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Troubleshooting my low-beam relay was very enlightening

A few years ago, I wrote a detailed four-part series for Hagerty about how relays work (you can review those here: 1, 2, 3, 4). You’ll find other stories that provide more detail, but basically, a relay is a remote-controlled switch that uses low current to turn on high current by employing an electromagnet to pull together a pair of switch contacts. The general idea is shown in my primitive sketch below.


This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/06/10/troubleshooting-wiring-low-beam-relay

This was a great read. I recently had to check a relay when a fuel pump would no longer pressurize. Now I’ve learned more about the specifics of how a relay works.

One thing I would add to the story as the first step:

  1. Have you done any work that might affect the problem you’re currently having? If so, check that work first. :smile:

Very useful column. Will have to print and save it. You do show promise too as an illustrator.

A poor ground seems to be the cause of all sorts of electric gremlins. I run into ground problems occasionally working on old motorcycles and they are always puzzling. The hardest part, on cars, is finding them all. Once you do if you can clip a jumper from the ground fitting to the negative terminal on the battery, that’s usually diagnostic.

Nice article, but trim those nails before the next photo shoot.

Good article but it’s always better to start at the end and work backwards:
check lights, check grounds, check connectors, check relays and then check switches as the switch is usually the hardest piece to access. Also, American cars don’t have a standard for relay pin designation so a Service Manual and wiring diagrams are essential for finding where the critters are installed and locating the ground points and correct pins in the connectors

Electric Gremlin? Hmmm. Now there’s a project!

A dim headlight is a sure sign of a loose, open or dirty ground. As he had cleaned all the grounding connections, he should have coated each with an anti-oxidant compound to prevent further corrosion of the ring lugs. When two dissimilar metals are joined like this, electrolysis sets in (however gradually) and the weaker of the two metals will corrode. In this case the copper or the aluminum of the ring lugs against the steel of the frame of the car. The anti-oxidizing compound will prevent this and keep air out of the connection. The fewer the air gaps, again however small or minute, the better.

I should have said grounded connections…

Wiring diagrams are gold, NOT the enemy. Of course it depends on who did it, rough pencil sketches can be misleading. I think too many builders today want relays on everything. How did older cars ever live without em? You’d think the highways from the 30s to the 70s would look like a nuclear apocalypse, cars smoldering every few miles on the side of the road. No? Well, surely I digress, but too many relays equal too many sources of trouble. And too, as was found here, they can give up at the worst possible time. I think racers needing relays for high current things like 500 GPS fuel pumps and electric fans transferred the thinking to restorers and rodders from coast to coast. Not to be a hypocrite, I have 2 in my upgraded 39 Ford, but I’ve been told i need as many as 6 (!) for everything from the heater motor to the amp in the trunk. Uh, no thanks.
I’m sure I’ll be just fine without, and nice tech on how to sort it out.

This is very helpful and timely. The high beams on my recently acquired 1979 Fiat Spider 2000 (2.0L 124) come on but very dimly. The word on the street is to check the ground connectors first. Having this article’s info gives me a lot more confidence to move forward it if isn’t the ground connector. And I now know much more about relays whether in lighting or other circuits. Thanks.

I found that when working to flush out the electrical gremlins from my 1972 Mustang Mach 1, the original OEM wiring diagrams proved to be a how the Mustang was originally wired but not how it was in its current state. The horn didnt work, the turn signals didnt work…the stereo wiring was jury rigged. Cleaning this mess up was a nightmare. I replaced the turn signal switch which included the column wiring. I wired it up back to the OEM configuration set in the original electrical wiring diagrams. The first turn on the key and smoke billows out from under the dash. Whoever owned thus Mach 1 before me did a number on the original wiring and it was far from original. Am currently spending a chunk of deneros to get this beauty completely rewired.

Bad connection to ground should be one of the first things you check. Especially when you have done something in the past.
Boat trailer lights seem to be very prone to this.

Worst problems are the ones where it is intermittent. Like a wiring harness going to a door for power windows and locks. Insulation can look fine, wire is broken inside. Opening and closing the door can randomly make or break the connections.

I couldn’t disagree more that wiring diagrams are your enemy. If I have a wiring diagram there’s nothing I can’t fix in whatever circuit I’m working on. If nothing else, color codes can be used to trace the wire. They’re also absolutely critical to find out what some back yard mechanic did 5 years ago. With that said, I found your article well written and I’m sure it’ll be helpful to a lot of people.

Even factory mechanic’s wiring diagrams can be crap. In the early eighties, I had a '77 XJ-6 that developed a problem: the car would start and run, and, a few minutes later, die. usually in an intersection. A few minutes later, it would start and run for a while longer, repeating the process until I got it home.

I spent hours after work each day for 3 weeks, trying to run down the problem with the factory wiring diagram used by the local Jag dealer’s mechanics. Then, as I was checking the fuel pump relay against the wiring diagram for maybe the fifth time, I noticed a little wire coming off the bottom of the terminal that was NOT on the diagram. Curious, I followed this wire into the bowels of the engine bay and low and behold, discovered that it was attached to a second oil pressure switch, wired to shut off the fuel pumps if the engine lost oil pressure. And the insulation, of course, was shot, causing a short when the wire got hot. Replaced the wire, and it ran like a… 1977 Jaguar.

Moral of the story- remember the Lucas refrigerator factor when checking wiring on vintage British cars!

Many years ago I received some very good troubleshooting advice. “If it worked yesterday, but not today, then look for something simple.”
Also from high school electronics we were taught that clean electrical connections are necessary for proper circuit operation.
Remember your dealing with 12 volts DC.
Any high resistance connection due to corrosion
will result in faulty circuit operation.
My two cents worth.

Sorry, no, wiring diagrams are great. Well, as long as they match what is in your car, they are.

When I picked up my Lancia Delta Integrale in Seattle and drove it home, it overheated the first time I got stuck in traffic. After getting towed back to where I bought it from, we started going through it and, working our way through the wiring diagrams, found that the fan worked and the thermoswitch worked and the relay worked and the fuse was good and … wait a minute. Which fuse?

There was a cooling fan fuse in the main fuse block, but it turned out that there was another cooling fan fuse near the cooling fan relay under the glove compartment. The one under the glove compartment had fallen apart and the radiator fan worked as expected after that fuse was replaced, so problem solved. But what is the other cooling fan fuse for? Maybe some day I will figure that out.

Having owned lots of Lotuses and TR7s as well as an Alfetta, X1/9, Delta Integrale and Ducatis, I say that British wiring is better than Italian wiring.

My British experience was limited to the Jag, a TR-3A, and '65 MG Midget.
No Italian cars, so I’ll yield to you on that!

Thank you! Great article! I made a PDF out of it for future reference!