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Alternator voltage drop under load

 
Old 04-07-2019, 09:27 AM
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merchauser
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Default Alternator voltage drop under load

I started an AC thread with voltage questions that morphed into more about charging procedure and pressures, so back to voltage:

when using AC with blower on high (read: load), driving along, approaching a stop, clutch in, and voltage gauge takes a dive along with RPMs. sometimes voltage and RPM recover, and sometimes car stalls. does not happen all the time, and happens with fan on high only. I can make things worse by adding high beams (more load). in garage at idle, alternator measures at 13.8. sometimes, with car at rest, I can turn AC on with fan to high, and gauge will stay in the normal range, and other times it will dip below 12? hoping my alternator is not ready for the grave?
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Old 04-07-2019, 11:51 AM
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This can be easily narrowed down to only about 362 different things. haha.

But seriously, it is a long list of 'could be' items. I'll hit a few from the top: 1. clean grounds. 2. battery ground cable. 3. Engine to chassis cable. 4. High resistance at jump post. 5. High resistance at starter lug. 6. High resistance at the sense wire from batt to alt(this senses the load on the alt). 7. weak alt.

The electrical description is called 'current crowbar'. As you demand more current, the alt is supposed to ramp up the delivery of amperage to maintain constant voltage. At some point, you reach a load factor where the alt can no longer keep up the voltage and it sags. In reality, all cars do this, but as cars age, things get worse. If you've already done all the maint items above, start looking at the alt output. These cars are hard on alternators.
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Old 04-08-2019, 12:11 AM
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Originally Posted by merchauser View Post
I started an AC thread with voltage questions that morphed into more about charging procedure and pressures, so back to voltage:

when using AC with blower on high (read: load), driving along, approaching a stop, clutch in, and voltage gauge takes a dive along with RPMs. sometimes voltage and RPM recover, and sometimes car stalls. does not happen all the time, and happens with fan on high only. I can make things worse by adding high beams (more load). in garage at idle, alternator measures at 13.8. sometimes, with car at rest, I can turn AC on with fan to high, and gauge will stay in the normal range, and other times it will dip below 12? hoping my alternator is not ready for the grave?

We can start with "never use the gauge as a gauge"

What are your voltmeter readings at the jump point and battery.

Also worth noting that unless all grounds, and all pod connections have been removed, cleaned, and foil connections to the gauges have been cleaned, then maybe the gauge is sort of accurate...if it's also been calibrated.

A nauseating about of 'search' data on voltage readings, panic, and diagnosis.
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Old 04-08-2019, 12:51 AM
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Put in a battery capable of supplying the necessary current when the alternator isn't charging at idle RPM. Second purpose for the battery, right after cranking the engine with the starter motor.

Make sure the battery is getting full charging current when engine RPMs are up above idle. Clean connections and a good ground strap will help.

There are a couple alternator options that do a better job of supporting voltage at idle. Part of the installation processs includes the same clean connections and cable updates that can help while your current (sorry...) alternator is installed. Do the maintenance, then look at other options.
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Old 04-08-2019, 08:18 PM
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Originally Posted by Speedtoys View Post
We can start with "never use the gauge as a gauge"
.
not relying upon the gauge at all. I am certain it is not accurate. voltage while running at jump post is 13.8. what I can tell you is that with AC fan high, the car will stall upon depressing clutch, coming to a stop. not always.....tried to force situation this AM with high beams, fogs, AC high, etc, and could not replicate. also, gauge stayed in a "normal" range. seems to be most prone with car at full warm temp. posted awhile ago about heat soak, and not sure if there is a relationship? just seems to be electrical in nature.
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Old 04-08-2019, 08:42 PM
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Originally Posted by merchauser View Post
not relying upon the gauge at all. I am certain it is not accurate. voltage while running at jump post is 13.8. what I can tell you is that with AC fan high, the car will stall upon depressing clutch, coming to a stop. not always.....tried to force situation this AM with high beams, fogs, AC high, etc, and could not replicate. also, gauge stayed in a "normal" range. seems to be most prone with car at full warm temp. posted awhile ago about heat soak, and not sure if there is a relationship? just seems to be electrical in nature.
Going out on a limb here...thats not the alternator.

If the alternator red light comes on, and then it does...sure, yer probably below 9v and things will just shut down.

But if you can turn the key and it starts right up, its not the battery or alternator.
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Old 04-08-2019, 09:08 PM
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Originally Posted by Speedtoys View Post
Going out on a limb here...thats not the alternator.

If the alternator red light comes on, and then it does...sure, yer probably below 9v and things will just shut down.

But if you can turn the key and it starts right up, its not the battery or alternator.
starts fine and no red light when car stalls. I just happen to notice the alt gauge take a dive and car stalls. but sometimes, it recovers, and proper voltage resumes
and car idles fine. immediately restarts.
is this a fuel issue, like a form of vapor lock? exacerbated by loads? not sure I understand the relationship or function of the
fuel cooler. any correlation there?
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Old 04-08-2019, 09:10 PM
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Originally Posted by merchauser View Post
starts fine and no red light when car stalls. I just happen to notice the alt gauge take a dive and car stalls. but sometimes, it recovers, and proper voltage resumes
and car idles fine. immediately restarts.
is this a fuel issue, like a form of vapor lock? exacerbated by loads? not sure I understand the relationship or function of the
fuel cooler. any correlation there?
If you can help draw the line between the voltage gauge and vapor lock, and the fuel cooler (bonus points, it involves the AC), it will save me dropping an edible with my evening meal.
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Old 04-08-2019, 11:08 PM
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Find out why the idle is irregular and low, and the low-voltage symptom should go away. Vapor lock is not in the cards, as it doesn't magically go away when you bump the key to restart. It may be a fuel issue, but more likely related to air/fuel mix than a fuel supply issue.

Follow all the normal troubleshooting steps for the car stalling with erratic idle. The low-voltage issue is an effect, not a cause.


Jeff, pop that edible anyway. Life's too short.
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Old 04-09-2019, 03:10 AM
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I had a very similar issue in my S4. Voltage indicator would drop when I stopped at the lights an idle woul become erratic and sometimes stall. I could feather the accelerator to keep the idle up. I'm not exactly sure how I resolved it as I fixed a number of possibly unrelated issues.

One thing I would suggest is check/clean/replace the battery ground strap. Check the battery terminals and ensure a good tight connection. I was removing the battery connection when the car wasn't being used. On one occasion I noticed that the positive was very hot. Clearly the connection was not as good as I thought.

Other things I fixed was a small fuel leak at the pump, huge hole in the cam cover oil breather, replaced a broken sensor on the cam and did a water bridge refresh with new thermostat replacing the long forgotten inner seal which was fossilized. If the oil breather was broken then you will probably have a big pool of oil under the intake. You can use a torch to follow the line from the cam cover to the oil filler and look for holes.

So after all that my running issue went away but I'm not sure what resolved it. I kind of suspect battery terminals but not sure.
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Old 04-09-2019, 08:02 AM
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Originally Posted by dr bob View Post
Find out why the idle is irregular and low, and the low-voltage symptom should go away. Vapor lock is not in the cards, as it doesn't magically go away when you bump the key to restart. It may be a fuel issue, but more likely related to air/fuel mix than a fuel supply issue.

Follow all the normal troubleshooting steps for the car stalling with erratic idle. The low-voltage issue is an effect, not a cause.
I wouldn't say I have an irregular idle; idle is smooth and spot on all the time. as mentioned, only under a heavy load, and seemingly with heat in the mix, when depressing clutch, coming to a stop, the car will sometimes a.) behave as normal, b.) take a dive down and quickly recover to smooth idle, or c.) take a dive and stall. car starts up immediately and back to perfect idle. I guess my eyes were focused on the volt gauge and why I was thinking it was electrical. if A/F related, a bit confusing about rapid recovery and restart

fuel pumps are new, and FPR and FPDs test fine so agree I don't think it fuel supply. O2 and MAF are only a few months old, so can I put air/fuel mix in the rear view mirror? I have gone thru most, but not all, of the grounds, but have heard the battery ground is a weak link: isn't there an upgrade cable?
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Old 04-09-2019, 08:32 AM
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I'm thinking the voltage regulator is going bad. My car demonstrated similar conditions, and a new voltage regulator fixed it. Don't just buy a new alternator, try to fix the one you have first. Note, having it tested may show the alt is fine, and the test does not show weakness in the vr. Anyway, clean the alt well, replace vr which is not expensive at all, clean all ground points and contact points, including at the 14 pin connector, then try it out again.
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Old 04-09-2019, 09:18 AM
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Prescription: "Check, loosen and retighten, clean if needed, every dot on the attached chart, and call me in the morning."

Seriously, Double-check the engine to ground strap and the battery post ground strap, and the ground nearest your A/C fan. The factory battery-to-ground strap may "look good", but be corroded under the insulation.
Easy way to find out: Give up your daily coffee for three days and spend $12 for a 2 or 4 gauge battery cable at your local parts store.

You seem to believe you have found a correlation between your issue and the A/C fan on high. That could be coincidence (the A/C fan is a "tipping point", not a lone gunman) or possibly a bad fan. Drive around for a couple days without turning the fan on at all and see if your issue is truly eliminated.

Based on everything you've said, I agree with others that it does sound like an electrical issue, not fuel related. Not vapor lock. Heat soak affecting a failing component is possible, but typically has different symptoms (eg. no-starter actuation at all when car is warm and then parked for 10 minutes or so, then will start fine once cooled down after 30 minutes or so.) Neither vapor lock nor heart soak would cause your "at the stop light" conditions.

Keep us posted,
Jason




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Old 04-09-2019, 10:28 AM
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Going another way, could be a sticky ISV. If the idle under small power load is ok, and not ok under higher Alt load, then the ISV may get a flat spot in it.

Remove, clean, test and install the ISV, see what happens.
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Old 04-09-2019, 10:43 AM
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I don't know how the 928 ecu handles it, but in other makes I would be suspicious of the idle air valve as well.
It could be a sometimes sticking valve, or an intermittent power or ground connection to the valve, but the symptoms match what I've seen on other makes.

edit: what he said! ^
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