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?

[carolina69]

Did you find out what the issue was? I am experiencing the same problem.

[LH Lead-Foot]

In the beginning of this "Electrical Forum" are good wiring diagrams. It is in color but uses legal paper to print. Either way, V8 or In-line 6, the diagram is about the same.
These where paper service manuals with electrical schematics in the back of the book that folded out onto 2 pages. Headlights on the left and tail lights on the right.

The OEM fuse box under the dash is held in place with two (2) 5/16" screws (Maybe 1/4"). You have to remove the 4-way flasher to get to the top RH screw. Get a good light under there to look. Let screw is low / right screw is high. Goes into the firewall to hold in place. Bulk-head connector pulls apart after that with one top half & one bottom.
The starter wire circuit begins at the ignition switch. B+ to the switch is battery voltage. The back of this switch is marked with "S" as start. That connector has a 12 AWG wire that is PPL. It passes behind the fuse box, goes thru the bulkhead / firewall connector. This where the problem may be.

I have a Fluke 87 Type-V with a life-time warranty...It's about $500, plus Vetronix Master-Tech with lab scope / ignition module.

Once again, do you have a digital Volt-Ohm Meter (DMM)? If not, they are cheaper than ever before and the more you spend, the better that are. But even lost cost ones will work.

Electrical Testing:
A test light to ground, will tell you is you have voltage. Just not how much. Is the light bright or ok? Who knows...but a good place to start. A DMM will allow you to see if battery voltage is at the ignition switch. Test battery, then ignition switch B+ RED wire to a good ground, like un-painted E-Brake bracket for ground.

The PPL wire in the crank position should read the same as the B+ at the switch. If not, you have a bad 50 year old ignition switch. If it is the same reading while cranking, then re-position your meter led to PPL at ignition switch and PPL at the starter. (Note: If Radio Shack is still around / they sell "Meter led Wire" on a roll. It is soft covered wire and you can solder clips on both ends / I have 5 different lengths in my tool box) Try cranking, use a buddy to crank & disconnect the distributor so it won't start for safety.

If it does not turn over, the voltage reading from switch to starter solenoid will read less than 12 volts. This indicate a loss of voltage or "Voltage Drop". Now you remove the inside fuse box, check the PPL wire in the top half middle of the bulk-head connector. It just pulls apart. Look at both sides.

Testing with Fuse box in place:
If the voltage reading is less than 1 volt from switch to starter, the starter solenoid/starter is the problem. This is a series circuit & the solenoid & starter will use all of the voltage in the circuit. But at this point, is not working.
Inside the starter or a mechanical problem with then engine. Socket and breaker bar on front pulley to rotate clockwise to see if it turns...not locked.

Check the battery for clean ground connections. Look for the ground wire tight & clean on the engine, usually at the water pump or alternator bracket. Look for a ground wire from the battery to the fender for chassis ground. You should have one next to the battery. Proper up-dating, is to add one from the engine to the frame also. Don't forget the battery connections as well. (But you told us it will crank with screw-driver)

Hope this gets you going. Ignition switch, bad PPL connection behind fuse box, Bad starter solenoid (But you said it would jump with screw-driver / so good), but check engine mechanical for crankshaft movement.

Otherwise, you have a bad switch, PPL fuse box connector, or bad wire. The male to female terminal connections fit should be tight. They can be removed, pinched with pliers on edges, then fit tighter. This is loose terminal, loss of voltage with current going thru it (For test voltage drop) resulting in loss of current to operate the starter's solenoid.

Best of luck. Sorry to be long, but have done this for years & taught it for years.

[carolina69]

Thanks lead foot. Have been experiencing a no crank/ slow crank for awhile now, especially after driving the truck. After using your trouble shooting advice along with some others helpful hints , the ignition switch was the culprit. Even managed to buy a bad new switch and had to go get a second one. The 6 volt issue I had was intermediate, it would come and go. The first new switch installed gave me 0 volts on the purple wire to the S terminal and made me go insane trying to find an wiring issue. The second new ignition switch was spot on.