I’m pretty handy with a multimeter. I should be. After all, I spent 15 months working at Bentley Publishers, writing an automotive electrical book for them. I would’ve sworn I wrote several “How to use a multimeter” articles for BMW CCA, but I can’t find them. The pieces I wrote for Hagerty, though, can be found here (introduction, measuring voltage, measuring resistance, measuring current, measuring voltage drop).
While there’s a lot of useful detail in those pieces, honestly, most of the time, when something electrical is misbehaving, and you whip out a multimeter to help you, what you’re using it for is to simply confirm the presence or absence of voltage at a device like a light bulb or an electric motor. Set the meter to the DC voltage measurement, hold the tip of the black probe against a good ground connection, and touch the tip of the red probe to the connector on the wire supplying voltage to the thing you’re testing. If the meter reads about 12 volts, then there’s voltage present there, so if the thing isn’t working, it’s likely due to something else, like the part itself being broken. Yes, it could also be the ground connection, and you need to verify that by checking for continuity (less than one ohm of resistance) between device ground and battery ground, but if that’s the case, most of the time, Thing Not Working + Voltage At Terminal = Thing Is Broken.
Still, there is another test you can and should do before you conclude that the Thing should be tossed in the trash, and that’s to wire it directly to the battery. Take a length of two-conductor lamp cord or even thick speaker wire, safely connect one set of wire ends to the terminals on the Thing (crimping on connectors is the best way), then temporarily touch the other ends of the wires to the battery terminals, paying attention to + and -. If the light lights or the fan spins, whatever it is that the Thing is supposed to do, then it’s good. Therefore, if it wasn’t working when it was wired in the car, the problem lies in the wiring, switches, and connectors.
It’s rare that these two tests contradict each other, but it does happen. And therein lies today’s story. I apologize for the lack of photos. It was found and fixed on the fly.
During a late fall drive in my red 1973 E9 3.0CSi coupe, I was nearly blinded when low sun came through the dirty windshield. I pulled over, tugged on the wiper stalk to fire up the windshield washer pump, and nothing happened—that is, there was fluid in the reservoir, but nothing squirted out the jets. When I got home, I tested it in the garage. Sure enough, I heard no sound from the washer pump motor. I dutifully whipped out the multimeter, set it to measure DC voltage, pulled the two connectors off the pump, slid the red probe into the positive connector and the black probe into the negative one, saw the presence of 12 volts, and therefore concluded that the pump was dead. Geez, what kind of world is this when all you get is 51 years out of a little VDO washer pump motor?
I have a box full of old washer reservoirs and pumps that I’ve accumulated over the decades. I pulled out another pump, tested it directly off the battery to make sure it spun, installed it, cracked the key, pulled in the stalk, and…
Nothing.
Hmmmmn.
I took the original pump I’d removed and tested it on the battery, a step I shouldn’t have skipped the first time. It spun immediately.
Okay, so two good pumps, neither of which worked when installed.
Let’s try this another way. I still had Hampton, the ’73 survivor 2002, at the house. I cracked its key, tugged on its washer stalk, and heard the pump spin, so I took out the washer bottle with the pump attached, brought it over to the E9, re-checked the E9’s washer pump voltage and ground with the multimeter, connected the pump back up, pulled the stalk, and…
Crickets.
What was going on?
I pulled the connectors off the pump to test the voltage a third time. This time, when I did so, the crimp-on connector detached from the end of the positive wire. When it did, I could see that it had been held on by just a single strand. All the other ones were broken and corroded.
A-HA!
So what had happened was that one strand of wire was thick enough to allow the minuscule amount of current that the voltmeter needs in order to read voltage, but when the motor was connected, it demanded a higher amount of current. Even though a windshield washer pump motor is tiny compared to say a radiator fan, the single strand of wire was too thin and therefore had too much resistance to support that much current. To use the water analogy for electricity, one person can drink water through a straw, but an entire city can’t.
I crimped a new connector onto the end of the wire, and the washer pump worked flawlessly. I then returned the pumps and reservoirs to their original cars.
I’d had this same thing happen once a number of years ago. A fellow came by my house asking if I could help him troubleshoot why his alternator wasn’t charging his battery. When that happens, the first thing to check is whether the alternator warning light comes on when the key is cracked to ignition, then goes out when the engine is running. This needs to work because that alternator warning light actually provides the excitation current to the alternator. If the light doesn’t come on when you crack the key, it’s usually either because the blue D+ wire connecting it is broken or because the bulb has burned out. There’s a temporary way around this—you can run a wire from the D+ connector and touch it momentarily to the battery to deliver the excitation current. More details about all this can be found here. On this fellow’s car, the wire and its connector looked fine, and we swapped bulbs, but the light still wouldn’t come on. I did a temporary jumper wire trick, and the alternator sprang to attention and began to charge the battery, but was still mystified as to why the warning light wasn’t working. It turned out to be the same single-wire-strand problem. I would’ve sworn I wrote about that, too, but I can’t find it either.
I did, however, write about the five different kinds of circuit failures, and the single-wire-strand problem is a subset of #5. High-resistance failures. You can find that article here.
So. When a device has voltage but doesn’t work, don’t throw it out until you test it independently. If it powers up with a direct connection to a battery using a good set of wires, look at the connectors that normally power the device. One of the links I posted above was to an article I wrote about voltage drop testing, the bigger umbrella under which testing for high resistance strangling the amount of current falls. But on a 30, 40, or 50-year-old car that exhibits this problem, it’s a good bet that you can actually see corrosion and/or broken strands of wire without needing a voltage drop test.
—Rob Siegel
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All eight of Rob’s books are available here on Amazon. Signed personally-inscribed copies can be ordered directly from Rob here.
