More Obscure Electrical Brain Teasers

304065 · 03-26-2002, 06:37 AM

Ladies and Gentlemen:

I'm getting down to the final stages of my fuel pump install and have a couple questions re: wiring of relays:

1. Someone recommended that I wire the "Hot" or +12v to terminal 87 of the relay (the normally open contact) rather than to terminal 30, so that when the relay is de-energized, current won't flow across to terminal 87a (the normally closed contact)-- because if anything contacts 87a, even though it's not being used, it could cause a short. I doubt this is really relevant and PORSCHE does it opposite, e.g. the factory wiring diagrams show +12v to terminal 85 and then over to terminal 30.

2. The same source recommended that I use a "Quench" diode in parallel with the relay coil (that is, parallel to terminals 85 and 86) in order to prevent a backwards voltage through the circuit when the energizing current is shut off and the field breaks down. Just like an ignition coil. . . does this really matter? And which end of a diode is the positive end, is it the one with the stripe?

These types of questions are so obscure that I would ask them ONLY to this board. . . the quantity and quality of automotive knowledge here continues to blow my mind.

03-26-2002, 06:55 AM

john,
They look like this.

current flows in the direction of the point of the triangle and is blocked from flowing the other way

03-26-2002, 06:59 AM

Quote:

Originally posted by Inari77S
john,
They look like this.

current flows in the direction of the point of the triangle and is blocked from flowing the other way

first try uploaded huge.

wckrause · wckrause's Garage

Don't the fuel pump relays already have the diode internally?

wckrause · wckrause's Garage

Like this

Jim Smolka · 03-26-2002, 07:17 AM

Current flow depends on whether one believes in the 'hole flow' theory (positive to negative) or the electron flow (negative to positive) theory. Installing a diode in parallel with the coil is Ok, but it is easier to install the red round Fuel pump relays that were designed with a diode inside. Yes, the diode is a ‘good idea’, but necessary on a car??? Well, it cannot hurt if installed correctly.

304065 · 03-26-2002, 07:43 AM

Hey, this is great information. I didn't know that the red fuel pump relays have an integral diode, that's a big help because I already have one (and a socket) and the clip of the later car elec diagram should give me the colors for the harness on the socket- saving valuable time.

My '71 didn't have a relay for the fuel pump- it's only supposed to draw something like 3-5 amps, but I think it's a good idea-- I hear that fuel pumps draw more amperage as they get worn.

Thanks again for all the info.

JFC

Early_S_Man · 03-26-2002, 07:51 AM

John,

I don't know whether you made a typo when you referred to a 'multiple' going from "+12v to terminal 85 and then over to terminal 30. " ... but, Terminal 85 of all DIN spec relays is normall the ground terminal of the coil, and this becomes VERY IMPORTANT when talking about quench diodes and relays with built-in quench diodes, such as the round, red fuel pump relays! If you reverse terminals 85 and 86 on a 'normal' relay ... it doesn't really matter, but on a red relay, it becomes a fire hazard, because the diode represents a short when connected backwards from normal!!!

As far as your first question, it doesn't really matter, because Porsche does it both ways for different relays in different circuits, as demonstrated by the same circuit Bill 'clipped' the relay from!

304065 · 03-26-2002, 10:25 AM

Warren:

Thanks for your excellent reply. Nope, that wasn't a typo, that's what I meant and I was wrong-- was thinking that the connection to the coil wouldn't matter, and since I was originally considering an external diode, figured that I would just hook it up in parallel with the correct polarity. But your answer makes sense- don't want to cook the internal diode which will be sitting 12" from my fuel cell. Nice catch and thanks.

So I've got:

85 Ground
86 Switched +12v (jumper from 30)
30 Switched +12v
87 to fuel Pump +
87a not used (remove pin or clip+ insulate wire from circular relay socket)

The color of the wires on the socket that Pelican sells don't correspond to any socket I can find in the wiring diagrams, so I'll just check them myself. The male pins on the relay are labeled and it only fits one way into the socket, so it should be simple to correlate which male pin against which female pin, and then use my VOM to match up the wires against the female pins.

By the way, I just ordered the Bosch Automotive Handbook which I hear has all kinds of good stuff in it, like the DIN pin assigments etc. Will be nice- I suppose I could hook stuff up any old way that works but it's nice to conform to a standard, even if only to limit frustration by future owners.

wckrause · wckrause's Garage

Why are you connecting switched 12v to 30? If you want to isolate the fuel pump current from the ignition switch, then you would connect unswitched 12v (through a fuse) to 30.

304065 · 03-26-2002, 01:24 PM

wckrause:

Yep, you're right- the whole point of HAVING a relay is NOT running current through the ignition switch. I've got a 10 amp circuit breaker that I plan on running inline with the current from the battery- the spec for the pump (a Mallory 4060FI) says it shouldn't draw more than 5.0 amps.

Gotta stop pulling these all-nighters to get the car together.

Seriously, thanks for the clarification and your advice above.