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2006 Mercury Montego Overcharging

Old 11-18-2014, 09:13 AM
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Default 2006 Mercury Montego Overcharging

Hi everyone. Looking for some much needed advice on the issue I'm trying to diagnose/fix on my personal vehicle. I have a 2006 Mercury Montego Premier. Currently, the vehicle is operating normal at an idle. At idle the voltage from the alternator to the battery is 13.8V-14.2V. When I increase RPM's the alternator increases voltage to over 17V and happens at about 2500RPMS. When this overcharging condition occurs all modules shutdown (cluster, interior lights, ect). When RPM's decrease to idle all modules come back.

When problem originally occurred, prior to diagnosing with a meter, I replaced the battery and alternator. PCM was putting out a high voltage and low voltage code. I thought that this was going to be a simple alternator issue. After replacing alternator and battery, I still have the issue of modules shutting down upon increased RPM's, however it appears now that is only an overcharging condition, but I'm not 100% confident on this. We might be dealing with the same exact issue prior to replacement of alternator and battery.

The alternator has a 3 pin connector. There are two wires that communicate to PCM and 1 wire that runs from the alternator to the battery junction box and then to the battery. This is the 1818 circuit and I believe this wire detects what voltage the battery is getting from the alternator and then the internal regulator increases or decrease voltage.

I checked the 1818 wire for any voltage drop and all appears good. Also, I checked the voltage the battery was getting at the battery and checked the voltage at the alternator connector. It was the same. The wire is good.

Based on the diagnosis, I can only assume that the alternator I put in is bad. The only concern I have is if the other two wires that run to the PCM actually regulates voltage verse only the 1818 wire. I think those other two wires are used just to report to the PCM what the voltage is and then reacts to high voltage by shutting down modules to prevent damage, and do not actually regulate voltage as some newer vehicles have "smart alternators". I also read that there is an internal battery temperature sensor that regulates voltage and this is communicated through the 2 wires that run to the PCM.

I'm trying to rule out the PCM as the culprit that is inducing the overcharging event...

Anyone have any ideas?

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