An Electrical Puzzle

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An Electrical Puzzle

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An Electrical Puzzle

Postby kmam » September 29th, 2018, 5:17 am

Looking at the circuit for my M151A2

1. Power runs from the battery via lead 6 to the starter switch.
2. From the starter switch it goes via a second lead 6 to the starter when the switch is operated
3. From the starter switch it goes via lead 5 to the alternator (a permanent connection, presumably the voltage sensing input), to the ignition switch (which connects to nothing when in the off position), to the horn (which is only connected when the horn button is pressed) and to the lights switch (which is connected to nothing when in the off position).

These are the only connections providing power to the rest of the vehicle which means that if all switches are off there should not be a completed electrical circuit.

I have the positive of the 24V batteries connected to lead 6 and the negative passes through a battery isolation switch to earth. When the isolation switch is open circuit (i.e. switched off) and I place a volt meter between the 24V battery negative and vehicle frame it reads 24 Volts (or thereabouts) and 2.5 Amps. This tells me that there is something providing a path for current to flow. My assessment is that it can only be one of six things:

1. The ignition switch is faulty and passing current when turned off.
2. The starter switch is faulty and passing current when turned off.
3. The lights switch is faulty and passing current when turned off
4. The horn switch is faulty and passing current when not operated.
5. There is leakage from the alternator sensing circuit.
6. One of the leads carrying power to the switches/circuits has poor insulation and is a creating a path for current to flow.

There is another possibility. The turn indicators work even when lights are not turned on so there may be an issue there.

I only noticed this today so tomorrow I will start isolating these to see if that causes the voltage reading to fall to zero. Before I start I will be interested to hear if there is any possible cause that I may have missed.

Thanks, Howard

PS It just occurred to me that the RT524 is also permanently connected to +24V.
Howard

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Re: An Electrical Puzzle

Postby rickf » September 29th, 2018, 8:41 am

Ok Howard, you are in an area where I know a little bit about what I am talking about now but you have me a bit confused there at the end. First lets get to the way the wires are routed which does not sound right. The #6 wires which are the heavy battery cables go from the batteries to the starter switch and from the starter switch to the starter. That is the extent of the #6 wires. The #5 wire SHOULD attach at the starter switch on the same side as the #6 wire coming FROM the batteries. This #5 wire in turn feeds the light switch, the ignition switch, the alternator and the # 25 breaker which goes to the horn. The horn wire is hot all the way up to the horn switch which when you push it grounds the circuit and blows the horn. So basically your assessment of the passage of the power is pretty close to spot on. You just left out the alternator. If you have a bad voltage regulator that could cause a draw on the system. Here is what I would do. First, pull the radio out. Check again and see if that changed anything. Then unplug the number 5 wire from the alternator (make sure batteries are disconnected during disconnect and reconnect of this wire so you don't spike the regulator, you can hook up batteries if you want after it is disconnected.) Check for draw again. Report back after these two tests. the starter switch can cause a draw but that is a pretty heavy draw, same for the light switch, you would have to have several lights on to show it.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone
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Re: An Electrical Puzzle

Postby rickf » September 29th, 2018, 8:43 am

You can also unplug the power wires from the back of the radio mount. That will eliminate the transformer as a culprit.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone
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Re: An Electrical Puzzle

Postby Fil Bonica » September 29th, 2018, 10:56 am

If the radio is connected directly to the battery ,both positive and negative ,the antenna shield is connected as it should there could be a complete circuit.
You may find that it would even turn over.
Had this happen on a Hawk missle carrier some years ago.
Disconnect the radio completely And see what happens.
The solution is to
Put the cut out switch in the positive line instead of the ground.
That’s what we did at that time and it worked.
Hope that this helps.

Fil Bonica
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Re: An Electrical Puzzle

Postby rickf » September 29th, 2018, 1:08 pm

Doesn't matter if it is the ground or positive, you are breaking the complete circuit to the battery, it goes out the positive and back the negative (actually in reality it is the reverse of that but we will leave that for electrical theory class) so break either one and the circuits are dead. If using the simple knife type switch the negative side is much safer.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone
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Re: An Electrical Puzzle

Postby Fil Bonica » September 29th, 2018, 1:32 pm

You. Miss the point,
Ground to the radio is still supplied through the shield of the antenna coaxial cable.
There is no way to break that unless you disconnect the cable .
Putting the power switch in the positive lead Will make a difference provided the radio power is downstream from the switch.
That’s what we had to do with the 524 that was on the Hawk missle loader
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Re: An Electrical Puzzle

Postby rickf » September 29th, 2018, 4:24 pm

Ok, We are talking about two different systems here. I am talking about the main vehicle system and you are talking about the radio. Either way, if you disconnect the batteries from either cable on the vehicle the radio is dead, coax or not. Unless there is another battery system in the radio pack somewhere like a PRC77.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone
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Re: An Electrical Puzzle

Postby kmam » September 29th, 2018, 6:20 pm

Rick - "Ok Howard, you are in an area where I know a little bit about what I am talking about now but you have me a bit confused there at the end."

Sorry, it may not read well. What I meant was that the +24V of the radio is permanently connected to the +24V supply but the -24V of the radio passes through the battery isolator so it could be the cause. Just took the dog for his (and my) morning walk and now off to the shed to start problem solving.

Thanks,

Howard
Howard

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Re: An Electrical Puzzle

Postby kmam » September 29th, 2018, 7:23 pm

OK The alternator is the main culprit. When I disconnect number 5 the voltage between negative and frame drops from 25V to 7.2 which slowly drops to 6.8V. Disconnecting the RT524 had no effect.

Now for the alternator gurus - what is causing this? I guess I need to get the alternator to an auto electrician for checkout/repair? There is an old timer (younger than me, though) at a local auto electrics business who knows all this stuff. He checked out the generator/regulator and starter on my Land Rover series 2A and the starter on my M151. When chatting to him the other day, he mentioned that the only other vehicle of this type he had struck with a 24V system was an Austin Champ so I am confident he can help me out if need be.
Howard

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Re: An Electrical Puzzle

Postby kmam » September 29th, 2018, 7:36 pm

Hmm. From digging around it look like a failed diode in the alternator. So, pull it out and off to the repair shop next week. Hope they can fix it...
Howard

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Re: An Electrical Puzzle

Postby kmam » September 29th, 2018, 9:50 pm

I have the alternator out and will take it for repair this week. But I now have a reading of 7V at 1A between the negative post and frame so there is leakage in another spot too. With a voltage drop of 17V and a current of 1A this means there is something in circuit with a resistance of about 17ohms. I will have to start disconnecting other items to track that one down. At least I have some time to do it while the M151 is out of action...
Howard

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Re: An Electrical Puzzle

Postby Fil Bonica » September 29th, 2018, 9:58 pm

Two separate issues here,
Kens problem seem to be related-to the failed diode in the bridge rectifier.
That can be proven by the shop repair man.

The issue of power to the radio is still not clear in my mind.
If the radio is connected directly to the battery terminals power will always applied to the radio
Regardless if the ground switch s closed or open.
As a consequence power could be fed to the main harness.
That is why I said the main battery cut off should be on the positive lead not the negative.
Where am. Inwrong?

Fil Bonica
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Re: An Electrical Puzzle

Postby kmam » September 29th, 2018, 10:30 pm

No worries Fil. Only the positive lead of the RT524 is connected to the 24V battery, the -ve connection goes to vehicle frame. The battery -ve post goes to the battery isolator which then connects to the vehicle frame. There is no circuit possible from the battery when the isolator is turned off. This means that the RT524, like the rest of the vehicle, has no power when the battery isolator is turned off. It was probably confusing when I said in the original post "It just occurred to me that the RT524 is also permanently connected to +24V." - I should have made it clear that it is not permanently connected to -24V.
Howard

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Re: An Electrical Puzzle

Postby kmam » September 30th, 2018, 12:49 am

After pulling out the circuit breakers (pita to put back), switches, connectors, etc I realised that the leakage I measured after removing the alternator was caused by me setting my meter incorrectly! So, once the alternator is fixed, all is good :)
Howard

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Re: An Electrical Puzzle

Postby rickf » September 30th, 2018, 8:10 am

Fil, Think about this as a big picture. All of the power for everything in the vehicle comes from the batteries. The radios are included in this big picture.. Now, If you take the positive terminal off of the battery you interrupt the power to the vehicle, including the radios. I think we are in agreement here correct? Keep thinking big picture here, power has to travel in a circle and return to the source. Out one terminal and back in the other. SO, If you disconnect the negative terminal instead of the positive terminal you have done the exact same thing, you have interrupted the circuit. Yes, the positive cable is still hooked up but the power has no way of returning to the battery so therefor the circuit is dead. The radio will still have no power because it cannot return to the battery.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone
rickf
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