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Ok, so here's the story. 2003 Focus LX, 120,000 miles, 2.0L, SOHC. A couple of weeks ago, I go to my car after leaving it parked all evening and it won't turn over. No clicking, no anything. I have a buddy jump me off and it won't charge enough to turn over. We take the battery to a shop, have them check it (checked out fine) and charge it, then threw it on the car. It started up fine, so I didn't think too much about it until I had another problem. A couple of weeks later, I was driving and the car just died. No warning lights or anything. Just the scary silence and lack of power steering, accompanied by my frantic search for a place to park within the range of the inertia of my vehicle.

Once again, it's a dead battery. Same friend jumps me off (lifesaver, I know) and this time it takes the jump. I take the car to an auto parts store and they test the alternator with their alternator/battery tester, and it tests bad. They suggest that I check my fuses and relays before replacing the alternator, so I did just that. Turns out that the R10 relay was bad (badly burned and melted housing). Replaced the relay, was the cooling fan 2 relay. They put the alternator tester back on, and lo and behold, it tested fine. I drove away, thinking I had just averted an expensive and PITA repair; this feeling lasted about 2 weeks. Once again, I was driving and the car began to show signs of dying again. This time, it happened when I turned the air conditioner on. When the compressor kicked on, the lights dimmed and the radio turned off. When I turned the air off, it came back on, then it finally just died. I checked the relays, and the one I had just replaced was fine. R16, however, was blown. This one is the Cooling Fan Low Speed Relay. I replaced it, took my battery to charge once again, and this is where I am now. A mostly full battery, two good relays, and 12.4 volts across the battery even at high RPMs.

I have noticed that the charging seems to be intermittent--sometimes it shows a healthy voltage (~14v) and sometimes it shows the 12.4. I checked all of the wires from the alternator and the cooling fan, and to a first approximation they all seem fine (no corrosion, no worn insulation, etc). I also checked the resistor inside the fan assembly, and that seemed fine as well.

I have a couple of questions about my problem. First off, does it seem like this is an alternator problem? My *guess* is that if I took the car to the auto parts store with it not charging like this, then the alternator would show bad. Why then did replacing the relay seemingly fix the problem? Would a bad relay cause the alternator tester to think the alternator was working? Next, what could be causing the cooling fan relays to blow? Is there any reason why they're not blowing consistently (first the R10, then the R16)? Lastly, do you think there's any way that a failing alternator could be sending voltage spikes through the system, frying the relays?

Thanks so much for your help in advance. I am sure I left out some critical detail, but just let me know if you need more information. I'm at my wits end; I hate electrical problems!!!
 

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DTC P0606
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IMO, it's likely the alternator's internal voltage regulator (VR). When these fail, they do not necessarily provide a low system voltage. As you've already guessed, they can fail in a way that's creates voltage spikes. Because modern car electrics rely on a consistent voltage within a given range, the symptoms you describe - dead battery, dimming lights, failing electrics, intermittent charging voltages - have been noted before on this forum as a sign of alternator problems. Before you replace or rebuild the alt though (VR is a non-serviceable internal part), check the plug that connects the alt to the wiring harness (known to break), check the engine grounds for corrosion, check the continuity of the the positive lead (in line fuse fitted in some models), and check the battery terminals for cleanliness and tightness.
 
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