Ask the Mechanic

By Steve Thompson, Contributing Columnist

STEVE: Warren Guidry, from Rayne, Louisiana, has a common problem when replacing the ignition switch on an older tractor. When the number of posts on the replacement switch is different, the colors of the factory wires are different, the harness doesn't match the switch, and the replacement switch posts are not marked. Here is Warren's story.

READER: "The ignition switch on my old Case 275 has died. I found a replacement switch, but the terminal identification markers are not the same as the OEM switch. Everything is different about the replacement switch. I know that making a wrong connection can do some pretty serious damage to components. Can you help me with getting the new switch wired correctly?"

STEVE: This is a simple task with a test light. On this tractor, you will need at least an off, on, heat and start post. One wire to the switch is hot all the time. With the test light, locate the wire going to the switch that is hot all the time. This post is usually marked with a B (battery) or I (intensity). If it is not marked, you will need to find it. When you find that post, with the key off, no other post should be hot. Next, turn the key to "on." With the light, find the post that is hot with the key on but no light when the key is off. This post is for the gauges and possibly the light switch, and a tractor with an electric solenoid for the injector pump. Install the wire on this post that turns on these functions. Then, we need to find the heat position for the glow plugs. Turn the switch to the heat position, and find the hot post. Put the wire (usually larger) on this post. You can put the test light on the nut on top of a glow plug to check for voltage. This function is usually spring-loaded. Finally, turn the key to start, and find the hot post that is hot only on start. This wire will activate the starter only in start with all safety switches (including PTO and neutral start switch) closed. Note: The "on" post will also be hot in start. Sometimes, it is easier to find a used switch from a dismantling facility (junkyard). You can find dismantled machines on tractorhouse.com. Scroll down to dismantled machines. All the machines dismantled will appear. Call and get a switch shipped. It will fit and also keep your old tractor original.

> Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email mechanic@progressivefarmer.com.

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