Relay / electrical help

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P man

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Help please

1997 tj 2.5 new to me. It has had questionable previous ownership. I found this relay in the engine compartment. It is wired as follow:

1 wire to back of alternator
1 wire to open pin in fuse box
1 wire to ground
1 wire to positive on battery.

What were they doing here? Wouldn't a relay be in line with the device it's relaying? Any ideas? I would like to remove it all but need the jeep operational. Thanks in advance

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The one wire from the fuse box is probably a batt+, maybe even an ignition switch batt+. This may be to turn the alt output "on" to the battery, with the ignition on.
I'm wondering if the PO had or thought he had a battery drain issue, thought it was through the alt and tried to hack it this way.
 
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You do have a really nice hot air intake system there, wow!

I guess that I should ask if that is the only wire coming off of the alt.

If not, it may be some kind of hack to power up the circuit that it is plugged into in the fuse box.

If you get a chance, let us know which wires go to which numbered terminals on the relay. Or take a close-up, after your hands thaw a bit.
 
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dbbd1 said:
You do have a really nice hot air intake system there, wow!
Click to expand...

If you think that's nice you should see the rest of the jeep!!! It was stolen and the tweakers were living inside of it with their dog. They spray painted it 10 different colors in the process. Insurance totaled it because it would cost more than the jeeps value to repaint it.
 
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Nevermind. I took a closer look and if it's the yellow wire on the alt that goes to the relay, they have been using for the field windings. Then you have to go with #Wildman.
 
The pin they’re grabbing from the PDC is ignition power from the window defogger relay spot. My guess is the PCM gave out on “turning on” the alternator (via its exciter) and so they are probably using an ignition switched relay to do so now.

What wire pin on the relay goes to the PDC and which wire goes to the alternator? And which location on the alternator does it go to?
 
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You'll have to trace the wires.

The relay is in line with the device it is switching. But the wiring can be routed wherever one chooses.
Relay pins 87 and 30 should go between the battery and the device to supply 12v power. 85 and 86 go between the signal power and ground.
 
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Just so I don't omit any info..there is another wire coming out of the fuse box and runs behind the battery to an inline fuse and then to the positive battery terminal. This is seperate from the relay just added information

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Yeah, they are using the relay to energize the alternator field (+) which is usually done by the PCM. The PCM appears to control the charge rate by varying the resistance to ground on the (-) of the alternator field. So I’m guessing the PCM usually supplies a simple battery voltage and for whatever reason that quit working.

K72 is the field (+) circuit and K20 is the field (-) if you want to search for FSM for more reading info.

Here is the alternator connector info.

IMG_3411.jpeg
 
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machoheadgames said:
Yeah, they are using the relay to energize the alternator field (+) which is usually done by the PCM. The PCM appears to control the charge rate by varying the resistance to ground on the (-) of the alternator field. So I’m guessing the PCM usually supplies a simple battery voltage and for whatever reason that quit working.

K72 is the field (+) circuit and K20 is the field (-) if you want to search for FSM for more reading info.

Here is the alternator connector info.

View attachment 491334
Click to expand...

So in your opinion that part of the PCM has failed? So if I was to take this relay out the alternator would not charge the battery?
 
P man said:
So in your opinion that part of the PCM has failed? So if I was to take this relay out the alternator would not charge the battery?
Click to expand...

That is my guess, I don’t know for sure. I know the PCM controls the alternator field. I don’t know exactly how it does it whether it simply energizes the field with power and ground off and on or if it resists the ground side. But yes I would assume something in the PCM went wrong, I just don’t know exactly what.
 
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There are aftermarket external voltage regulating kits that can do the job if needed. The main thing is an alternator’s voltage is supposed to fluctuate depending on factors which is part of why they use the computer to do it in many modern vehicles. I’m not sure if the relay for the power side of the field is screwing up the ability for charge rate to fluctuate or not. But then, the aftermarket external kits have you turn a screw to set the voltage and that’s the voltage you get all the time, so maybe you don’t need fluctuating charge voltage too badly.

The kit I’m referring to is ERCK
 
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machoheadgames said:
There are aftermarket external voltage regulating kits that can do the job if needed. The main thing is an alternator’s voltage is supposed to fluctuate depending on factors which is part of why they use the computer to do it in many modern vehicles. I’m not sure if the relay for the power side of the field is screwing up the ability for charge rate to fluctuate or not. But then, the aftermarket external kits have you turn a screw to set the voltage and that’s the voltage you get all the time, so maybe you don’t need fluctuating charge voltage too badly.

The kit I’m referring to is ERCK
Click to expand...

So let's say I take the relay and wiring out. Could I verify proper voltage with a meter? I did notice the PCM has junkyard yellow marker on it? I'm hoping they replaced the pcm and fixed an issue but we're too lazy to remove the relay and wiring?
 
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My guess is PCM is original and still bad and the relay is keeping things working.

I don’t know for sure but I would assume K72 from the PCM is no longer supplying the voltage it’s supposed to for whatever reason. You could use a multimeter with the jeep running and see if your voltage is at charge level or not with the relay removed.
 
This got me interested and so I started digging in further. I referenced both YJ and TJ manuals since the early TJs use the same alternator and basically an identical charging setup as the YJ save for a few small details.

On a YJ, the pin 1 (K72 - the one you have tied to the relay) comes straight off a splice after the ASD relay. On the TJ models, they decided to run that ASD relay output to the PCM and then the PCM turns around and gives that voltage to the alternator pin 1 as field supply voltage.

Pin 2 (alternator field (-) K20) is how the PCM controls the charge rate. The PCM varies the resistance to ground on that circuit. I found this out by going into the troubleshooting/diagnostics section in the YJ FSM. TJ might have that same section but I missed it. For the diagnostics, you put a jumper on the K20 alternator pin and connect it straight to ground. This throws a DTC and should set the CEL off for alternator malfunction. So when things are good, the alternator field negative from the PCM will have resistance to ground at least somewhat.

SO....the issue at hand - the relay. There is nothing wrong with this setup, other than it being pretty ugly. If you can make it clean then it will continue to operate the alternator exactly as it is supposed to. Essentially you are wired up like a YJ, and pretend that your relay is a secondary ASD relay since the YJs powered that alternator field (+) terminal right off of the ASD relay output. Instead of that or the PCM, you have an added relay being used as a fix.
 
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