Why is a diode used on the Brake idiot light? On page EL-43 of the 1994 Q45 service manual it shows a diode used right before the alternator and parking brake switch. Not on the low brake fluid switch.

I don't get the one on the parking brake switch, but the one for the alternator is because most L terminals are push-pull and not open-collector. That means if one of the brake switches went to ground while the L terminal was high, the push output in the alternator would be shorted and blow. The diode basically makes it act like an open-collector.

Thanks for the reply so far. If you look at the active suspension part of the foldout section of the 1994 Q45 service manual. You will see maybe a little more of the parking brake switch.

I am trying to retrofit the active suspension on my Q45.

If you look on the first page of the foldout schematic you'll see that the parking brake line 20 drives a relay coil in the auto lights section. That relay will kick back EMF when it releases, so the diode is there to block the spike from migrating back into the rest of the system.

Now it makes sense because they used the same harness maybe whether you had a Canadian car or American? So is it safe to say that I can not worry about it? I thought it had something to do with keeping the active suspension module safe.

I am cutting up the wiring harness from my old active Q45 to swap into my hicas Q45.

If you're keeping that headlight system active then you can't remove the diode, but as long as you're leaving it in then no worries. EMF "kick" from a relay is only a worry if the line is attached to ECU inputs, because the "kick" can go above 100V and needs to be blocked or clamped. If there are no ECU inputs on the circuit then the kick is irrelevant, the battery will absorb it. If you need to duplicate the diode because you're altering the circuit or the hardware, a common 1N4004 off of eBay will work fine.

Not to join in late, but you'll never regret keeping a diode. If you need it and don't have it you'll regret it.

Q451990 wrote: ↑

Wed Dec 15, 2021 4:29 pm

Not to join in late, but you'll never regret keeping a diode. If you need it and don't have it you'll regret it.

Big +1. Some of the diodes on older stuff are big power diodes and not weeny 1N400x's like you'd use for a relay clamp. Since OE's don't specify the characteristics, finding an equivalent from Digi-Key or Mouser becomes a guessing game if the OE part isn't available. Never trash one, and look up the p/n and tape it to the part.

Thank you all! Merry Christmas.

You're most welcome, Merry Christmas and Happy New Year to all!