I have a problem with my disco and it goes like this. I went to my baby last night after classes and she was dead. I had a jumper box with me so I jumped her off. I got about 4 miles with the tach going crazy the anti lock light and the SRS light on and then she died while I was driving on the freeway. Lights out and all! Very scary. I got a tow home and when I got up this morning I jumped the battery again and put a multimeter on the battery and it was above 13V so I let it run while I watched the multimeter go down, down , down. Disco died around 8V. So I checked the wiring to the top wire on the alternator (+) and the negative on the battery and it lit up my light tester. I removed the alternator and took it to autozone and it tested good. I cleaned my wires and put the alternator back in and jumped the battery. Tach was good for about three minutes. It went crazy and then voltage drooped and she died again!

Any suggestions? Its a Bosch AL9347X alternator.

Thanks,
Jason

 

Well, could be two things:

Replace alternator or take to indy local alternator/starter shop to rebuild. Rebuilts from most auto parts stores have a high failure rate, if you go that route save the receipt with your important papers for warranty later.

Or something is making a good alternator not have output:

First, disconnect battery. Clean every terminal and cable end. Open fuse box under hood, remove and clean with wire brush the two large wires bolted down in front. Also remove and clean with wire brush the 100 amp fuse, which is inline with the alternator output. If any of these points had a build up of corrosion, it can act like a high resistance, that will vary with a mind of its own, and reduce voltage output from alternator.

If you have a volt meter you can check for this by reading (-) battery to various other (+) points mentioned, if bad connection you will see a change in voltage. Do this with truck idlling , AC and headlights on, which provides some load on the system.

Also do battery under load test with truck turned off, voltage will drop very very slowly. If drops quickly, bad cell in battery.

The alternator has three wires from it, the big one is output power, the smaller one is the "excite" voltage to run the alternator (measure with voltmeter - controlled by Fuse 14 in fascia fuse panel, which also runs some cluster items). There should be good steady volts there. The flying plug in lead is the altenator to tach wire, has a series of pulses on it that tach counts. Will show up on AC scale of some meteres, volts not very high there, what "counts" is the pulses. About 270 hertz (old skool cycles per second) at idle. The tach lead is not required for charging, but it is an indicator that there is no charging. See page B1 - 9 in the electrical troubleshooting manaual of the RAVE.

 

Obviously when your alternator heats up the voltage regulator quits working.
You need a new alternator.
Bench testing it will not allow it to heat up.

 

Spike has a good point. The air duct function of the radiator cowl pushes hot air into the alternator, way hotter than just under hood air. See pix - over 230 degrees....

 

I appreciate the replies. Before I had read what you have written I took my battery to have it tested and they said it was bad. So I got another one. I put it in the car to see what would happen and it is stays running, battery voltage at 11.8_. However, my tach still isn't working right, the SRS light and antilock light are still both illuminated. Do you think it's the alternator still?

Thanks.

 

Yes - either bad alternator, or bad wiring connections to back of alternator, blown 100 amp fuse, etc. You are simply draining the battery on "reserve" running time. If you have a battery charger, charge up battery so you can drive truck short distances. Charging voltage should be very high 13's up to 14.2, or so.

 

Well, Im in the damn warrantee cycle as well, running on the third one now.

 

I feel your pain!

 

You can get out of that cycle, buy a used alt, keep it in your garage.
When this 3rd alt fails install the used one and demand your money back.
You dont want to keep replacing alts and they dont want to keep giving you "new" ones.
I got my money back, plus my core charge.

 

Yeah, its on the list... How do I determine by sight if its factory?