crank no start

[WaRadman]

Starter pulses but won't crank enough to run

 

[MrMikey4026]

Low voltage somewhere. My issue of the same was caused by loose battery cable at the reverse polarity relay. You don't say if it is an A0, A1 or A1R? Does make a difference for trouble shooting.
Cheers

 

[Ronmar]

The pulsing starter is probably accompanied by pulsing relays? Relays will react this way to the voltage falling off as you try and pull current thru a bad connection.

This was my first LMTV problem, pushed the button and it pulsed at first, then it just went click click click… had a bad connection on the 12 V lead at the polarity box. Was readily apparent with a volt meter at the power panel test points while trying to start. When the voltage would drop, the main relay would release and interrupt the start circuit, once open the voltage would rise to normal, the relay would pull in and the starter would engage again…

a 24V coil relay mistakenly placed in a 12V position will make this situation worse as it will operate marginally with perfect 12V and only takes a very small drop in the 12V circuit power to release the relay…

 

[Guruman]

Starter pulses but won't crank enough to run

This is on my list to troubleshoot too. Mine will pulse three or four times, then finally crank and start.

I found that if I hook up a trickle charger to the batteries (I used 2 12v chargers), once the batteries were fully topped off, it did not pulse at all.

 

[Ronmar]

This is on my list to troubleshoot too. Mine will pulse three or four times, then finally crank and start.

I found that if I hook up a trickle charger to the batteries (I used 2 12v chargers), once the batteries were fully topped off, it did not pulse at all.

You can lift a relay out of the socket twist a small copper wire around the coil terminals so they stick out past the relay body and re-insert the relay. With these wires in place you can check the voltage reaching the coil. Relay K2 controls the switched 24V ignition power, which feeds ALL the 24V control power used in the start circuit. Ut it uses 12V power from CB70 via the main switch to power it’s coil. This is one of those places where if a 24V coil relay is installed, it will only operate marginally with perfect 12V.

The 12 and 24 pass thru diodes in the polarity box, so they drop 1/2-2/3 of a volt just getting to the power panel. Any other connection issue or a weak 12V batt, and 12V will have trouble holding a 24V coil reliably, or if the voltage drop is low enough, even holding a 12V coil reliably(i think they drop out under 9 ish volts…

if you can re-create the situation, by letting the batteries get a little low, you can prove this situation by unplugging relay K2, and making a small jumper wire with 1/4” spade terminals on the end. Install a jumper between pin 30(top center when looking down at the panel) and pin 87(bottom center when looking at the panel) in the K2 socket. This will close the same circuit that K2 does when you turn on the main switch. Then try and start the truck… if it still pulses, then you have a connection issue somewhere else in the circuit.

The pulse is the give-away that something is interacting with an inductor(relay coil or motor winding)… and if you can crank reliably by jumping across the aux start contacts, like the OP stated, then that pulse interaction is occurring somewhere in the start control circuit…

I really need to do a relay troubleshooting video someday…

 

[Guruman]

You can lift a relay out of the socket twist a small copper wire around the coil terminals so they stick out past the relay body and re-insert the relay. With these wires in place you can check the voltage reaching the coil. Relay K2 controls the switched 24V ignition power, which feeds ALL the 24V control power used in the start circuit. Ut it uses 12V power from CB70 via the main switch to power it’s coil. This is one of those places where if a 24V coil relay is installed, it will only operate marginally with perfect 12V.

The 12 and 24 pass thru diodes in the polarity box, so they drop 1/2-2/3 of a volt just getting to the power panel. Any other connection issue or a weak 12V batt, and 12V will have trouble holding a 24V coil reliably, or if the voltage drop is low enough, even holding a 12V coil reliably(i think they drop out under 9 ish volts…

if you can re-create the situation, by letting the batteries get a little low, you can prove this situation by unplugging relay K2, and making a small jumper wire with 1/4” spade terminals on the end. Install a jumper between pin 30(top center when looking down at the panel) and pin 87(bottom center when looking at the panel) in the K2 socket. This will close the same circuit that K2 does when you turn on the main switch. Then try and start the truck… if it still pulses, then you have a connection issue somewhere else in the circuit.

The pulse is the give-away that something is interacting with an inductor(relay coil or motor winding)… and if you can crank reliably by jumping across the aux start contacts, like the OP stated, then that pulse interaction is occurring somewhere in the start control circuit…

I really need to do a relay troubleshooting video someday…

@Ronmar do you have a good source for the relays? Especially the 24v ones. I'd like to just replace them all, but the 24v ones I ordered from Amazon are physically too large in size.

 

[Guyfang]

Starter pulses but won't crank enough to run

Have you ever seen it work right? Is the truck new to you? Or it ran before and now it has the problem?

 

[Ronmar]

@Ronmar do you have a good source for the relays? Especially the 24v ones. I'd like to just replace them all, but the 24v ones I ordered from Amazon are physically too large in size.

I wouldn’t bother replacing all of them, that is a waste IMO. Unless they have been abused, they have a very reliable track record and last a VERY long time…

They specify 4 types, 12 and 24v, 4 and 5 pin, but you only beed to get the 5 pin versions.

 

[WaRadman]

As normal you guys are spot on. Newer batteries but one was defective and dragging down the other. Since they were new I figured it couldn't be them but back to the basic testing uncovered it. Should have mentioned it's an A0 1079 that I've had for 3 years with very few problems.

 

[Ronmar]

As normal you guys are spot on. Newer batteries but one was defective and dragging down the other. Since they were new I figured it couldn't be them but back to the basic testing uncovered it. Should have mentioned it's an A0 1079 that I've had for 3 years with very few problems.

Let me guess it was one of the 0-12v batts… Drop to two batteries, your alternator will thank you for it. It is way undersized for the monster bank they saddled it with, and it has trouble keeping the batteries charged. This leads to it basically running in an overload condition whenever it is online…

Add in a vampire load on the 12V side and those batteries are always in an uphill struggle to get to full charge. Partially charged lead acids sulfate, loose capacity and fail long before they should…

2 batts are way easier to keep charged with less run time, they don’t overload the alt and are more than enough for the truck(cat specced a pair of group 31’s for these engines)…