dzywicki

Have a new problem on my 99 XJ. It's been running great lately, until today. I replaced the alternator with a 117A NAPA (different story) over the weekend and now I keep blowing the ASD fuse when I accelerate hard. I initially thought it was when I turned hard, but now it seems to be when I accelerate hard. I saw a thread where someone had an alternator cause the ASD fuse t blow. Has anyone heard of this before? I could see it if the amperage were to spike up for some reason. I have dealt with the fuse before when I moved an injector and it shorted against the throttle cable bracket. I know that the ASD system has the injectors, coil and O2 sensors. I checked all those wires and even added some extra electrical tape to the injector connectors where the old tape had gotten crusty. I have moved every wire, connector and harness I can get to in the engine bay and the O2 wiring underneath while idling and have not been able to reproduce the problem.
I am suspecting the alternator since that is the only thing that has changed before this problem started. Has anyone had this happen before with an alternator? The thread I saw said they disconnected the alternator to test and the problem went away. That's my next test but wanted see if the wisdom here had already dealt with this problem.

dzywicki

After some more reading, I tried an experiment just revving the engine. It blew the fuse as soon as I hit about 5000 rpm. Looks like I'll be taking that alternator back and getting another. I didn't like the brand selection on amazon or rockauto so went with NAPA since they are close and was the only one I could find with a lifetime warranty.

jordan96xj

I would be more concerned about the system voltage being produced by the alternator than its total amperage capability. It would have been good to have a multimeter measuring system voltage when you did the 5000 rpm ASD fuse blowing test.

Do you have a stock ignition coil? or have you added a high-performance coil at some point?

dzywicki

Stock, original coil. The alternator is this one. I have used that NAPA store pretty regularly over the years with all the vehicles I have so I know them pretty well. I'm going to take it back out and try a different one. Here is an image of the test sheet that came with it.

jordan96xj

Those are good numbers to have - and generally look healthy. Unfortunately, they might not show what we need to see. Almost everything related to volts is missing from the sheet. That amperage load test is performed using a fixed voltage of 13.2v (shown just below the 2nd table). They draw as much amperage as possible as long as the alternator can keep the volts up at 13.2v. When it can't keep the volts up - that determines the amperage output limit at that speed (ultimately producing the bottom table/graph).

What you want to know is what is happening to your voltage when you increase your RPM - and the alternator's voltage is being controlled by the PCM. This is as easy as putting the multimeter probes on your battery terminals and watching the value as your manipulate the throttle body by hand. Does it get higher than 13.5v ? How high is it when the ASD fuse blows? Let us know.

dzywicki

Yep. I ran out of time last night. Now that I know how to reproduce it, I'll do one more check with a voltmeter. I already called NAPA and they said they can exchange it. I have owned this XJ since new. Also have a 98 so I know them well. I'll do the test and then pull the alternator and exchange it at NAPA. Luckily the alternator replacement is about 30 minutes (unless your bimmerjeeper ). I am highly confident that the alternator is the problem here. Probably should have just gotten a new one of a different brand instead of the reman from NAPA.

lawsoncl

Does it blow the fuse if you unhook the alternator control wire?

97grand4.0

Why is it putting out so much current? 120 Amps is crazy unless you have something using that much. Is this a load test? Where is the voltage to rpm chart?
And what jeep engine regularly does 6000 rpm>?>

Scotty will be on the intercom "I can't push the engines much further Captain"

Thanks

lawsoncl

I think you missed the part where he said "Here is an image of the test sheet that came with it." During a load test, the field coil is fully energized and the electrical load is adjusted during the rpm sweep to hold a steady voltage (13.2 it says on the sheet). The alternate has a smaller pulley than the crank, so it runs much faster than the engine.

dzywicki

I don't regularly rev the engine that high. It was happening during regular driving such as accelerating out of a corner. I couldn't figure out what was causing it to blow and stall while driving. I read a few threads with a similar problem and then just tested it by revving it in the driveway and it blows the fuse every time it get near 5k RPM. I took out the alternator and going to exchange it at NAPA this morning. Will have the replacement in this evening and hopefully have this problem gone.

dzywicki

Swapped the alternator and all is good again. Revved it with every accessory on. Drive it in 1st gear to get the RPMs high, punched it a few times and no blown fuse. In the future, I' think I'll stay with new over remanufactured. I went with NAPA for the lifetime warranty, but that probably means they have spotty quality control and expect them to fail. If something happens to this one, I'm going to go with this one: . Rockauto shows it with a limited lifetime warranty for non-commercial use. The ones with 12 month warranties just make me nervous.