Glow plug relay getting too hot?

Hey everyone, I’ve got a 1996 F250. I just replaced both batteries and all glow plugs. I started the truck just to check if the batteries were good, and after about 1 minute, I turned it off and noticed one of the studs on the relay was smoking. Is it normal for the stud to get that hot, and what am I missing? I can’t find any clear diagrams online, so I’m hoping someone can explain.

That solenoid/relay shouldn’t get hot. Something downstream could be causing resistance, or maybe there’s corrosion or a loose connection. You can see the stud is discolored from heat. The relay is probably cheap, around $20-30, so you should replace it.

@Akira
I’d also check to see if the relay is stuck on. The two little red wires shouldn’t have 12 volts after running for a minute. If it still has 12 volts, it’s staying on too long and could burn out the glow plugs. It’s probably just a bad relay, but double-check. Make sure the 12 volts shuts off, and check across the studs to confirm the power is turning off for the glow plugs.

@Vick
Good idea. I’ve had one fail and stay ‘on’ before.

Akira said:
@Vick
Good idea. I’ve had one fail and stay ‘on’ before.

Yeah, I’ve seen it too. Usually, the relay pulls in, but after a while, it gets stuck. Sometimes the glow plug controller fails and keeps them on. It’s an easy fix either way though.

@Akira
Thanks! Do you know what the stud connects to? It seems to be the battery side of things. Can the relay itself cause the resistance? I got burned when I touched it, so it was definitely really hot.

@Amory
One side is always powered, it connects to the battery. The other side powers the glow plugs (or other high-power parts). The second side should only have power when the glow plugs are on. You might need someone to help check if the relay is stuck on. If you find power on both sides, the relay is stuck. Also, check for any loose or corroded connections, especially battery connections and engine to frame grounds.

@Akira
Thanks a lot, that helps!

You could replace it with the Stancor White-Rogers relay. Here’s a link: White-Rodgers Stancor 586-902 relay

@Lin
Wow, that’s a big one. I’d have to extend my wires, they’re already tight with the stock one.

That’s just a bad relay. Replace it and everything should be fine.

Looks like you found the reason your batteries and glow plugs needed replacing! Try checking continuity between the two big posts with the key off.