M1088A1 cat 3126 wiring questions alternator & batteries

[Jeep Nasty]

All right, I’m a bit stumped on some alternator and battery wiring on my truck that I just got.

It’s an M1088A1 with the cat 3126.

The previous owner has done some tinkering with it and I’m just trying to figure out a baseline before I do anything worse.

The wiring on the alternator had the black wire shown connected to the ground but that has the metal tag on it still prior to the extension that was added of TL 35, which, according to the wiring diagram, shows that that should be connected to the regulator and not the ground .

Then I have this white wire, which seems odd since most of the military truck wiring is black with metal tags, and there are no numbers, stamped or tag onto this wire, which was connected to the energize circuit of the regulator.

With the ignition on, I showed no voltage to the black wire and I show approximately 16 V to the white wire with the engine started and running. I don’t show voltage to either of them.

if I go into the circuit board or power, distribution panel, the LED for the K 11 excite circuit does not illuminate when the truck is running or when the ignition is turned on, but it will illuminate while the engine is cranking.

The circuit breaker 77 has been replaced with a 10 amp fuse, which is the correct load but I’m only showing 6 V when the ignition is on to that fuse but when the engine is running I am showing 24 V to that fuse

In addition to the alternator which could be part of the problem or completely separate the batteries have been changed to a two battery set up, and there is a manual battery disconnect switch, which does not look military in nature to me, but it’s only set up to disconnect the 24 V side off of the batteries leaving the 12 V side constantly connected .

I’m not sure if this truck was one of the ones that was supposed to be equipped with no battery disconnect or if it should have a different style one that disconnects both the 12 and 24 V circuit or does a manual disconnect of the ground.

I know on the cat 3116 motors you can manually excite the regulator and the alternator by bypassing the two wires coming out of the wiring harness and hooking into the 24 V side of the fuel solenoid.

I am wondering if this is the appropriate action to take with the cat 3126 motor or if this does not apply and I am wondering if both of those wires need to be connected to the regulator, and that neither of them was designed to be a ground.

At least that’s what it looks like according to the wiring diagram that I’m looking at for an A1 series truck any tips guidance links to relevant articles would be incredibly helpful. I would just like to ensure that the wiring is all done correctly before I start replacing parts

Additionally, if I place the white wire onto the excite side of the regulator with the ignition off engine, not running, I will get a red flashing light for the 28 V LED on the alternator regulator and I will get an amber flashing light for the 14 bolt LED on the alternator regulator but when the engine is running no power is going to that white wire so none of the lights illuminate.






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[Suprman]

Maybe something caused the truck to loose excite voltage. The white wire on the front regulator terminal needs to have 24v when running or the alt will just spin and not make power. I believe the way the truck was intended to operate, when oil pressure comes up once the engine starts it triggers the excite relay.

 

[Jeep Nasty]

Maybe something caused the truck to loose excite voltage. The white wire on the front regulator terminal needs to have 24v when running or the alt will just spin and not make power. I believe the way the truck was intended to operate, when oil pressure comes up once the engine starts it triggers the excite relay.

That’s what I was thinking and I was wondering if I could tap into voltage from somewhere else similar to your suggestions on the 3116 to see if that gets the alternator and charging system function properly.

I have another question for you that you may be able to answer cause I know you’re pretty familiar with these trucks do you happen to know what type of harness connector I could order that fits on the timing sensors? I am missing the harness side connector on the lower sensor.

I can find part numbers for replacement sensors, but not the connector side on the harness .

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[Ronmar]

When the oil pressure sw hits 15 psi, it deenergizes K11 which powers the regulator thru its de-energized contacts 30-87A and brings the alt online AFTER the engine is running. K11 should energize(led on) with the oil idiot light in the dash and de-energize when oil light goes out. On the A1s it also will de-energize when you select neutral. I think K11 also enables the starter, and disables it when engine is running, so you mayhave some other hack-jobs to deal with...

They did this so that horrendously expensive alt isnt trying to power the starter while spinning below idle RPM... i would fix that circuit and not do the bubba ignition excite trick some do on the 3116...

My .03

 

[Jeep Nasty]

When the oil pressure sw hits 15 psi, it deenergizes K11 which powers the regulator thru its de-energized contacts 30-87A and brings the alt online AFTER the engine is running. K11 should energize(led on) with the oil idiot light in the dash and de-energize when oil light goes out. On the A1s it also will de-energize when you select neutral. I think K11 also enables the starter, and disables it when engine is running, so you mayhave some other hack-jobs to deal with...

They did this so that horrendously expensive alt isnt trying to power the starter while spinning below idle RPM... i would fix that circuit and not do the bubba ignition excite trick some do on the 3116...

My .03

OK that’s definitely helps because when I turn on the ignition before I start the truck I’m not getting the idiot light for the oil so I might have something wrong on that circuit.


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[Ronmar]

Ok, you most definitely have some other hack jobs. K11 does indeed provide the power to the start button(pin 30-87 when energized) to energize the starter, and K11 must be energized to allow the starter to function. There is a backup circuit on the ground side that insures K11 stays energized during long cold cranks where it might reach enough oil pressure to de-energize K11 and interrupt the crank. So K11 energizing/LED coming on during a crank is a good thing.

i would disconnect the wire labeled TL35 from alt ground, then determine if it has a connection/circuit up to K11 pin 87A. I suspect it does not because if it did it would be shorting power from CB77 to ground. There are 2 connectors between K11-87A and TL35. Up in the PDP, P302 pin14, and down on the main engine connector P191 pin 30. Once you figure out this circuit, you can go back to powering the alt by it’s normal system…

when you turn on the ignition switch the transmission powers up and provides power to energize K26(neutral start). K26 provides power to tell the ECU you are in neutral to allow it to start the engine, and it provides power to the coil of K11. The other side of K11s coil goes to the oil pressure switch on the drivers side of the engine, and when it is below 15 PSI it completes a circuit to ground. The line to the switch also branches up to the idiot light panel. This path to ground causes K11 and the oil light to energize When IGN is switched on. K11’s contacts 30-87 provide 24v ign power from CB77 to the start switches.

so somewhere, perhaps under the power panel, power has been hacked to bypass the de-energized K11 and reach the start button/switch. There are 3 screws in front of your knees when seated in the pax seat, and 3 more up under a lip above the power panel toward the window. This will allow you to lift the power panel up out of its hole ao you can get to the bottom side.

That white wire on the regulator. You need to disconnect it and follow it back to wherever it comes from…

I hate hack-jobs, hopefully the amateur didn’t butcher your wiring too bad…

 

[Ronmar]

Does that white wire on the regulator have any numbers stamped on it? that might be able to give you an idea where it runs to. On the A1 they installed a LBCD to disconnect the battery, but you do not appear to have that equipment(box behind batteries with manual and remote disconnect switches). I call trucks wired like this an A0.5. basically an A0 with a 3126 engine and the circuit board power panel… At any rate they ran 2 wires to the regulator. One is the excite wire from K11 to engage the alt. The other was an RPM sense wire that runs from the AC terminal on the reg to the LBCD(where the polarity box should be, between spare and air filter). This wire sent alt RPM info to the LBCD. The LBCD used that and voltage to determine if the alt was overloaded, and remotely disconnect the batteries if it was overloaded.

the hacktivist may have pulled power from the vicinity of the polarity protection device using this wire, to power the regulator

 

[GeneralDisorder]

There appears to be a burnt PDP circuit board trace in the vicinity of CB77 as well.

 

[Ronmar]

There appears to be a burnt PDP circuit board trace in the vicinity of CB77 as well.

Oh yea there is, good catch. I didn’t see that, despite it being blatantly obvious in the pics

Grounding that TL35 to the alternator ground could cause that, along with over-fusing CB77 at some point… Pretty easy to repair with an external wire from CB77 socket to relay socket 30 on the under-side of the circuit board. Anyone who can solder reasonably well could do this in about 10 minutes on the bench. Thats a drawback of the printed circuit power panels, they do not handle overcurrent well, so DO NOT OVER-FUSE, or short past/bypass fuses in troubleshooting…

this of course would also have disabled power to the start buttons/switches, so he will still need to figure out where that has been hacked…

 

[Ronmar]

On a brighter note, the wire down to TL35 at the alt probably has a good circuit