Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I have a manual Cadillac CTS 2007. My speedo nose dives to zero whenever I turn on my AC. At times, It will also hesitate back and forth and drop to zero even if the AC isn’t on. A few times a week, the Speedo will surge to around 20mph or more even at idle. Sometimes to “fix it” temporarily, I’ll turn my AC on, which will make the make Speedo drop to zero as mentioned before, but when I turn the AC off, the Speedo will “work” again for a minute or so. Has anyone had the problem or heard of this? I think it may have something to do with the alternator not supplying enough power but I’m not sure. I’ve already bought a brand new battery but that didn’t do anything. Please help!
That sounds like a problem caused by a failing alternator due to excessive AC ripple voltage.
Do you have a volt meter with capability to measure AC voltage?
That would be the first thing I would try to verify/eliminate
I don’t personally have a volt meter. I’ve taken it to several auto electricians and they’ve all bit the dust, and bad. Not one of them that I know of has mentioned anything about AC ripple voltage. They’ve hooked it up to their scanners and what not and once they see that the battery is charging at above 14v, they think it’s not the alternator. But I’ve learned that an alternator can charge a battery above 14volts and still be bad.
Hello, sorry for the delay. I thought I would get an email every time someone responded to my post. Guess not! But yes, by AC, I mean hitting a button to make the air cold. But yes, a mechanic had his scanner hooked up to my car while driving and he said the VSS didn’t drop out. Im not sure a full scan was done just yet. I’m leaning towards not yet though. But mechanics have had scanners galore hooked up to the car while driving and idling, but to no avail. Also, don’t know if this helps, but their will be hours, if not days, where everything (Speedo, cruise) works even with the air on. It’s as if nothing is wrong with the car. Then at the drop of a hat, the Speedo starts wobbling up and down and then is just sitting at zero even when on the highway.
