by a fusible link, pull that and check as you did with fuses.

 

Did you check for draw after removing alternator?
Back to your thread title; there is a lot of helpful info here:
http://www.jalopyjournal.com/forum/threads/how-to-use-a-multi-meter-volt-meter.960762/

 

Rfraze, thanks. No I didn't. I haven't talked to him today. I'll check in.

 

Wow,78 and still cruising!!!! Gives me hope I'm 64 and just now having a garage built @25000bux!Hope I live long enough to finish n drive my projects.

 

Well, after disconnecting the alternator, the draw continues.......now what?

Can a bad relay or a sticking relay cause these symptoms? He has the gear reduction starter that came of the later 90's SBC. It sounds like it is dragging to me. I was suspect of a solenoid having some draw.

 

Sure. It's not likely, but you can unplug any relays and see if it stops.

It sure is hard to troubleshoot someone else's car over the internet, through a third person...

 

I get that Squirrel, but I can't afford to feed you and put you up in an apt.

 

Interesting problem.

What conditions have to be met to make it arc again????

Just wait??
How long????
Start tries???
Connects/disconnects???

 

Good questions above.

Something else to check: How about voltage in and out of IGN switch poles? The only pole that should have any power is input to switch. You may end up disconnecting all wires to and from IGN switch and checking each to be sure they go to correct pole. One thing you should always check is that wire to coil stays HOT while in START position (ACC side should temporarily have no power during START) START pole should only have voltage when switch is in START position.

I don't think that starter should have any ignition bypass wire, so if, for some reason it does, that could be the problem. START wire to solenoid should have no voltage (at either end) until energized by switch. Voltage at starter end of that wire should be close to battery voltage during cranking.

 

At this point, I'd start at the battery and work my way back. Disconnect each wire from the battery at the 'away from the battery' end until your draw disappears. If it doesn't disappear, then you probably have a wire with a bare spot on it or a connection rubbing against something. Disconnect the other ends to identify the bad wire/connection. If it does go away with the 'away' end disconnected, now you know which 'sub-system' part of the harness has the problem. Reconnect that wire, and start disconnecting 'downstream' wires from that connection to localize it. Keep 'following' the draw until you locate either the wire or component causing the problem. Not a fast or always easy process, but this method will locate the problem.

 

When the battery is disconnected, if I take the connector and read the draw across the terminal and the connector, using a light or meter, the draw is present. Once I touch the cable back to the terminal, it will arc and then the draw stops. I have checked it with a light and meter after discharging. It baffles me. That's why I was wondering about relays.

He has an old 80's Cadillac column in it. We have changed the factory ignition switch out. I haven't checked for any draw at the switch itself. I told him to check on the starter to see if he has any draww there first. I agree with the statements above that I should start with everything attached to the battery first.

 

what is the ot engine set up

 

Like Crazy Steve said, just start at point zero, disconnect everything and start from there. With only the ground cable connecting the negative battery post to the frame you should have no draw on the battery. Now, start connecting one cable back up at a time. You probably have 1 large positive cable connected at the positive post on the battery, then connected to the large terminal on the starter solenoid. You should remove any other wires at the solenoid at this point and only connect the positive cable to it. CHeck for draw, should be none, if there is, you've found the short. If there isn't, move on. Connect 1 of the other wires to the solenoid and follow that 1 wire where it goes. Check again for draw, if you have draw then you've found the short. No draw, move on to the next one. Etc, etc. You asked for a methodical method of trouble shooting, that's it. Start at ground zero and work you way out slowly, until you find the culprit.

 

Well, the draw isn't stopping or the battery wouldn't go dead. You MUST check for draws by connecting your test gear in series in the circuit; once the wire is reconnected the only way to check for a draw is with an inductive ammeter, and very few (if any) of those will read low enough. With the wire connected, your test lamp or meter are a parallel circuit and that won't read a current draw.

 

Does this thing have a computer in it??
If it does check all grounds associiated with it, case & circuits.
Go poking around on various connections with a voltmeter & see if there is voltage in places where or when it shouldn't be. You can't have current flow without voltage to push it.

 

You mention it has a/c, have you disconnected the a/c clutch? It sounds like your loading and unloading a relay or magnetic clutch staying engaged by changing your battery connection.

 

What about a interior light staying on. If you have a door jamb switch check to make sure you don't have one stuck. This truck has fuel injection doesn't it? you may have a hot wire all the time going to the computer.

 

It's a 1997 Vortec out of a C1500 pickup. It went through a house in the May 3 tornado, so he pulled the engine, computer and all the wiring out. This was way back before people made harnesses for engine swaps.