Lotus Elan and Lotus Elan Plus 2 Forums

Conflicting opinions regarding the polarity for relay coil. I am wiring Bosch style relays. The "black cube" type. I was told that pin 86 should be +12V and pin 85 to ground. I have seen the opposite used in designs by reputable companies. Eaton being one. So is it pin 86 + 12V or ground? Or does it matter.

Thanks

Bob

It generally doesn't matter. Most relays just have a coil of wire inside, which works with both polarities.


However some relays also contain a diode and such relays need to be wired as specified. They generally show a diode symbol on the case.

I understand the flyback diode. Also a resistor can be used. I do not beleive the resistor is polarity sensative. In any event it would be good practice to use +12V to terminal 86. Given any relay with, diode, resistor or none, can be inserted.

I see applications with terminal polarity both ways. I assume that it is not critical with most functions. I was told by an "expert" that 86 must be +. I was confused as I see the opposite in many credible schematics. He may have been speaking of diode protected relays only.

I have these "Bosch Style" relays in my +2. They were added by Autsparks when they made new harnesses for me. They did a great job.

I use a diode protected relay with the radiator fan motor to protect the temperature sender.

Thanks

Bob

According to https://www.12voltplanet.co.uk/relay-guide.html

"According to DIN 72552 the coil should be fed with +12V to terminal 86 and grounded via terminal 85, however in practice it makes no difference which way around they are wired, unless you are using a relay with an integrated diode (see more info on diodes below)."

As far as the one relay working is concerned, the polarity doesn't matter. However, in one complete wiring harness, it's best to choose one convention and stick with it, rather than flip polarity back and forth from relay to relay as it seems to be the most convenient at the time.

Switching polarity can result in some interesting "sneak circuits". Electricity will find alternate paths to ground that may not be obvious to you while looking at a schematic... or 'your' creative wiring job. Sneak circuits can result in odd things happening like the interior lights turning on whenever you honk the horn. Things that shouldn't be related suddenly are. In a friend's Esprit, the ABS pump wouldn't run while the vent fan was on. The fix was simple, but it took a while to 'see' it for what it was.

The 'relay' itself doesn't care about polarity, so true, 85 & 86 can be wired either way. But the car's overall wiring circuit might care if you don't select one option, and stick with it everywhere. If the 'book' says one terminal should be positive, that's about setting a precident and being consistent, not about does it work or not. Be inconsisent, and a circuit may work even when you don't want it to.

Good luck,
Tim Engel