Cat 420eit Alternator charging problem?

[justsomedude]

2010 Cat420Eit DAN00662

Charge light on / no tach / battery voltage only

I replaced the alternator but it seems like its not charging / no tach. with the key on motor off the exciter post reads 2.6v.

I touched a wire from the battery to this post with the motor running and the charge light went off and tach started to work until i removed the wire so I'm assuming it really wants 12v. I'm seeing all sorts of conflicting answers about what voltage this exciter wire should read and at what time. Maybe its machine dependent? From looking at the schematic i see that there is some kind of resistor that plugs in somewhere on the harness but the parts manual does not show a call out for it so i have no idea where that is or if it could be my problem. If someone could point me in the correct direction on if exciter voltage is supposed to be 12v or 2.6v and / or where this resistor is hiding out i would appreciate it.

 

[justsomedude]

So i have been looking into this and the only thing i can think to do is test the excitation resistor (238-9397) or check the connection in case its bad and causing voltage loss but i cant locate where it is. I've been digging around in the wiring for days now trying to find it with no luck at all. Has anyone ever had to replace or find this thing before?

 

[heymccall]

It looks like this

It says it's on engine harness.
Should be with 1 foot of alternator.

 

[justsomedude]

yep thats the part i have been looking for. I was expecting it to be close to the alternator but i cant seem to locate it. The area above the alternator just has wiring for coolant temp and the area below seems just to have wiring for the AC compressor and the horn. The wiring schematic seems to show that this should be real close to the alternator though. I'll give things a 5th look to make sure i'm not crazy once the rain here stops though.

 

[Nige]

In the Parts Manual it is listed as an integral (but serviceable) part of the 290-6751 engine wiring harness, so expect it to be buried beneath the harness sheath somewhere.

I dunno, look for a "bulge" in the harness close to the alternator maybe.?

 

[justsomedude]

Hi Nige!

ya i was looking at the parts manual and was a little disappointed that there isn't a call out on that diagram that shows where that component lives. It must not fail very often i guess. I've been working off of the wiring schematic you directed me to previously when troubleshooting the auto up function a few months ago. I ended up changing the alternator just recently and trying to get it to work. Odd thing is when i click on the part name in the pdf for it to show me the location on the machine it seems to point at the area where the platform harness sits under the floor pan on the passenger side. I've taken up that floor panel and been looking in there also. I see lots of other end points but none that have the correct wiring colors or have something plugged in that looks like the part above. I'll see if i can find a bulge in the sheath though.

 

[heymccall]

On my KMW 420Eit, the stick drawing shows it below alternator.

 

[edgephoto]

Can you post a wiring diagram? I can probably come up with a few tests to perform so you will know exactly what is wrong vs. guessing

 

[justsomedude]

On my KMW 420Eit, the stick drawing shows it below alternator.
View attachment 298126

Ya that’s what I’ve been looking for but it does not seem to exist on the machine itself. Should be a grey wire and a white wire going to a cylindrical endpoint.

 

[edgephoto]

@justsomedude A member was kind enough to send me a schematic for you serial. You stated you applied 12 volts to a terminal on your alternator and the light went out.

From reading the wiring diagram if you applied 12v to the terminal with the grey wire and it charged you have one of two things wrong. Either a wire issue from the charge light (L5 in diagram) to that terminal or you have a bad regulator. That bulb is an LED according to the diagram.

I have a few questions:

1. When the engine is running is the Alternator (charge) bulb lit up?

2. when the engine is running do you have 12v at terminal A at the data link connector? It is a White wire.

If you answer yes to number one then you have a bad regulator.

If you answer yes to number two the exciter resistor is good.

Don't worry about the yellow wire. That is for the tach. It won't work until the alternator is working correctly.

Make sure to use a decent meter not a test light.

 

[justsomedude]

@justsomedude A member was kind enough to send me a schematic for you serial. You stated you applied 12 volts to a terminal on your alternator and the light went out.

From reading the wiring diagram if you applied 12v to the terminal with the grey wire and it charged you have one of two things wrong. Either a wire issue from the charge light (L5 in diagram) to that terminal or you have a bad regulator. That bulb is an LED according to the diagram.

I have a few questions:

1. When the engine is running is the Alternator (charge) bulb lit up?

2. when the engine is running do you have 12v at terminal A at the data link connector? It is a White wire.

If you answer yes to number one then you have a bad regulator.

If you answer yes to number two the exciter resistor is good.

Don't worry about the yellow wire. That is for the tach. It won't work until the alternator is working correctly.

Make sure to use a decent meter not a test light.

Hi Edgephoto. I really appreciate the help here.
1. when the engine is running the charge bulb is lit up. Seems to flicker a little though.
i did note that when i ran a wire from the battery to the exciter post and measured the output it seemed high 14.8 - 15v which maybe supports a bad regulator? This is a new alternator. I replaced the previous one because it was showing the same symptoms and thought it had failed.

2. i have not tested that. Is this the round service connector in the cab near the floor on the passenger side?

For clarity the grey wire with engine off and key on should read 12v? Currently 1.98 but will fluctuate if i slap the alt case a bit.
with engine on should also read 12v?

How I'm reading this schematic is that with the key on engine off 12v comes down the white wire
123 EN12 WH18, passes through the resistor onto 309 en1 / en3 gy18 which applies voltage to the exciter post as well as back to the other side of the charge bulb which shuts it off but that kinda makes no sense or the bulb would be off all the time. I'm missing something for sure here.

 

[edgephoto]

the 12volts comes through the bulb which is actually an LED(diode). The resistor is fed from 309(grey wire) and goes to Pin 3 of the water in fuel sensor. With the engine running you can see if you have voltage at pin 3 of the water in fuel sensor. If the sensor is connected I would expect the voltage to be around 5 volts and jump to 12 volts if you measure it disconnected. If you have voltage here then the resistor is good and who cares where it is located.

You could have a corroded wire between the charge light and the alternator. Easy test is to run a jumper wire from the bulb to the alternator.

 

[justsomedude]

Ok that makes sense. I was definitely reading the flow of electricity wrong here. I will dig into that side console and see if one of the connectors is oxidized and causing voltage loss and a couple other tests now that I understand that circuit better.

 

[justsomedude]

With key on engine off I went to the circuit board for the battery bulb. The input voltage for the bulb is the same as the battery through wire 133rh1 but where it outputs from the circuit board onto 309rh72 it’s only 2.8v. From what I understand I should be expecting 12 or somewhere around there? Just want to confirm that before I call it bad and replace / repair it

 

[justsomedude]

SOLVED - So after much fooling around I've found that the alternator that was supplied and part matched by number from a 3rd party company is either the wrong part or is not the correct design or faulty. The original alternator is externally excited however the part that was supplied, while it looks the same, has the same posts in the same layout and is claimed to be a direct swap for my cat part number is actually internally excited and charges correctly if only the battery cable is connected. Any other cables connected causes the alt to not charge. All of this trouble for nothing it seems. I appreciate everyone who kicked in their ideas thoughts and troubleshooting methods on this. I hope all these ideas and methods can help someone in the future.