Need help with weird issue

[stang_gang]

i just bought a 2006 mustang gt with 57,000 miles on it all stock. I’m having an issue where the car will have a surge of some sort where the lights on the gauge cluster light up and the car revs up to about 2000 rpm for a split second and then the rpm gauge works but at about 1000 rpm higher than it should be. When I shut the car off the rpm gauge sits at 1000 instead of 0. I opened the gauge cluster thinking it was just a weird hiccup and set it to 0. All was well and working till this morning the surge reappeared everything mentioned above happened again. Any ideas what it could be? I drove this thing all the way from Houston to dallas with no issues and the only thing I think was different was the lights were on versus off. Could it be the alternator?

 

[ChewyR]

I would start there, the alternators on these cars are failure prone, and the ecm is quite sensitive to voltage.

 

[stang_gang]

That’s what I thought it might be. I also checked the fuse box on the passenger side to make sure it wasn’t damp since it’s been raining a lot. But it was dry as a bone and when I drove from Houston to dallas with no issues it was pouring the entire time so I highly doubt it’s that.

 

[Dino Dino Bambino]

Yup, it's quite possible that one of the alternator diodes is intermittently failing causing an AC ripple current to leak.

 

[stang_gang]

Yup, it's quite possible that one of the alternator diodes is intermittently failing causing an AC ripple current to leak.

Would autozone be able to test if the alternator is causing this

 

[1950StangJump$]

I’m not saying the alternator isn’t causing this ..... but know going on that it’s a shot in the dark. As long as you can absorb the cost knowing it might not be the cause, go for it. Maybe you’ll get lucky.

Seems like immediately blaming the alternator for a wacky electrical gremlin is a little capricious.

 

[stang_gang]

Today I had some time to work on the car. I found the negative terminal was corroded. I cleaned it off and lord be hold the gauge is back to normal and the I haven’t had another surge yet. Really hope it was the weak connection.

 

[Juice]

Today I had some time to work on the car. I found the negative terminal was corroded. I cleaned it off and lord be hold the gauge is back to normal and the I haven’t had another surge yet. Really hope it was the weak connection.

That makes perfect sense to me. What most ppl don't realize is you NEED a good battery for the alternator to work properly. With a corroded connection to the battery, the system voltage will be all over the place and that can will cause havoc with computers.

 

[stang_gang]

Just goes to show you even the smallest cheapest thing can cause bigger issues than you think. Glad I didn’t go out and buy a alternator.

 

[SLIKSKILZ]

Glad you corrected the issue. The alternator, to my knowledge, should be able to run everything on it's own WHILE charging the battery. Not battery dependent as the engine keeps it rotating and generating energy. Faulty ground, however, would make the difference. I had a similar problem and it ended up being a small piece of protective rubber insulation on the alternator power wire was worn down and the power cable was shorting to the coolant crossover. Just another possibility for someone in the future. Upgraded to PA cable and reinforced that prone area with some extra rubber heater hose cut and wrapped around.

 

[Dino Dino Bambino]

Would autozone be able to test if the alternator is causing this

Glad you found the actual cause of your problem but for future reference, you can easily perform an AC ripple test of the alternator with a digital multimeter. There are several YouTube videos to guide you. If you don't already have a digital multimeter, I strongly recommend that you buy one. You'll find good ones for less than $30. You can also use it to test the battery and other electrical components so it's a very useful tool to have.