Schooled by a Flasher

I got an interesting call for help this last week that turned out to be very educational and a little humbling. A friend of mine called with what sounded like a simple electrical problem on a 1951 (yes, older than I am) Chevrolet Pickup truck. He said that he had installed a retro-fit turn signal system on it, complete with a nice chromed turn signal switch that mounts to the steering column with a stainless steel hose clamp, and replacement sockets for the lights on all four corners of the car to accommodate the new 1157 double filament bulbs .

So far it sounded simple and straight forward. Further, he said the system utilizes a typical “old school” 3 terminal flasher, which was wired in series with the battery and the “new” turn signal switch. He described the problem in a few simple words; “If you run the turn signals with the engine off, they work perfectly. But, as soon as you start the engine, they stop blinking, and stay on continuously.”

Well, I have diagnosed and fixed electrical problems in Cadillacs and Corvettes with a dozen computers in them, and built wiring harnesses from scratch for burn jobs, so I really figured, “How hard could it be to fix a simple 3 wire flasher circuit on a ’51 Chevy?”   That’s when the fight started.


When I got to his house and initially inspected the vehicle, here’s what I saw. The truck was unrestored, but in great shape for its age. Along the way in its 64 year life, it had been “updated” in a few places, getting a nice Chevy 3 wire alternator in place of the old generator, and a “hot-rod” intake manifold and 4 barrel Quadra-Jet, which looked great, but is obviously overkill for the old straight six engine, and a few other minor improvements. The new wiring that was added for the turn signals, was very clean and professionally run, and the rest of the mostly original wiring was in excellent shape, considering its age. The only odd thing I noticed is what otherwise looked like a typical generic 3 wire flasher had an additional pigtail sprouting from the top  labeled, “Ground,” and, it was securely connected to a good chassis ground.

So, I set out to duplicate the concern. I climb into the driver’s seat, and flip the turn lever to the left. Instantly, I was rewarded by the traditional “Click-Clack” of the flasher and I could see the signal bulbs’ reflections blinking evenly and steadily. So, I reach over and start the engine. Immediately, the flasher stops clicking, and the bulbs just stay on steady.

My first inclination was to do a few basic voltage checks. Using a trusty Fluke 87, I tested for voltage at the input terminal of the flasher; 12.7V with engine off, 13.8V running. Normal! Next, I did some voltage drop checks on both the feed and ground sides of the various circuits, and found everything well under .5V both with engine off, and engine running.

Suspecting that maybe some spurious AC voltage was worming its way into the system from that old non-stock alternator, I ran an AC ripple check (around .6V AC under load), and then disconnected the alternator entirely. Still, whenever the engine was running, the turn signals refused to flash. By then, I was sure I knew what the problem was, but since we had the equipment handy, I performed one more quick test.  I disconnected the output terminal of the flasher, and substituted an old 12V sealed beam headlight bulb to simulate the load of the turn signal bulbs, but effectively eliminating all the vehicle’s wiring, and the newly installed turn signal switch. Key on, engine off…. Blink, Blink, Blink, just like normal. Then, immediately upon starting the engine, the bulb stopped blinking and stayed on steady.

I came out from under the dash, and said, “I know what’s wrong. You need a coil wire!” My buddy looked at me as if I’d lost my mind. But,  convinced I was right, we ran off to NAPA for a nice new $8 coil wire and installed it on the old straight six.

We hooked everything back up like it was, turned on the flashers, started the engine, and the problem was gone. He looked at me for a second time with that confused, “you’re crazy” kind of look.

Well, that explained the extra little ground wire on the flasher, and taught me something new. It turns out, that the manufacturer of that retro-fit turn signal system used to include a nice heavy duty standard 3 wire flasher. But, since the proliferation of LED light bulbs, people started to complain that the turn signals would never flash if they used LED’s. So, the manufacturer dutifully started shipping an electronic “load independent” flasher that uses a conventional little relay inside to flash the bulbs and provide the desired “Click-Clack,” but is driven by a solid state sensing circuit and what looks to be a typical 555 timer IC to provide the on /off delay times. But, they chose to house it all in a plastic case, the same shape and size of a typical old bi-metallic flasher, so you could still mount it into a conventional flasher bracket clip under the dash.

The rest is partly speculation on my part, but based on observed fact. The old coil wire had developed extremely high resistance, and although the engine started and ran just fine, required substantially higher voltage from the coil. This higher voltage, combined with the coil wire’s high resistance caused a dramatic increase in radio frequency interference radiated from the ignition wires. The unshielded 555 timer IC in the flasher had its circuits scrambled by the flood of RFI ignition pulses, “confusing” the timer. Since it could no longer “count” out 1/2 second intervals, it never triggered the flasher’s relay, leaving the bulbs on all the time.

If I’d have known, I could have just wrapped it in tin foil. (Insert mischievous smile here.)