Another electrical system issue thread

[longrowe]

I have a 1998 ROW C2 coupe tiptronic.  For the past couple months (since it is hot enough to make AC mandatory) I have noticed that the voltage will start at 13.8 and then as I drive will steadily drop to rest at about 12.8/12.5. This is with the AC on. With AC and headlights, it is closer to 12.3 volts. The engine temp usually stay just past the 0 in 80C. If stop and go, it will creep to the high speed fan point but not to the 100C line. The issue I am having is that with the lower voltage, sitting at a stoplight, the idle will stumble and sometimes stall. If I keep the car in Neutral, the car will not stall and I can drop it into drive when the light goes green. This only happens when the temp is way up and the voltage goes down close to 12.

 

I found Ahsai's electrical troubleshooting guide and measured the voltages today: Batt- to Alt case is 0.07V, Alt Out to Batt+ is 0.2V, Alt Out to Charge Post is 0.71V, Post to Batt+ is 0.12V, Alt out to Alt case is 14.00V. 

 

The battery is only about 2 months old and I replaced the Y-Cable in January because I had a slow, hot-cranking issue. That turned out to be a worn out starter which I replaced about a month ago. Cranks like a champ now, hot or cold.

 

So I guess my question is what could my issue be? If I drive with no AC the voltage stays at 13.8 no matter the engine temp. Once i add AC, the voltage immediately drops to 13 and then creeps down to 12.5 ish. It doesn't appear to be the ground cable and the reading point to the Y-cable but it is not that old. The car drives excellent except for the odd stall when the voltage gets really low. Thanks

[DBJoe996]

Does the engine RPM's increase when you engage the AC?  Perhaps it is more an idle speed issue than an electrical issue.  Have you ever cleaned out the idle control valve?

[JFP in PA]

4 hours ago, longrowe said:

I have a 1998 ROW C2 coupe tiptronic.  For the past couple months (since it is hot enough to make AC mandatory) I have noticed that the voltage will start at 13.8 and then as I drive will steadily drop to rest at about 12.8/12.5. This is with the AC on. With AC and headlights, it is closer to 12.3 volts. The engine temp usually stay just past the 0 in 80C. If stop and go, it will creep to the high speed fan point but not to the 100C line. The issue I am having is that with the lower voltage, sitting at a stoplight, the idle will stumble and sometimes stall. If I keep the car in Neutral, the car will not stall and I can drop it into drive when the light goes green. This only happens when the temp is way up and the voltage goes down close to 12.

 

I found Ahsai's electrical troubleshooting guide and measured the voltages today: Batt- to Alt case is 0.07V, Alt Out to Batt+ is 0.2V, Alt Out to Charge Post is 0.71V, Post to Batt+ is 0.12V, Alt out to Alt case is 14.00V. 

 

The battery is only about 2 months old and I replaced the Y-Cable in January because I had a slow, hot-cranking issue. That turned out to be a worn out starter which I replaced about a month ago. Cranks like a champ now, hot or cold.

 

So I guess my question is what could my issue be? If I drive with no AC the voltage stays at 13.8 no matter the engine temp. Once i add AC, the voltage immediately drops to 13 and then creeps down to 12.5 ish. It doesn't appear to be the ground cable and the reading point to the Y-cable but it is not that old. The car drives excellent except for the odd stall when the voltage gets really low. Thanks

 

I would suspect your voltage regulator is on its way out.  A cheap and easy fix.

[longrowe]

The idle RPM does increase when the AC comes on.

 

I initially suspected the voltage regulator as well so I got a new one. No difference. I have tried swapping alternators also since the one in the car was a cheap one. No luck. I still get the same readings on the dash voltage meter with the stock alternator as well.

[Ahsai]

The 0.7v voltage drop between the alt output and +ve term post in the engine bay points to the y cable. I had one turned bad after a yr of installation so it's not impossible.

[longrowe]

Well that sucks. I guess that is next on the fix list. That is what I suspected as well but was hoping for a better answer. Thanks for the confirmation 

Edited September 23, 2017 by longrowe

[longrowe]

Ordering a new Y-cable will take 1-2 weeks so I figured in the meantime, lets try and put the old cable back in. I removed it, but maybe it is still good (since it was unrelated to my slow starter). Sure enough, 1.5 hours later, new cable out, old cable back in and the volt meter is at 13.5 with both AC and headlights and with the engine at or above normal operating. Sucks that the new cable went bad in 8 months, glad that the group here convinced me to swap out the cable in my 100F garage.

[Ahsai]

Glad you got it sorted. It's a great demo of the voltage drop test.