There is no starter relay on 55-59 GMC 100 series. The solenoid performs like a relay except it has an added mechanical linkage attached to the electromagnetic component. The switching activation of the relay is low draw, then it's big contacts inside handle the big draw. Two different windings.
The only starter relay for 55-59 GMC hydra-matic is for a MW500 tractor truck.
Don't know why it is there to begin with. If a stock 1/2T, your automatic transmission has nothing to do with it.
If someone has replaced the motor from an earlier year, they may have brought over all the wiring with it. It may have an older 6V starter and etc. We don't know. If it has a 55.2 starter and electrical system, it should not have a starter relay. If so, wire your ignition per the 55-59 GMC schematic in the maintenance manual. It could have something to do with 1955, this was a transition year; 55.1 to 55.2. So could be a left over hybrid system. The 1954 schematic posted previously seems to associate the starter relay with larger trucks? and Buicks?
I have done the Ford relay/starter jumper trick several times. Nice setup. Eliminates all wires down to starter except the big cable. Prevents hot wires down there which cause resistance and loss of current carrying capacity. It's not done because the system "needs" it.
