The ST85 is a continuous-duty relay. Nothing about it makes it particulary good for glow-plug use. It is rated at a max of 80 amps. It cost more for two reasons. #1 - because it's at NAPA. #2 because it's a low-draw relay meant for full-time use, not just momentary. They are often used for trailers that you want to have power when being towed and the two vehicle is running. When the tow vehicle is shut-down, this full-time relay shuts down power. Also used for vehicles that use starting batteries AND a aux batteries. Full time relay allows charging all when runniing, but disconnects them when the key is off. This way the starting batteries are isolated from the aux batteries.How come the st85 are so much money? napa wants 65 dollars. What is the difference from using a dog head relay or any other starter relay?
You can buy better rated relays for less money. I use the relays rated for 150 amps continous and 800 amps momentary. I buy them new for less then $20 each. Often used on Western plows with electric-powered hydraulic pumps. Work great for high-draw glow-plug systems. Some places sell them for as high at $60 but many others for less then $20. I buy them from Surplus Center.
Surplus Center - 12 VDC SPST CONT DUTY INSULATED SOLENOID
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