Help Idle Surge and Voltage problems

sj9ers · Mar 9, 2011

Ok so a few weeks ago I started a thread about having an idle surge and stuttering problems with my 95GT after doing a tune up. Well I took it to a Mustang shop to get looked at and they told me the timing was set wrong so the reset it to 11 degrees and they adjusted the TPS so it was within spec and they said the MAF and IAC were fine.

The stuttering problem went away, but now I'm having an idle surge like I did before and the voltage seems low at idle. When I turn on the headlights and fogs and use stuff like the window switches and hold them down the voltage drops down low and the lights dim, and it gets even worse when I turn on my radio and sound system on. Also when I turn the A/C on the idle will surge up and down about 100 or 150 RPMs.

So right now I think I have a voltage issue somewhere or something is draining my system. The battery is new and only a couple of weeks old and I had the alternator checked twice on the car and off and it tested fine.

Right now the car is idling around 700 or 750 so I think i may need to bump it up a little bit and I was wondering what are some voltage test I could do on the car with a multimeter to make sure everything checks out?

toyman · Mar 10, 2011

The idle speed is fine. I suspect that even though the alternator is good it may be underrated for your application. I don't recall the specific rating but it should be in the 150+ amp output range. I'm certain someone will post the correct rating info.

sj9ers · Mar 10, 2011

toyman said:
The idle speed is fine. I suspect that even though the alternator is good it may be underrated for your application. I don't recall the specific rating but it should be in the 150+ amp output range. I'm certain someone will post the correct rating info.

I don't think it's the alternator because the problem occurs even with the radio and sound system completely turned off and this problem wasn't occurring a few weeks ago. I don't know the specs on the system because I didn't install it but there is a capacitor in the back which should help with alternator and voltage issues.

sn95gtvert · Mar 10, 2011

it seems like once a year i have this same voltage problem. every time it ends up being an alternator dying. good thing napa has a good parts warranty.

CC'S95GT · Mar 10, 2011

Do you have UDP's
When you had the alt tested, did they do a load test or just ck the voltage?
How many times did they run the test? Sometimes it will fail only after the alt warms up from being tested 2-3 time consecutively
Nothing should cause the voltage to drop except maybe the radio.

sj9ers · Mar 10, 2011

CC'S95GT said:
Do you have UDP's
When you had the alt tested, did they do a load test or just ck the voltage?
How many times did they run the test? Sometimes it will fail only after the alt warms up from being tested 2-3 time consecutively
Nothing should cause the voltage to drop except maybe the radio.

No under drive pulleys. The motor is stock with the exception of a BBK CAI. The alternator was bench tested at 2 different O'Reilys where they put it on a machine and one time on the car when it was running. It failed the first time, but I didn't trust the guy because I over heard him asking others for help and that he didn't know how to use the alternator checking machine so I took it to get tested two more times at another O'Reilys and once on the car and it passed. Can I do a load test by myself at home with a digital voltmeter?

CC'S95GT · Mar 10, 2011

No not really.
but you can start the car and measure the voltage at the battery. should be about 14.5 ish vdc.
Hold or wedge the meter probes to the terminals then turn on the a/c and h/l, and watch for the voltage to drop.
The voltage may drop for a second and only slightly then pop back up to the origional reading. and stay steady there.

sj9ers · Mar 10, 2011

So I did the voltage tests today at the battery. With the car off at the battery I got 12.9V. When I started the car I got 14.08v. With the heater blower on full blast and headlights turned on it dropped down to 13.85v. I let the car run and sit for a little bit because the problem usually gets worse the longer I drive and when I did the test again the voltage dropped down to 13.77v with the headlights and heater blower on. Also I noticed when I had the heater on full blast my A/C pulley would spin and the pump would turn on for about 2 or 3 seconds then turn off. It would do this about every 10 or 15 seconds and cause the RPMs to go up and down and cause a surge. Is this normal?

So before I go out and get a new alternator could a bad ground be causing these problems as well?

CC'S95GT · Mar 10, 2011

Your alt def sounds like it's not up to par. Mine puts out 14.8 consistantly.
You had the heater on. or was it really the defroster. The defroster turns the a/c comp on, and the extra load from the a/c kicking on and off will make it seem like a surge. Low freon can also cause the the a/c comp to kick on and off.

sj9ers · Mar 10, 2011

CC'S95GT said:
Your alt def sounds like it's not up to par. Mine puts out 14.8 consistantly.
You had the heater on. or was it really the defroster. The defroster turns the a/c comp on, and the extra load from the a/c kicking on and off will make it seem like a surge. Low freon can also cause the the a/c comp to kick on and off.

It was the defroster. I'll try the test again with the heater. Is there any other tests I can do just to make sure it's the alternator before I go out and get a new one?