2006 Mercury Montego Overcharging Condition

[montegoman]

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. 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".

Anyone have any ideas?

Thanks.

 

[Tmoose]

I there is resistance and thus voltge drop in the wiring or connections that the regulator gets a whiff of the system voltage then the regulator will crank the control voltage too high. A higher level repair manual will likely describe test points right on the alternator/regulator to confirm system info is good.

 

[montegoman]

There are 3 wires coming out of the alternator connector GEN COM, GEN MON, and A-SENSE. I checked the A-SENSE wire and it is good, only .5ohms. I'm trying to figure out if it is my PCM or a bad alternator. I just don't know how to test the GEN COM and GEN MON to find out if the PCM is operating within the correct parameters. I did find out that the voltage regulator has a battery temperature sensor that reports to PCM and can increase and decrease voltage based on temp. I'm hoping this or the voltage regulator has failed verse the PCM. It's odd. At idle I'm getting 13.8V-14.2V at battery and everything is functioning properly. When motor is revved up to over 1500RPMS everything goes dead then rebounds once back to idle. Thank you for the response. Any and all suggestions are welcome.

 

[MikeHalloran]

You have not eliminated one possible plague: missing/ corroded/ broken ground straps, everywhere.



Mike Halloran
Pembroke Pines, FL, USA

 

[oure543]

I was wondering if you ever found out what was causing the overcharging problem. I also have a 2006 Mercury Montego with a very similiar issue. It is intermittent and I have not been able to track it down. So far we have changed out the alternator and battery about 3 times a piece and the PCM has been changed once with no luck. About once a week or two when starting the car I notice the tach and other gauges go to 0 and the alternator voltage goes to as high as 20Vdc. If I shut off the car and disconnect the battery and restart everything is back to normal and the car runs fine. Any help with this would be greatly appreciated.

 

[GregLocock]

sounds like a bad earth

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[waross]

Something that is often overlooked in automotive trouble shooting is the return path, or ground. You may have a loose or corroded ground connection somewhere. Try checking the resistance between the PCM and the battery negative terminal. Check for voltage first. That is a NO VOLTS condition before doing a resistance check.
I agree with Mike.
Is the battery voltage as measured at the battery going high or is it just the PCM reporting a high voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter