Carl Heideman
Carl Heideman
6/26/08 8:10 a.m.

When it comes to the electrical system on a car, a lot of people shake their heads in despair. Even experienced mechanics can shy away from electrical work for fear of the unknown. While some components do use computers and advanced circuitry, solving most electrical problems just isn’t rocket scie…

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friedgreencorrado UltimaDork
9/27/09 9:55 p.m.

Articles like this are why this is the best magazine for people like us on Earth. I'm still a little afraid of going through my Alfa (the PO replaced a lot of the GTV6 switch gear & gauges w/Alfetta parts), but at least now I know how to start looking.

boxedfox (Forum Supporter)
boxedfox (Forum Supporter) Reader
12/8/20 8:32 p.m.

In reply to friedgreencorrado :

If it's any consolation, you can't make it any worse than it already is. From my experience, the electrical systems on those things randomly release electrons into thin air even when everything is wired properly.

David S. Wallens
David S. Wallens Editorial Director
12/8/20 10:45 p.m.

Hey, guess what article (and article author) helped diagnose an electrical problem tonight? laugh

Thanks, Carl!

Vigo (Forum Supporter)
Vigo (Forum Supporter) MegaDork
12/8/20 11:54 p.m.

I would just like to say that we are lucky to live in a world where a decent multimeter now starts at around $12. 

Martt New Reader
12/9/20 9:02 a.m.

This is a marvelously well-written article and Carl has provided a resource that should help many puzzled and electrically-inexperienced mechanics and owners.   I know for sure that if I had carefully read, understood and absorbed Carl's tips 60 years ago when I got started working on cars, they would have saved me lots of time and having to learn from hard experience.   And tips like the one about the starting system 'finding' an alternative ground path via control-cable sheaths are marvelous.    My only caution for the inexperienced is, (1) read what Carl says carefully, probably several times; then (2) ensure you understand what the test is actually measuring.   Once the underlying logic becomes clear, e.g. putting the test light across the power and ground terminals of a device to determine whether its ground is OK or faulty, then Carl's advice will suddenly make great sense and you, the novice electrical trouble-shooter, will have learned a lesson that will stand you in good stead over the years to come.


David S. Wallens
David S. Wallens Editorial Director
12/9/20 12:54 p.m.

In reply to Martt :

Thank you for the kind words. Carl is the best. 

The M3 has a battery drain. It has had one since we bought the car. Figure we should figure out why. Carl's easy tests confirmed that, yes, it has a drain and, no, it’s not a few likely culprits. So now we're digging deeper--and more to come as an update once we know more. But, yeah, this is a methodical approach vs. simply throwing parts/money at the car. Tools use so far are shown above: test light, volt meter, 10mm wrench, flashlight. (Not shown: cell phone to call Carl.) 

Thanks again. 

californiamilleghia SuperDork
12/9/20 1:14 p.m.

Anything on changing tail light and turn signal bulbs to LED bulbs and  how the turn signals flash ?

Also that antitheft systems are always on , and always draining the battery a little , at least the older ones.....

fearlesfil New Reader
2/14/21 7:58 p.m.
  • When troubleshooting, don't jump around looking at all manner of things. Work logically from one end to the other (either direction works). E.g. with a bad start circuit (the small wire that operates the solenoid) first check for 12V at the start wire at the starter. If that's zero volts (and assuming the starter is probably good) move to the neutral safety switch, then ignition switch, and finally the fuse block. Between where the readings go from bad to good is where the problem lies. Things can sometimes (rarely) get weird, like when I went through 4 new starters and traced the above circuits and all appeared good. Checking the voltage at the starter told the story: it dropped from 12V (disconnected with key to start) to 5V (connected) when trying to crank. The starter manufacturer had the wrong solenoid resistance and it just drew too much current! I could have switched starter brands (the best idea), or do what I did: let the starter circuit (low current) operate a relay with a bigger fuse and wire (higher current) coming directly from the battery. Been starting well for many years.
  • Fuses: in most cases the fuse is there to protect the wire it feeds. You may sometimes find a wire diameter good for 10 amps with a device that only draws 2. The fuse isn't sized wrong, it's protecting the larger wire, not the end device. On some newer cars with 286 fuses in 4 panels, there may be some desire to also protect the end device. But those low amperage fuses usually tree up with other circuits to a larger fuse up stream. When you look at the schematic diagram you may see two or more fuses between the battery and the end device. Don't let that confuse you, as the logical one-end-to-the-other method still works.
  • Fuses 2: When adding a fused circuit, the fuse goes as close to the power source as possible, and again is sized to protect the wire. That way, if the wire is shorted anywhere along it's length the fuse will blow, saving the wire. When you find and fix the short, the wire can return to service.
  • With more modern cars, use a test light designed for "computerized" cars. These draw very little current and are less likely to damage a computer or other sensitive circuitry.
  • Once in my 45 years of troubleshooting, a wire DID break. Recently a knock sensor showed an open circuit per the DTCs. Doing a resistance (ohm) test from sensor end of the wire to the computer end (note the battery should be disconnected for most resistance testing) showed an open too. There was no visible damage to the harness, pins, tape or connectors. Using the sharp point on the tester to check along the wire's length, the open was located about 2" from one end. Spliced around it and it was back in business. I think there was just a lot of vibration there behind the engine, and after 13 years of very small but repeated bending it just gave up.
engiekev HalfDork
4/21/21 7:57 a.m.

I need to get a powerprobe! They help a lot with being able to test circuits and power them up remotely to diagnose issues at the sensor or part.

 That, in addition to a good multimeter should be in everyone's electrical toolkit.

For really advanced stuff, an oscilloscope is also useful. PicoScope makes an affordable USB 2 channel scope that runs on any PC, well worth the cost of $115.  WAY more portable than a bench o-scope.


Scott Skillman
Scott Skillman New Reader
4/21/21 4:19 p.m.

One exception to wires do not break is the NC1 Miata built from 2006 - 2008. The engine wiring harness can create lots of codes that are not fixed by replacing components due to bad wires in a loom. For me is the MAF sensor and O2 sensor heating circuits, and then a dead car. 

All was good once we replaced the wiring harness.

MrSmokey Reader
4/21/21 4:49 p.m.

I have seen breaks in wires where the insulation is completely intact and looks fine ... probably not very common occurrence but is can happen ... those are really hard to spot

ShinnyGroove (Forum Supporter)
ShinnyGroove (Forum Supporter) HalfDork
4/21/21 8:50 p.m.

It goes without saying that you should always disconnect the battery before doing electrical work, but a lot of people really don't think through how to go about doing that in the most safe manner possible.  The positive terminal is the one that can hurt you, but only when you're part of a circuit leading back to the negative terminal.  The negative terminal is connected to the chassis of the car.  If you're touching the positive terminal and another part of you touches the car's chassis while the negative terminal is hooked up, bad things can happen.  My protocol:

- always be the one-armed electrician.  If you've got B+ on one hand and B- on the other hand, the current goes right through your chest and heart.  Work with one hand, keep the other behind your back.

- disconnect the negative battery terminal before disconnecting the positive terminal.  That way if you short out B+ to the chassis you still have a dead circuit.

- when you're done working, connect the positive terminal before connecting the negative terminal.  That way you don't have to handle B+ when you have a live circuit.

OHSCrifle GRM+ Memberand SuperDork
4/22/21 5:08 a.m.

Terrific article!

For those who learn better by watching .. If you want to watch somebody troubleshoot and make use of wiring diagrams with nice clear explanation (and good humor) along the way, seek out Eric O from South Main Street Auto on YouTube. He is a master diagnostician in the land of rust. Example:

GCrites80s HalfDork
7/8/21 9:51 p.m.

I've been reading this one lately to help me find an amp draw. The words "Alternator Over- or Undercharging" (under the "Slow Battery Drain" headline) should be headlined for clarity as it seems was the intent of the editor. Editors are the ones that headline things, right?

GCrites80s HalfDork
7/10/21 8:32 p.m.

I am trying to find a drain on my IROC-Z. So far I have done as the article states, using a test light connected to the negative terminal. I have disconnected each fuse in the block one by one and the test light still comes on. I have also pulled the inline fuses to all three circuits that were added later (amp, radio always-on and alarm) at the same time. I also disconnected the alternator cable and the test light still lights. Then, in case there were two drains I started pulling two fuses and/or circuits breakers at a time of things that are most suspect -- lights (ACC), hatch pull-down motor, power antenna, power windows/locks, blower motor, power ACC. Test light still comes on. This car does not have a relay block, but rather individual relays in different places under the hood. But of course, those should get nothing if their corresponding fuse is pulled, correct?

I should mention that this drain is not very severe. I let the car sit for over 48 hours with the negative cable connected and the car started fine and only charged at the normal 13V. But after 10 days of sitting last time it did not crank and had to be charged for 1.5 hours to crank weakly. But it did start and the car began charging at 18V then slowly dropped to 13V as I drove.

GCrites80s HalfDork
7/10/21 9:11 p.m.

OK, now reading the M3 link David posted and am going to try those steps. My car doesn't have nearly as many fuses nor that resistor or a power actuated fuel door but there is good SCIENCE! in the link.

David S. Wallens
David S. Wallens Editorial Director
12/9/21 10:44 a.m.

Some additional content from our sister title for tracing those electrical gremlins:


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