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Blown resistor in automotive latching relay

Hi all. This is my first post to this NG forum, so please be kind.

I am trying to repair a 12v latching relay for my Volkswagen Beetle (~1975). It controls the dimming of the headlights (hi/lo).

The trigger is a momentary ground signal (S) which actuates the coil. The arm that the coil pulls on has a plunger which toggles a rather complicated see-saw arm which moves a contact between posts 56a &

56b. Also attached to the arm is a contact which connects to both outputs (56a & 56b) while the arm is being pulled (I can actually only see where it connects to terminal 56b on the actual relay). The diagram may be a bit off since only one of the outputs is "hot" during actual operation.

The below diagram came from the relay casing. Here is a pic w/ the circuit diagram screened on the outside of the casing:

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I've added all the terminal labels in parentheis. The BAT terminal has constant 12v(+). The IGN terminal has 12v(+) when both the ignition and the headlight switch are ON. The GRD is connected to the momentary switch on the turn signal (I've confirmed it grounds when pressed). The 56a & 56b circuits power the HI/LO beam circuits, respectively. These are fused after this relay which means this relay must flow all the current for the headlights. I believe the mark on the contact that swings between the two outputs indicates it is a latching contact which stays closed to one of the outputs until the next activation of the coil. The extra "swinging" contact to the very right of the diagram would seem to apply power to both outputs for the brief time that the coil is energized. The result is that you can dim the headlights w/o the IGN being on and the headlights will momentarily light up.

The HI circuit is fused w/ two 16A fuses in parallel (total 32A), one for each headlight. The LO is fused w/ two 8A fuses in parallel (total 16A).

+-----------------------+ IGN | | +(56)------|-------------+ | | | | BAT | | | +(30)>-----+-------+ | | | | + + | | \\ \\ R? COIL=====\\====----+-------+ + \\+ + \\ | | | +-----+---+ | | (56a) (56b) HI LO

My problem appears to be the resistor located in parallel w/ the coil. It is obviously burnt out on my relay. It looks like it "popped". It is mostly black and I cannot make out the colored stripes to determine the rating. I'm assuming the resistor was protecting the coil. If it were to burn out, wouldn't the coil then take the full current/voltage? I haven't taken the relay apart enough to test the coil separately, but wired normally it wasn't activating (which is why I opened up the relay).

While I could buy a new relay for around $20USD this one was brand new and lasted only a day (it replaced an old one that had lasted 30- yrs!) I'm not sure if that says the quality is bad, or my car's headlight circuit is at fault. The HI circuit was upped to 16A fuses by the previous owner to accomodate Halogen lights. I know the wiring can handle the current, but maybe the components can't?

Any suggestions for replacing the resistor? The original was very small - about 5mm long and 2mm in dia. What should the rating be? Would it help if I clipped the resistor and measured the current through the coil? Could I replace it w/ a diode (1A, 50v)?

AshMan40

Reply to
AshMan40
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Sounds like you have a bi-stable relay. One click one way another click the other way. Mechanical failure is common in those. Are you sure the device across the coil is a resistor? Some had a diode in reverse.

If you added Halogen lights to a car that did not have them and kept the stock relay you are asking for trouble.

Tom

Reply to
Tom Biasi

If the one blew out after 30 years, and then a new one blew out in a day, either the new unit was defective, or there's another problem in the electrical system you'll have to diagnose, or have someone fix it for you.

Good Luck! Rich

Reply to
Rich Grise

I thought it might be a diode since I have seen diodes wired in parallel across other relay coils. I'm not sure. It is shaped like a resistor, but is burnt/chared so badly I can't tell. It has a few stripes around the body that are charred black, but I guess there are diodes that look like this too.

The reason I said resistor was based on the below article I found while looking for a solution to my problem. It talks about using resistors, diodes and other things to prevent relay contacts from welding themselves together.

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The "cheapest" solution appears to be a resistor. And since this relay died in a day, I'd say it was REALLY cheap! :-)

I'm going to dig up my MM and test the continuity thur the coil. If it's open (fried) then there's no hope in fixing this.

Does anyone have any suggestions for building a solid state replacement for this type of relay? I was thinking I could use a cheap 40A SPDT relay to switch between the 56a & 56b terminals (that way it can handle the current flow of the Halogens), but have a simple solid state circuit handle the "bi-stable" portion of the circuit.

Or better yet, a completely solid state circuit w/ no moving parts! I could always add relays at each headlight circuit to reduce the current flow thru this circuit.

AshMan40

Reply to
AshMan40

Normally what you think is a resistor should be a 24V 1/2 watt zener diode, which is used to absorb the transient voltages when the relay coil is energized and de-energized. It is connected with the cathode(the bar) towards the +ve supply voltage. Sometimes an inverse 1N4004 is used instead of a zener diode, but a zener diode is better. The zener can absorb both the positive and negative transients. In recent years, a resistor is used instead of a zener diode in order to save money. They all serve the same purpose.

Reply to
Unkown

Tore my relay apart today. De-soldered the thing I thought to be a resistor and it was. It measures 98ohms on my MM, in both directions. Does that mean it is a resistor, or would a zener diode show resistance? If it is a resistor, since it is much smaller than my 100ohm/1W resistor I'm guessing the power rating is much lower (I still can't make out the colored stripes). Check out the pics below and maybe you can decide if it is a resistor or a diode.

With the resistor off I could measure the resistance thru the coil.... infinite. :-( My coil is shot. Time to dump the relay and go to Plan B. I've ordered a German-made dimmer relay which I hope to be more reliable.

In case anyone is interested, here are some pics of the inner workings of the relay. Quite interesting for a "bi-stable" latching relay.

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AshMan40

Reply to
AshMan40

You are having way too much fun with this problem. ;-)

Reply to
John Popelish

Nice pictures! It is definitely a resistor.

Ed

Reply to
ehsjr

Possibly some fault is holding the relay on for extended periods, burning the coil and the resistor out.

Reply to
GPG

I've tried replacing my dimmer relay w/ three cheaper 30A (Bosch- style) relays. The first relay below on the left is the relay for the headlight switch. The relay to the bottom right is for the "dimmer" which switches between hi/low beams. The third relay at the top right allows the dimmer switch to act as a "daylight flash" when used while the headlight switch is off. The single switch to the very right controls both right-hand relays via their coil ground.

BAT

+(30)>--------+ +--(30)>--------+ | | | (85)>--+ | +--(85)>--+ | | | + | | + GND | \\ | | \\ COIL==--+ + \\+ | | | | | | | | | | | | (87) (87a) HL SW (87) (87a) | +-|--+ | | | | + | | | \\ | | COIL==--+ + \\+ | | | | (87) (87a)

Problem, when wired as above the circuit works, but there is a loud buzzing from one of the right two relays. Above you can see I've added diodes between the two terminals (#85, #86) that are grounded by the HI LIGHT switch (one each) to limit this part of the circuit to negative flow only (cathodes towards terminals). This helped to prevent the relays from buzzing all the time. Still, the relays "buzz" when the HI LIGHT switch is turned on. If I remove one of the right two relays the remaining circuit works fine. I believe it has something to do w/ using a common ground switch between the two right relays.

Any idea how I can get this circuit to work? I was thinking of switching to a DPST switch at the right.

AshMan40

Reply to
AshMan40

Sorry there is an error in the above diagram. Here is the correct one.

BAT

+(30)>--------+ +--(30)>--------+ | | | (85)>--+ | +--(85)>--+ | | | + | | + GND | \\ | | \\ COIL==--+ + \\+ | | | | | | | | | | | | (87) (87a) HL SW (87) (87a) | +-|--+ | | | | + | | | \\ | | COIL==--+ + \\+ | | | | (87) (87a)

AshMan40

Reply to
AshMan40

Your diodes are drawn backwards, and the connection from 87 upper right to 87A lower right is wrong. You need a connection from 87 of the daylight flash relay (upper right) to 86 of the headlight relay (upper left) and 4700uF from 85 of the flash relay (upper right) to gnd. See modified diagram below.

I suspect that is not the end of the story. (For one thing, I left out transorb protection for the cap) As you drew it, it appears the HI LIGHT switch is not a momentary switch. If that's true, I think you're home free with the addition of a transorb across the cap and a series resistor. But first, try the circuit. My bet is that the HI LIGHT switch is momentary, and a different circuit will be needed for hi/low to work properly. I think that the high beam will come on when you activate the switch, and return to low beam when you release it, regardless of the daylight flash circuit.

Ed

BAT +(30)>--------+ +----+--(30)>--------+ | | | | (85)>--+ | 4700uF +--(85)>--+ | | | + | | | + GND | \\ Gnd | | \\ COIL==--+ + \\+ | | | | | | | | | | | | | | (87) (87a) HL SW | (87) (87a) | +->|----+ | / | | | | 1Ax2 | | / | | | | +->|----+---|----------+ +--GND | | +---+ | | HI LIGHT | | | | SW +---- | ----------- | ----------+ | | +---------+---|----(30)>--+ | | | | (85)>--+ | | | + | | \\ | COIL==--+ + \\+ | | | | (87) (87a) >

Reply to
ehsjr

Thanks Ed. Actually the HI LIGHT switch is a SPDT non-momentary switch. What is the function of the 4700uF cap? It would appear to store a charge to keep the flash relay on even after the headlight relay is on and power is cut to the flash relay.

After posting my last diagram and staring at it for 10-min I realized that the power going to the lower right relay's 87a connection would also continue on and power the HI/LO RELAY (lower right). As soon as teh HI/LO RELAY went on it would cut off it's own power and switch off. It would then toggle back and forth on/off really quickly. This is the buzz I was hearing. I've come up with this new design that disconnects the HI/LO RELAY while the FLASH RELAY is on. I've also corrected the diodes (thanks Ed).

BAT

+(30)>--------+------------(30)>--------+ | | (85)>--+ | +--(85)>--+ | | | + | | + GND | \\ | | \\ COIL==--+ + \\+---------------+ | | | | | | | | | | | | | (87) (87a) | HL SW(+) (87) (87a) | +->|----+ | / | | | | 1Ax2 | | / | | +---+ +->|----+---|----------+ +--GND | | | +-----+ HI LIGHT | | | | SW | +-------------|----(30)>--+ | | | | | | (85)>--+ | | | | + | | | \\ | | COIL==--+ + \\+ | | | | | | HI BEAM (87) (87a) LO BEAM | | | | | +-------------------------------------+

It's raining now, so I won't be able to try this until later this week. Doesn anyone see a problem w/ it?

AshMan40

Reply to
AshMan40

the buzzing is probably caused buy currents flowing throgh the relay coil to the headlamp filament which you didn't show.

more info on the intended function and outputs of this cicuit would help.

Bye. Jasen

Reply to
jasen

Right. It will give you roughly 1/2 second until the cap discharges low enough where the daylight flash relay drops out - so a roughly 1/2 second flash of the lights. That is for the old circuit.

Yes. Since the HI LIGHT sw is not momentary, then it will not flash your lights when the headlight switch is off - it will turn them on and they will stay on until you turn the HI LIGHT switch off.

Ed

Reply to
ehsjr

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