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Wiring....would like to get it right the first time

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zork Avatar
zork Gold Member Gary Robinson
N.ft.myers, FL, USA   USA
Am adding relays to the headlights for obvious reasons. I would really like to get
the wiring right the first time with no smoke or flames.
Battery negative goes to the starter solenoid, so the system is positive ground, correct ?
So if the power for the relays come from the starter solenoid as suggested, where is
the correct connection for the relays (black) ground ? ? Was thinking just connect to
somewhere on the engine (a bolt that holds the generator is close by) Am I missing
something here ? ?
Thanks a lot as always !
Gary

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Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
shouldn't a '67 be negative ground? I thought that was the year they started using alternators in the US, but don't have my notes handy to be sure

Anyway, if the battery negative is attached to the solenoid, and the battery positive is attached to the chassis, then it is positive ground.

That means everything is positive ground and you simply treat ground like ground and hot like hot (the electrons are still going the same way).

The relay should have come with a schematic (some have it molded on), it will have one terminal that gets connected to the switch, and a terminal "across" from it that goes to the chassis. Connect those like you normally would (one to the switch and one to the ground).

The relay will also have another pair of terminals which are connected together when the relay is turned "on" by the switch. One goes to the power supply (in your case, -12V) and the other to the headlamps.

Finally, the headlamps will be grounded by the black wire from each bulb.

Hope this helps.

Also, you'll want to use two relays, one for high beam and one for low beam.


N

Midgies Dad Avatar
Midgies Dad Ben M
York, ME, USA   USA
Started using alternators in '72 or '73, I believe.

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jamesm5858 Avatar
jamesm5858 James Moore
Kent, OH, USA   USA
Don t know where you are with your project but an appropriate manual and a Prospero model specific wiring schematic are a must. JMcool smiley

58custom Avatar
58custom Tom Crocker
Santa Clarita, SoCal, USA   USA
See below a simple schematic of an automotive relay. For the trigger coil, hook positive to contact 86 and negative to contact 85. So, on your positive ground car, hook 86 to ground. The other contacts do not care what polarity of DC is used or if you are switching AC.





1969 MG Midget with Datsun A15/5 speed



Edited 1 time(s). Last edit at 2018-03-12 09:02 PM by 58custom.

NYCCharlie Avatar
NYCCharlie Charles Linn
Lawrence, KS, USA   USA
My '67 was positive ground, Norm. Don't ask how I learned what that means. sad smiley

Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
doh!

11/'67 was the change to negative ground (and an alternator) for the US market, for the '68 model year

rusty memory

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Midgies Dad Avatar
Midgies Dad Ben M
York, ME, USA   USA
Norm, I've had Spridgets for decades, and I'm convinced that they had generators through 1972. The earliest I've had with an alternator was a '73. I searched Horler's book and he mentions that they switched in Dec. '72 for home market cars, but I couldn't anywhere find a reference to when they switch for North American cars.

Red midget Avatar
Red midget Robert G
Honesdale, PA, USA   USA
My '70 has a generator.

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Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
OK, this is embarrassing, because I now remember where I got that bad change over date idea:

Horler's book is clear about the change to Negative earth, for the '68 model year (11/'67), but regarding when the change from dynamo to alternator was, his text and tables only are clear about home market (RHD/UK) vehicles, as being from December, '72.

Re-reading that chapter just now, I find a statement, "For the North American market, an alternator was fitted to all 'round-arch' models", and I'd missed that before. That would put the start of production of alternators in US at October, '71 (beginning of the RWA).

I will update my notes to correct this point and stand corrected guys!

thx,
Norm

S1 Elan Kurt. Appley
Akron, Ia., USA   USA
Haven't done this in a while so looking at Tom's diagram I don't think it matters which end of the coil is ground....85 or 86. And if you are just using this as a single pole switch that you would wire 30 as the supply and 87 as the circuit you are energizing. Am i thinking correctly on that???

Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
yes, you are correct!

85 and 86 can be hooked up either way (a switch doesn't care which way the power is going through it), just so one goes to the power supply (in this case, -12V) and the other to the device (in this case, the headlights)

30 goes to the headlight switch and 87 goes to the body (ground, which is, in this case, +12V)

66Sprite Avatar
66Sprite David R
Sydney, NSW, Australia   AUS
In reply to # 3696949 by S1 Elan Haven't done this in a while so looking at Tom's diagram I don't think it matters which end of the coil is ground....85 or 86.

No it does matter, you should connect 86 terminal to the +ve side
- if the car is +ve earth connect 86 to earth and 85 to the switched signal
- if the car is -ve earth connect 85 to earth and 86 to the switched signal

This is because when a dc relay is de-energised the collapse of the magnetic field generates a voltage spike. Good dc relays have a diode wired internally across terminals 85 and 86 to block this reversed voltage spike. I've attached a diagram below.

This is brief non technical description - if interested google "back emf"


Quote: And if you are just using this as a single pole switch that you would wire 30 as the supply and 87 as the circuit you are energizing. Am i thinking correctly on that???

Yes, that's correct.


Attachments:
Relay_with_diode_across_coil.jpg    14 KB
Relay_with_diode_across_coil.jpg

Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
how would the back emi affect the headlights?



Edited 1 time(s). Last edit at 2018-03-14 06:12 PM by Kerr.

66Sprite Avatar
66Sprite David R
Sydney, NSW, Australia   AUS
In reply to # 3697052 by Kerr yes, you are correct!

85 and 86 can be hooked up either way (a switch doesn't care which way the power is going through it), just so one goes to the power supply (in this case, -12V) and the other to the device (in this case, the headlights)

Please see my comments in the above post.

Quote: 30 goes to the headlight switch and 87 goes to the body (ground, which is, in this case, +12V)

N, this would be a short circuit and I don't think you mean this smiling smiley 87 goes to the headlights, 30 is the supply that the OP was getting from the solenoid. This (87 - 30) circuit is the high current side that you are using the relay to take the high current away from the dash switch. I would fuse the supply from the solenoid.

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