Headlight "Combi" Relay - 111 941 583A, A Closer Look at a “Latching" Relay Options

[anderssj]

During a headlight check before a sanity drive a couple of weeks ago, I found that our car's headlights would not light up at all. The fuses checked good, but I couldn't hear the relay click and there was no power to the fuses themselves.

The relay sits on top of the fuse panel:

(picture courtesy of malcolm2 in this thread: http://www.914world.com/bbs2/index.php?showtopic=339629)

Our car is a 1972 914/4 and uses a 4-pin headlight/combination relay. Because it can be accessed more easily than the column switch, I decided to check the relay first.


In addition to all the schmutz, this picture shows the 4 pins/terminals: "56" (+ from 56 on the headlight switch), "S" (momentary ground "signal" when the steering column switch is pulled back), "56a" (switched + to fuses 3 and 4 for low beams), and "F" (switched + to fuses 1 and 2 for high beams). If this relay is working, it stays ("latches") in one position until it is momentarily "signaled," then it switches--and stays--in the new position until it is signaled a second time.

You can check the relay with 9V battery (like the one in your smoke alarm) and a multi-meter that reads ohm's (resistance)

1) place one lead of the multi-meter on terminal 56
2) place the other lead on terminal 56a - read the result on your multimeter
3) remove the lead from terminal 56a and put it on terminal F - read the result on your multimeter

One of two things will happen. Either 56 -> 56a will show continuity (~0) or no continuity (~1), or 56 -> F will read 1 or 0. If 56a shows continuity, then F should read no continuity. If pin 56a reads no continuity, then F should show continuity.

Next, momentarily touch two posts of the 9V battery to relay terminals 56 and pin S. You should hear/feel a click as the relay switches and latches. Use the test leads again. You should get exactly the OPPOSITE reading from what you received in the first pass. If this happens, your relay is fine and you have a problem somewhere in the wiring. If you don't get the exact opposite readings, you have a problem with your relay...just like I did. In my case, the relay didn't always click (switch), and I got no continuity between 56 and either 56a or F--even after cleaning up the cruddy terminals (IMG:style_emoticons/default/barf.gif)

If the relay doesn't work, it's time to open it up. Note where the terminals are in relation to the cover, then peel back the cover's bottom edges. Gently withdraw the guts and take a look:


This picture shows the relay latched in the "up" position.

When energized/signaled, the silver loopy thing is pulled down and latches the relay in the "down" position, opening the upper contacts:


Looking at it from the side, you can see how the lower set of contacts are closed when the latch is down.

There should be continuity between 56 and 56a when the relay is in this position...but none in my case. Dressing up the contacts with a tiny piece of wet-or-dry sandpaper fixed mine.

When the relay's latch is up, you'll have a similar view, except now the upper contacts are closed:

There should be continuity between 56 and F...but once again, none in my case. Another gentle application of wet-or-dry sandpaper and it was OK.

AFAIK, this is my car's original headlight relay--and only the second time it's failed in almost 51 years and nearly 245,000 miles (last time was in 2004). Pretty high MTBF--and it's repairable!

Hope this helps!

[Superhawk996]

(IMG:style_emoticons/default/aktion035.gif)

Nice write up. That’s the charm of these cars - so easy to fix stuff.

In a modern car, that would now be a black box module with SMD components. When it fails it would be tossed and a new one will cost a ridiculous amount of money for what little is in the black box.