MG Sports Cars

I just read a rewiring article in Classic Trucks -- I know this is BritishV8, but I read a couple of hot rod magazines because there's good info & interesting ideas in them... anyway, I was intrigued by a rewiring article in the Feb issue... there's a pic (page 77 for anyone with the magazine) that shows a very neat install with 14 relays. I can make out:
Fan 1
Fan 2
Ignition
Starter

then there's:
ACC -- what's this accessory circuit? doesn't make sense... A/C compressor clutch maybe?

Headlights -- looks like the 1st two left most relays are tied together, what's up with that?

Can't read the others, but can think of:
Horn
Power Windows
Power Locks
Amp for stereo system

That accounts for 11 of 14 relays. What else draws enough current to warrant using a relay?

Quote:

ACC -- what's this accessory circuit? doesn't make sense... A/C compressor clutch maybe?

On most cars there are three busses of circuits:
1) "Battery" circuits are always powered, regardless of the ignition-key switch position.
(examples: clock, radio memory)
2) "Accessory" circuits are powered when the ignition-key switch is "on" OR in a designated "accessory" position.
(example: the main radio circuit)
3) "Ignition" circuits are only powered when the ignition-key switch is in the "on" (running or starter) position(s).
(example: fuel pump)

Note: on most cars the accessory circuits are ON when the ignition is "on" but they're temporarily switched "off" when the starter is engaged.

Evidently, they're switching the whole accessory buss with a relay instead of running them directly through the ignition switch, which might make a lot of sense if the switch contacts or the wiring to the switch isn't up to the job. Are they using some sort of gimmick ignition switch instead of a relatively heavy-duty OEM switch? Are they powering any high-current circuits that are only on when the key is in the ignition switch - such as fan motors or electric windows?

Besides actual current level, there's a second rationale for using a relay to switch on-and-off electric motors. When you switch a motor on or off, you tend to get a spark at the switch contacts. That sparking can ultimately cause the switch contacts to "pit" or even to weld together in an "on" position. Relays preserve the switch (which is relatively expensive) and they also often have better constructed or better plated contacts than the main switch. There are lots of strategies to switch contact design that can also be used to eliminate or reduce the effects of sparking (e.g. designing them to "make before break" or to have a "wiping action") but sometimes it isn't practical to include these...

Good info Curtis. The article doesn't have a lot of detail on the vehicle the pic i'm referencing came from... but you may be on to something with the 'evidently they're switching the whole accessory bus with a relay instead of through the ignition switch' comment because i'm pretty sure i recall an article in a recent prior issue about wiring a start button... maybe that necessitated the relay to switch off the accessory bus.

What about having two relays tied together for headlights? In the pic, you can see wires that come off one relay & go into the one next to it... maybe something to do with switching from low to high beams?

Possibly something to do with high-beams... or maybe they're carrying the actual headlights on one relay and the marker/parking/tail lights on a second relay (in parallel.)

Rob, what type of headlights are they running? The newer style High Intensity Discharge units pull a lot of current, perhaps the double relays are for that reason.

There's really no additional info about it in the article... it was a pretty basic general wiring article... just that pic intrigued me because there were soooo many relays & the two were tied together. Sometimes they name the shop that helped them with the article -- if they did in this one, i'll try to call & see what I can find out... will update here if so.