Another no crank

[prodjay10]

Sometimes the Truck won’t crank with key. If I use a screwdriver to jumper the red battery wire and the purple wire it will crank and start. Then later it might work with the key.

When it wouldn’t crank I used a volt meter to test voltage on purple wire from battery ground and only got 6 volts.
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[prodjay10]

1971 c10 with 250 cid
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[ray_mcavoy]

Was the 6V measurement on the purple wire taken at the end down where it attaches to the starter solenoid?

If so, try measuring it again on the other end of the purple wire right off the back of the ignition switch. If you have full battery voltage (or close to it) there at the switch, but it drops down to around 6V at the other end of the purple wire, there are a couple places to look for a poor connection that's causing the voltage drop:

(1) If your truck has an automatic transmission, check the neutral safety switch. Try shifting to neutral (instead of park) and see if that makes any difference. Also try moving the shift lever a little while attempting to crank the engine. And finally, for testing purposes, you can temporarily bypass the switch by unplugging the 2 purple wires and connecting a jumper between them. Don't leave it this way because it will allow the truck to start in gear.

(2) The purple wire passes through the bulkhead connector on the firewall (located behind the fuse panel) on it's way from the ignition switch to the starter solenoid. A poor connection there could be causing the voltage drop.

However, if the voltage is also measuring low on the purple wire right off the back of the ignition switch, you could be dealing with an ignition switch that's getting bad and not always making good contact. But one last thing to check before replacing the switch is to measure the voltage on the red feed wire to the switch while attempting to crank the engine. If the voltage on the red feed wire is low, there is likely a bad connection somewhere in that circuit between the ign sw and battery.

[prodjay10]

Thanks ray_mcavoy!

I still haven't fixed this since I rarely drive the truck, but now I know where to look.

[LH Lead-Foot]

What Ray said is the best way to get started as there is considerable amperage going thru the PPL wire to the starter solenoid. Don't forget to look for loose or burn't terminals like the one behind the fuse box. This is the male to female plastic connector that may result in terminal push-out causing loss of voltage. The metal terminal is only held in by a small metal tang that locks it into the plastic connector. So, past repairs or pulling on the harness could cause the terminal to pull apart enough to create resistance to cause your loss of voltage.

The 12 AWG RED wire to the ignition switch is battery hot, so with a decent digital volt/ohm meter, it should be current battery voltage. Try it with head lights on, blower fan on high and wipers. A small amount of voltage drop or loss occurs in connectors and the length of wire, but should read real close to the battery with things turned on.

The PPL starter solenoid wire supplies current to the pull-in coil of the starter solenoid and the holding coil at the same time when cranking. Once cranking, the pull-in coil becomes de-energised once cranking begins. The solenoids movement alone completes the large amperage switch inside to connect the battery with the cranking motor. At this point, the solenoids amperage draw drops lower.

Best of luck and let us know what you found.

[Yooper truck]

Don't forget to look for loose or burn't terminals like the one behind the fuse box. This is the male to female plastic connector that may result in terminal push-out causing loss of voltage. The metal terminal is only held in by a small metal tang that locks it into the plastic connector. So, past repairs or pulling on the harness could cause the terminal to pull apart enough to create resistance to cause your loss of voltage.

How do you check behind the fuse box? I assume you have to pull the fuse box off. How can I do this without really messing things up?