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Discussion Starter · #1 · (Edited)
Here is the lowdown on 12V Fan and Starter Relays vs. Headlight Relays

SPST relays use terminals 30, 85, 86, and DUAL 87/87 (for cooling fan and starter)

SPDT relays use terminals 30, 85, 86, 87, and 87A (for the headlight)

I was having some problems locating spare Single Pole Single Throw (SPST) relays with an internal shorting diode to prevent voltage spikes for my starter and cooling fan. After looking at the wiring, I realized that I can just use a common (SPDT) relay w/diode just like our headlight uses.

This works because pin terminal #87 is always "normally open" on both SPST and SPDT relays when they are de-energized. As is the case with my '05 Boss Hoss trike, neither the fan or the starter relays utilize the center terminal of the relay which would correspond to pin #87A on a SPDT relay. This means that a SPDT relay w/diode can be substituted for both the cooling fan and starter relays.

To confirm, just pull a relay out of its plug and look closely at the socket plug - is there a wire or connector lug used in the center of the socket? If not, just substitute a common SPDT relay w/diode like the headlight uses. Just always use relays like Boss Hoss uses with a diode (to prevent voltage spikes), and at least a 40Amp contact rating. They are much cheaper and simpler to find than a SPST w/diode.

Circuit component Font Carton Packaging and labeling Electric blue
Cole Hersee 40amp SPDT RELAY (w/Diode)
Rectangle Font Circle Diagram Logo
SPDT Relay Terminal Numbers
Rectangle Sleeve Font Circle Window
SPST Relay Terminal Numbers

Rectangle Font Parallel Slope Technology
SPDT Relay Wiring (with voltage spike suppression diode)

Rectangle Font Slope Parallel Circle
SPST Relay Wiring

Hand Automotive tire Finger Thumb Automotive wheel system
Relay plug with center terminal unused

Link to relay photos: http://www.v8bikeriders.com/forums/album.php?albumid=1376
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