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Just purchased a new Speartech Gen 3 LS harness. 67 C10 LS swap.
I’m confused on how the harness is set up. According to the instructions, the harness has a built in fuel pump relay and I simply connect the fuel pump wire in the harness to my 12v + lead wire to my fuel pump. Seems easy enough.
However, I don’t see any ignition/start relays in the harness. It has a simple 20A fused block with 3 fuses in it. 1 brown wire, 1 small pink, 1 thick pink, 1 thick orange feeding out of the fuses.Thick orange is labeled battery, thick pink is labeled ignition. I’m assuming the ignition wire will need to be on a relay.
I called Speartech and they stated the ignition relay is handled by my chassis wiring and fuse block.
I’m reworking the entire wiring system from Painless and I don’t see anything outlining or referencing ignition relays. The LS harness from Painless uses a small integrated fuse block with relays similar to the OEM setup whereas the Speartech doesn’t.
Do I need to order a standalone LS relay block to integrate into the Speartech harness to handle the ignition and any other high current needs?
Orange is battery power and always on. That heavy pink is ignition on typically in most cars that gets power with key on. So you should have a ignition on circuit in your painless harness. make sure you fuse them both. if you don't have one in that painless harness, easy enough to add it and try it out. you can try something like this...
Orange is battery power and always on. That heavy pink is ignition on typically in most cars that gets power with key on. So you should have a ignition on circuit in your painless harness. make sure you fuse them both. if you don't have one in that painless harness, easy enough to add it and try it out. you can try something like this...
Thanks for the reply! I’ve looked at a few of those boxes like the one you linked on Amazon.
Are you saying it’s possible I don’t need an ignition relay? I’m using the Painless Pro Truck harness https://painlessperformance.com/prod...part-no-10416/ but I don’t see anything in there to handle the ignition part of the harness.
if you run your power through that, I think you will be ok. You need to use the key on circuit for that ignition. looks like circuit 931 does that. or 935.
Last edited by merim123; May 27, 2026 at 09:36 AM.
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Help with Speartech LS swap harness 
