Technical - relays, why do they work when hit?

lowpugV2

Member
I have no idea about relays. Specifically the 306 rear wiper assembly and the relay in there. It works some of the time, I can hear it click when I turn the stalk switch. More often then not, no wipe. But, if I go round back, take the cover off and tap the relay the wiper swings into action.

I pulled the relay apart and it all looks ok, a coil, a resistor I think and also a tiny bit of corrosion on the copper coil.

Basically, how does this work? And can I fix it.
 

seasink

1000+ Posts
A relay is a device to allow a small current to switch a larger one. If energized by the wiper switch the coil, now a magnet, will pull into itself an electrically connected spring blade which normally lies against an arm over to another arm, switching the power for the wiper motor on.

The moving part is sticking, and the tap fixes that. If you can clean it, good. I wouldn't bother - new ones are very cheap. The internal circuit and standardised terminal numbers are usually printed on the case. Get another the same from most auto suppliers - simple ones cost less than $20.

This is typical (note diagram - it is normally open). Two terminals are for the low current switch circuit, and two (note code numbers) are for the high current.
relay .jpg
 
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lowpugV2

Member
A relay is a device to allow a small current to switch a larger one. If energized by the wiper switch the coil, now a magnet, will pull into itself an electrically connected spring blade which normally lies against an arm over to another arm, switching the power for the wiper motor on.

The moving part is sticking, and the tap fixes that. If you can clean it, good. I wouldn't bother - new ones are very cheap. The internal circuit and standardised terminal numbers are usually printed on the case. Get another the same from most auto suppliers - simple ones cost less than $20.

This is typical (note diagram - it is normally open). Two terminals are for the low current switch circuit, and two (note code numbers) are for the high current.
View attachment 132229
Thanks for the explanation, that helps me understand the function better. Attached is a photo of the relay in question. I haven't been able to find a match for a replacement on Google apart from eBay links from Ireland.
 

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jaahn

1000+ Posts
Hi lowpug :)
I always thought it was a poor contact and a hit just made the contact but who knows. That one looks like it has been damp and has some light corrosion possibly. Are you sure it does not have the same plug in arrangement as most others at your friendly auto shop ?? Go to an auto electrician if it is unusual and he will have a catalogue with the correct one listed.:cool: The numbers are usually beside the pins on the base.

Just two months ago on my trip home we free camped at the back of a country servo for the night. There was a Falcon playing around under the lights after they shup up. I went over after a while and asked if I could help him. His lights stalk was stuffed and he could not get the lights working, Only a young bloke with a long drive. I suggested he pull the top off the relays and we checked they worked OK by pressing the contacts closed. So we put a small piece of wood in to hold them closed. Then did the tail and parkers also the same. He went off a happy chap. ;)
 

schlitzaugen

1000+ Posts
Your relay is not as simple as Seasink thought based on your initial description (or lack thereof) so you can't just walk in the shop and hope to find a replacement blindly. You need to check the diagram on the casing and the terminal number and position on the casing you removed and match that.

If you want to give it a go at saving that one (looks possible to me), read on.

Those electronics are subject to some serious shaking in a car and often the soldering on the PCB goes "cold" so they don't make contact anymore. Add condensation and you have your answer. Tapping it can move the terminals minutely but enough to coax the relay back to life.

The coil itself and the moving contacts rarely go bad, but it is not impossible. If visual inspection doesn't reveal corrosion on the moving contacts I would suggest you need to reflow the soldering on the PCB (add a minute amount of solder and reflow all of them).

As matter of course, inspect the hinge point of the moving contact and make sure it's healthy. Clean with isopropyl alcohol (brake cleaner for instance) and apply some D5 spray to protect and lubricate. Don't use WD40 anywhere.

Also last but not least, clean the relay spades with isopropyl alcohol and a stiff toothbrush (or scotchbrite) and apply D5. This goes for the mating plug as well.

Relays are also used when you want to protect a switch (or whatever) or make its life longer (say because it is expensive) whereas a relay is cheap so you put a small current through the switch to control the relay coil and let the relay take the abuse (large current). It is also often more convenient to have a long wire going to a switching element (say on the dash) and back to a relay very close to the load (the main consumer of power, e.g. headlights, cooling fans, etc.) so you keep power wires short to minimise losses.

Or you can use a relay to control one circuit with another but keeping the two completely separated (no electrical connection between the two).

DeoxIt D5 (Altronics, Jaycar, etc) - this actually works even without mechanical cleaning of contacts/electronics:

https://www.mektronics.com.au/pub/m...c7311230da32edc21fe9c686a0b/d/e/deod5s-6a.jpg
 
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seasink

1000+ Posts
It looks as though it has intermittent wiper control built onto to a standard on/off relay. The diagram and numbers on the cover may show this. These relays are also available at auto electrical places.
 

schlitzaugen

1000+ Posts
It looks as though it has intermittent wiper control built onto to a standard on/off relay. The diagram and numbers on the cover may show this. These relays are also available at auto electrical places.
You're much better than I. Just had a look at the picture (opened) and you may well be right, but I would expect at least two transistors or some small IC for that function. Probably hiding behind that coil.

That relay looks crusty as hell and it's exactly in the hinge of the moveable contact AND the relay spade terminals. Neither helps a lot. Give it a good scrub, re-solder the PCB and I bet it will work fine for a long time.
 

Peter C

1000+ Posts
Yes, rust and electricity are never a good combination. Needs a thorough clean and a spray with Deoxit or Servisol Clean And Lube.
 

schlitzaugen

1000+ Posts
Yes, rust and electricity are never a good combination. Needs a thorough clean and a spray with Deoxit or Servisol Clean And Lube.
Yeah, makes us all kinda look like fools when trying to help with a problem like this and it turns out we're trying to diagnose a crusty old piece of junk anyone would at the very least clean before even wondering why it's not working properly. Lack of understanding how things work is only secondary to understanding that rusty things old or new can't work properly and you can't fix them by tapping.
 
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The Gonz

Member
I really can't add anything to the already excellent advice given. Understanding that a mechanical relay is definitely not solid state means accepting that the moving parts are going to be the most likely cuplrits, along with environmental effects on the electronics, such as corrosion and dry (cold) joints. I would certainly dive into some scrubbing before anything else.
 
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