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MG MGB Technical - Headlamp relay question

I am in the process of installing some headlamp relays using standard 30 amp ISO relays. I have installed the ground wire to position 85, the power feed from the light switch to position 86 and the power feed to the high and low headlamps from each relay to position 87. Prior to installing the main power wire from the starter solenoid to position 30 I checked the switch to see if I could hear the relays click and noticed the lights worked. Is the current jumping from the coil in the relay from 86 to 87 or should this not be happening?
Frank

Frank - the current is not jumping from the coil to to pin 87, but you do have a sneak path for the current to flow from the original path from the switch to the headlights. Without looking at the wiring diagram and the actual wiring in your car, I would not be able to guess where that path is, but it is there somewhere rather than the current making a jump in the relay. Good luck - Dave
David DuBois

Frank,
From your description, you have the right wires going to the correct relay terminals. However, based on your description, I suspect that you may have crossed the control voltage with the lighting supply voltage. Each relay should be controlled by the blue/white or blue/red wires that feed the 4 way headlamp connectors on the passenger side. Relay terminal 85 as you indicate is the coil ground terminal. The output (Terminal 86) should be jumpered directly to the passenger side 4-way bullet feeding it's respective bulb filaments (hi or low) I ran thru several possibilities to get the headlights to come on as you describe, and crossing the control and supply voltage was the only way to duplicate your description.
The supply input to the switch contacts, which you have sourced from the starter terminal, should be on terminal 30, ("common" terminal). Standard 4 terminal DIN/ISO relays designate terminal 87 as the normally open contact.
As an aside, five terminal DIN/ISO relays also have a switched terminal 87A which is normally closed (when the relay is not energized) and shuts off whenever the relay is energized.

Mark Childers

Frank,
I erred in that the output from the relay to the 4-way bullet recepticle to feed the headlights is terminal 87 not 86 as I indicated above.
Mark C
Mark Childers

Sounds like you have left the switch powering the lights as well as the relay. You need to disconnect the blue/white and blue/red in the main loom from the double-connectors at the headlight and feed them back to operate the relays on terminal 86 (no additional wire from the switch to the relays). Then take the outputs from the relays from terminal 87 and feed that back to the double connectors. See http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on 'Lighting' and 'Uprated Headlamps ...'. This shows fuses for each filament and has the connections to 85 and 86 the other way round but is imaterial.
Paul Hunt

Wanted to post a follow-up to the lighting problem as I was able to track down the problem. It seems the DCU (me) when disconnecting the wires at the right front corner assummed the single wire feeding into the four way connector was from the switch while the two on the other side of the connector(both of which were of larger gauge as I noticed later) were going to the lights. Obviously the feed wire and the wire carrying the current to the left headlight are the larger gauge ones as I subsequently discovered. At any rate this was as hooked up to the relays allowing current to reach the left front headlamp and making me think both headlamps were lit even though the power wire had yet to be connected to position 30. Now all is hooked up properly and I am amazed at the difference. Thanks to all!
Frank

This thread was discussed between 24/05/2003 and 26/05/2003

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