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Mark, 22 Year Factory trained technician
Category: Jeep
Satisfied Customers: 11332
Experience:  Jeep factory training........25 years in the industry.
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I have a 1990 Jeep Wrangler YJ. The altenater is not charging

Customer Question

I have a 1990 Jeep Wrangler YJ. The altenater is not charging the battery. I just had the battery tested it is fine no dead cells. The alternater is putting out 12.7amps.When i start it and disconnect the battery it dies immediatly.This problem has been ongoing and i have a Hanes repair manual.I just cant understand wiring diagram.The voltage regulator is in the alternater and is ok.Is ther anything else in the components that can be changed or checked? Battery terminals have been changed also.
Submitted: 7 years ago.
Category: Jeep
Expert:  Mark replied 7 years ago.

Do you have a volt meter to check a couple of things?

Customer: replied 7 years ago.
yes sir a digital
Expert:  Mark replied 7 years ago.

Ok, The large battery wire at the alternator.....can you tell me the voltage reading on that wire.


Also, the 2 wire plug that goes into the alternator, can you tell me the voltage on each of those wires with the key in the on position.

Customer: replied 7 years ago.
Ok Mark lets see if i can explain. At alt plug red large wire is 15.8 the small gray or black wire at plug is 14.7 to 14.8 the bolt on back of alt is 15.8 .Now my battery cable has 2 leads large red goes to starter and small red goes to a terminal block on fire wall and is bolted at top of terminal. The 2 leads from alternator 1 from plug and the one from the bolt on back of alternator are twist tied together with a black changable 2 prong fuseable link and then bolts to that top bolt on the terminal on the fire wall. This concerns me that both alternator leads tie together this way. With jeep running meter test on battery is 12.2
Customer: replied 7 years ago.
Mark im sorry i did test to wrong. With key in on position at plug red large wire is (O.OO) the gray or black small wire is 8.9
Expert:  Mark replied 7 years ago.

Here is a pic of the charging circuit.


The brown wire at the alternator is only supposed to have 2 to 4 volts on it because they are using a resistor wire to lower the voltage.........unless this has been bypassed?? if you have 8.9, then we'll go with that.


Now if you do not have 12 volts on the other wire. then according to the diagram, you probably have a bad fusible link up near the starter relay




Also be sure and test all your fuses. There should be one or two fuses in the underhood fuse box that relate to the alternator.



Customer: replied 7 years ago.
Mark i do not locate a under hood fuse box or like a power ditribution box so im not sure where those would be located is it possible the starter relay is bad even thought the jeep will start when i charge the battery.
Expert:  Mark replied 7 years ago.

Ok, the power distribution box was brought into play in late 90 or early 91 so maybe you don't have that. The fusible link I'm referring to is a section of wire that is designed to burn or melt and according to the diagram it is between the alternator and starter relay on that wire with zero volts.


No, I'm not expecting to find a bad starter relay here.

Mark and 5 other Jeep Specialists are ready to help you
Customer: replied 7 years ago.
Ok i will try these things and thanks for the diagram.
Expert:  Mark replied 7 years ago.
You're welcome.

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