86 K10 starter won't stop cranking

[gassmann]

Hey all,

I am helping a friend rebuild a Frankenstein of a truck. A 1986 K10 with a 1998 350.

The current scene is - We totally rebuild the engine - balanced within a gram, new valve seats, headers and exhaust - the whole works. Engine is just about ready to start. The truck came to him like it was pulled out of a wrecking yard. Most of the hoses and wires cut at the easiest access point. The harness was a jumble but from the firewall connector, I got it figured out... or so I thought.

The starter solenoid housing was broken so I replaced it. The starter was bench tested and works as it should. From the wiring diags, I traced the purple starter wire, tested that I am getting 12v when the key is turned to start. No sparkplug wires installed yet, so the engine won't start.

The problem: Crank the engine and the starter will not stop cranking even if I pull the key out. I have to disconnect the battery to stop it. I tested the key again and the starter wire. Key to start - 12V on the wire. Key to run - 0V on the wire. Since the engine is not running, it isn't the engine locking the starter gear.

I have a feed wire from the battery to the junction block. I have a feed wire from the alternator to the junction block and I am wondering if the alternator could be driving power to the solenoid somehow... though I can't think of how that is possible.

Any ideas would be VERY welcome.

 

[Ricko1966]

Is the battery fully charged? The contact points in the solenoid stick sometimes if the battery is low.

 

[gassmann]

Brand new battery... trickle charged it to make sure it was topped off.

If the solenoid stuck (weld) it would typically stay stuck until you knocked it right?

 

[Ricko1966]

Brand new battery... trickle charged it to make sure it was topped off.

If the solenoid stuck (weld) it would typically stay stuck until you knocked it right?

They've always unstuck just by removing the battery cable. Do you know how to do a volt drop test? Volt drop test,both battery cables. You should only have 1 wire attached to the starter solenoid engage post. Is that how you have it. If so getting feedback keeping the solenoid would be almost impossible.I would say impossible but people say never say impossible.

 

[gassmann]

I have the purple wire connected to the S terminal on the starter. But a volt drop test is a great idea. The pos batt cable is original and that could be the culprit. The neg batt cable is new and connected to the engine block. I have frame grounds that were cleaned and reset and the same for body. But I will try a volt drop across all connections in the system to see where the challenge is.

 

[Ricko1966]

I have the purple wire connected to the S terminal on the starter. But a volt drop test is a great idea. The pos batt cable is original and that could be the culprit. The neg batt cable is new and connected to the engine block. I have frame grounds that were cleaned and reset and the same for body. But I will try a volt drop across all connections in the system to see where the challenge is.

You have ONLY the purple wire connected at the solenoid?

 

[Sad Sack]

Edit

 

[gassmann]

Yes... only the purple (ignition or starter) wire connected to the S terminal.

Also, I have read a bit about the solenoid sticking due to low batt or low volts. I understand ohms and power laws (or so I thought) but can't get my head around why it would weld (stick) when volts are low. When volts drop, amps drop as well so the current is lower, not higher. Do they stick because the coil moving the solenoid plunger isn't being pulled strongly enough to make good contact?

 

[gassmann]

Should there be another wire connected to the S terminal? The wire diags don't show anything but one wire (purple) connected to the S terminal.

 

[Ricko1966]

Should there be another wire connected to the S terminal? The wire diags don't show anything but one wire (purple) connected to the S terminal.

No there shouldn't be on HEI equipped vehicles.

 

[Ricko1966]

Yes... only the purple (ignition or starter) wire connected to the S terminal.

Also, I have read a bit about the solenoid sticking due to low batt or low volts. I understand ohms and power laws (or so I thought) but can't get my head around why it would weld (stick) when volts are low. When volts drop, amps drop as well so the current is lower, not higher. Do they stick because the coil moving the solenoid plunger isn't being pulled strongly enough to make good contact?

When volts drop amps increase,that's why 6 volt vehicles had much larger gauge wires.

 

[gassmann]

So, after a LOT of testing I found that after cranking, I am seeing 6 volts on the starter wire once it starts cranking but after the key is turned off. Am I wrong here? That wire, when the key is off, should have 0v.

It feels like a relay somewhere since it has zero volts once I disconnect the battery. And 6v is odd because if it was a short, it would be more like 12 v and be on constantly. I am totally stumped here.

 

[Ricko1966]

So, after a LOT of testing I found that after cranking, I am seeing 6 volts on the starter wire once it starts cranking but after the key is turned off. Am I wrong here? That wire, when the key is off, should have 0v.

It feels like a relay somewhere since it has zero volts once I disconnect the battery. And 6v is odd because if it was a short, it would be more like 12 v and be on constantly. I am totally stumped here.

Did this truck use to be diesel? Pull the solenoid wire off and check for voltage on the wire next time. If you are checking voltage at the solenoid post you are getting a 6 volt reading because the points are stuck closed and you are reading battery voltage from the starter main post through the points to the small post. Which a 6 volt reading in that case would also go along with not enough battery or a volt drop.

 

[Snoots]

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[Mike_82_Shortbox]

Did you try the simplest thing and just shim the starter?

It may be that when the starter gear engages the flywheel ring gear, the pressure against it is enough to overcome the force of the solenoid retracting spring, and hold it in place when power is removed from the solenoid, thus mechanically holding the disc that makes the electrical connection from the battery terminal to the starter power input terminal in a closed mode. As long as the battery is connected, this condition would cause the starter to crank forever, regardless of any electrical state.

The way to check it is just loosen the starter bolts, and crank the motor.