Mercury Repair Questions? Ask a Mechanic for Answers ASAP
That sounds like an internal regulator problem. Before installing the next one, check the wiring; Yellow/White and Black/Orange should have voltage at all times; Light Green/Red should have voltage only when the ignition is on. If those are OK, check engine ground for any possible voltage drops. If all checks out OK, then I would advise getting the next alternator from a different source.
Should not be that way... (see attached diagram from Ford) What's the VIN?
Let me do a little research on that VIN. Be back in a bit...
OK. Yours is a late production vehicle. These have the computer-controlled alternator, whereas the onboard computer serves as regulator.
Basically, the old alternator burnt the driver inside the computer, so now it needs a new computer (PCM) to correct the problem.
This issue can be easily confirmed with a NGS tester or equivalent scan tool capable of reading that PID.
The PCM would need to be configured to the VIN in order for the security/anti-theft system to work.
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An alternator puts out current, not just voltage or amperes. Current is measured in Watts (Watt = Amp X Volt), which is how electrical loads are rated. For example, a starter motor that pulls 60 Amps is actually pulling 720 Watts from the battery (720 Watts = 60 Amps X 12 Volts) As the bad driver in the PCM is erratically exciting the alternator windings, it starts producing erratic amounts of current, which in turn make the electrical loads act erratically as well (lamps will get brighter and dimmer, motors will speed up and slow down, etc.)
In reading your posts, I get the impression that you are not completely convinced of my diagnosis. I understand that, but you also need to understand that Ford designed this computerized charging system to be diagnosed using specialized tools. So, the only way I could tell you what the problem was without any doubts, would be if you had access to a NGS/WDS or equivalent scan tool so I could guide you through all the correct diagnostic procedures. As is, all I can do is offer you my knowledge and experience.
Please allow me to clarify that experts on this site are mostly OEM Master Technicians who have had OEM training and follow diagnosis and repair instructions set forth by the car manufacturers. In other words, being an expert does not mean we know how to diagnose and repair vehicles without using the correct tools.
Back to our Mountaineer, your mention of the Yellow/White wire makes me wonder if we're even talking about the same system. Could you look at the attached diagram and tell me if that fits the vehicle you're working on?
Here is that diagram.
OK. Then it's the one I'm attaching now. In which case, it is not computer controlled - PCM has nothing to do with it.
In this system, the Yellow/White wire is fed from the 30A Alt fuse in the Battery Junction Box and is supposed to have full voltage all the time. If you have fluctuating voltage there, check the fuse and the junction box to see if the connections are clean and tight. If all of that checks OK, then there is a problem with the internal regulator in the alternator.
In some cases, a bad battery will cause similar symptoms. Make sure the battery is load-tested and fully charged before connecting the new alternator.
Please let me know what you find.
Here is that last circuit diagram.
A battery that's fully charged will have 12.6 volts across the posts after a load test.
As shown on the diagram, the Yellow/White wire comes out of the battery junction box and should have 12 volts.
Do you have access to a Volt and Amp Tester (VAT) that could tell us what the charging rates are with different loads on?
Also, do you know the vehicle's service history that could tell us more on how this happened?
Just thought about something; how about running a jumper straight from battery positive to where the Yellow/White wire connects to the alternator? Can you try that and see what happens?
Hey, man. Just wanted to check and see how you were doing with that Mountaineer.
Thank you for taking the time to give me feedback on this thing - it was certainly one of those I don't hear about everyday.
I hope the jumper I asked you to run helped you and your friend diagnose the issue.
I'm very glad you hung in there and got that puppy fixed!