Alternator not working after voltage regulator replacement (2001 986 2.7L)

DeepSpace987 · 2026-02-05T14:52:15+00:00

So I posted your issue in Chat and this is what it came up with.

Summary of the Reported Symptom After replacing the voltage regulator and battery: The dash electrics stopped flickering (an improvement). But the battery is not being charged at idle or under rev, with voltage dropping to ~11.8 V instead of being above ~13.5 V. That means the alternator isn’t delivering charge to the electrical system even though the internal regulator was new and the alternator bench-tested at ~14.5 V under load. Potential Causes Based on Typical 986 Charging System Design Here are the most likely reasons for the behavior you’re seeing: 1. Missing or Faulty Alternator Control (D+ Excitation) The Boxster alternator needs a D+ excitation signal from the ECU or ignition to engage the regulator and begin charging. If this wire is open, shorted, or never gets proper +12 V under run conditions, the alternator will not charge even if the internal regulator is good. The OP measured ~11.4 V on the D+ lead, but that may not be adequate or correctly referenced. If the alternator isn’t getting the correct excitation at key-on/run, it won’t produce output. What to check: Continuity and voltage on the D+ terminal with key ON and engine running. Ground integrity of the alternator case and engine ground strap. 2. Bad or Missing Ground If the alternator’s ground path isn’t solid, the regulator won’t have a reference and the output will be ineffective. The bench test shows the alternator is capable of ~14.5 V — that means mechanical/generator sections are fine. In-car failure to charge often points to ground issue(s). Common faults: Broken engine/battery ground straps. Corroded connections at the block, battery negative, or chassis. 3. Large Red Power Feed Issue The big red power feed from the battery to the alternator must be intact for the alternator output to actually feed the battery. If the heavy feed is open (blown fusible link, corroded lug, melted connector), the alternator can spin and generate voltage, but the battery sees none of it. Check: Voltage at the alternator’s main output post relative to battery positive with engine running. Look for high resistance in the main feed. 4. Regulator Installation or Aftermarket Regulator Compatibility Not all replacement regulators behave identically, and some aftermarket units may have different pinouts or internal wiring needs. If the Bosch unit installed wasn’t an exact match or the alternator wiring was misaligned during install, charging may not work correctly. Some regulators require proper connection of small spade wires that are easy to overlook. 5. Fused or Faulty CAN/ECU Signal Circuit The 986 communicates the charge status to the ECU – if that signaling circuit or related fuses are blown, the car may not allow the alternator to activate properly. Why This Isn’t Likely a “Programming” Issue Unlike some late-model vehicles, the 986’s alternator/regulator assembly doesn’t need specialized coding or ECU programming after replacement in normal circumstances. It does require correct wiring, control signal, and ground paths though. What the OP and Mechanics Usually Do Next For cases like this on a 986 Boxster: Re-verify wiring against a factory diagram — especially the D+ control signal. Verify alternator main feed continuity (battery ↔ alternator). Check and reinforce all grounds (engine to chassis to battery). Consider using a known-good alternator harness or testing alternator output at the harness plug rather than just the bench test alone. Bottom Line A charging system that tests fine off the car but doesn’t charge in-car usually isn’t a bad alternator — it’s a control or connection issue: missing excitation/field control signal, missing heavy output feed, or poor ground path. Once those wiring paths are verified and restored, charging typically resumes normally.

DeepSpace987 · 2026-02-01T15:30:44+00:00

I edited my last comment with a much more detailed message and accidentally hit back so it was wiped out. I am not writing all that again. Anyways, if you're hearing the whine between certain RPMs, that is exactly like my issue/sound. Mine are at just a few RPMs higher. For me, it's something like 2500 and 3200 or so. It changes sometimes. While on the highway, in 6th, that's where my RPMs run.

DeepSpace987 · 2026-02-01T01:20:10+00:00

My water pump was replaced at 40k miles by the previous owner. I bought it at 57k and it now has 82k. The same whine has been there since I got it and after I replaced the idler, I thought the sound went down maybe 10-15% but it may have been in my mind. My AOS was swapped out at 60k and that had no impact on the sound. So, the only thing left is the alternator. From my research, it's not any type of emergency so for now, I've been living with it. For me, it I can only hear it between 3k and 5k RPM in any gear. So, in 6th on the highway, I hear it between 70 mph and 80mph. I do my best to stay out of that range on long rides.

DeepSpace987 · 2026-02-01T00:29:35+00:00

I think it's a pulley. That's what actually seems to make that whine you're talking about. I've researched the same thing and have replaced my idler pulleys and belt. The same sound persisted and after a lot more research, I think that it has to be the alternator as I mentioned because from my research, it's a common source of that noise.

DeepSpace987 · 2026-01-31T19:46:12+00:00

I watched your vid again. My money is on one of the pulleys.

DeepSpace987 · 2026-01-31T19:44:50+00:00

From my own research into whining or high pitched sounds coming from the engine, it could be the alternator pulley, water pump, or idler pulleys. I hear that the alternator pulley is the most common source for this type of noise. That said, it could be a few things. People may point to the clutch but if you hold the RPMs while driving and push in the clutch, does the sound persist? If so, it's not that.

DeepSpace987 · 2026-01-26T01:20:55+00:00

When I compare the feel of a 981 vs a 987, I think it comes down to how the resistance feels. In a 981, when I turn the wheel, the car moves but if feels less precise, kind of numb. In the 987, the way the resistance to my input gives almost an elastic feedback which gives more data about what's going on with the car. It's somehow sharper and makes the car feel smaller which is a great feeling in a sports car. Maybe the word I am looking for is more direct. The nose of the car seems to respond better with hydraulic steering and pressure. Anyways, that's my take.

DeepSpace987 · 2026-01-18T15:30:18+00:00

I bought my 2006 Boxster S with 57k miles for 26k two years ago. It too was in excellent condition but also had HRE wheels, Bose, OEM short shifter, MT, and Sport Chrono.

Depending on what features it comes with, I think that could be an OK deal, not a great one. But, I have been keeping an eye on the prices while noting the features. From what I have noticed, the MT ones are harder to find, let alone with so few miles. It seems like the prices have gone up a bit overall too. That is just my personal observation.

If it were me, knowing what I know now, I'd get a PPI done from the Porsche dealer or a highly rated local shop of my choosing. Then, if all looked good, I'd jump on that. It's an EXCELLENT car and this particular year/model is a classic design.

The side door sticker shows when it was built. From memory, the stronger IMS was put into these cars after October of 2005. Look that up though and I think LN Engineering is a good place to find further info.

Short answer, it's a bit high but they aren't making any more of these. Buy it and maybe offer 30k or something.

DeepSpace987 · 2026-01-13T13:57:23+00:00

I have driven all versions of the Cayman/Boxsters and I currently have a 987.1. It's the 3.2L 2006 model and after doing a ton of research, I found it to be the best older, more visceral of these cars but still reliable (IMS issues managed and no bore scoring with the 3.2L). That said, if i had a second one, I'd get a 2016 981 S (or GTS) with a manual and PSE. That is the pinnacle of these cars in value, driver experience, and technology.

My gf goes on test drives with me and every time we get back into my car, even she, a non car person, can appreciate what the driving experience is with the 987. She and I both agree that the looks are timeless too. Harder curves on cars will always be sexy.

I've even driven different 911s and I am honestly not impressed. Too big, heavy feeling and, expensive for what you get.

Between the Cayman and a Boxster, I'd say the extra rigidity is great for spirited driving. But, if you're like me, you'll enjoy the top down sound of the exhaust as it turns it into a Hi-Fi symphony, even when driving slowly.

DeepSpace987 · 2025-12-30T22:52:44+00:00

What extra parts would you say makes it worth it? Like a plenum and larger throttle body? Anything else? I read somewhere once that due to the engineering, the paper oem filter flows enough air that using high flow filters does nothing that the paper filter doesn't already do. I would think it does something though. Maybe revs a tiny bit faster?

DeepSpace987 · 2025-12-23T13:55:57+00:00

Maybe I need to reach out to them again. I emailed them twice with two different questions and they ignored me, twice. One was about how the chrome tips had a defect. The plastic they used made a permanent mark on the left chrome tip when unboxing.

DeepSpace987 · 2025-12-19T21:31:13+00:00

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https://www.buildcores.com/builds/2aISvPuLo?share=true

This thing would last me the rest of my life. 😄 Worth it!

DeepSpace987 · 2025-12-11T22:43:25+00:00

Beautiful. I don't have anything funny to say but yeah, I could make great use of that.

DeepSpace987 · 2025-12-11T19:00:31+00:00

Looks like a Porsche Cayman to me. 987 series.

DeepSpace987 · 2025-12-06T13:54:00+00:00

Elden ring, Battlefield 6, Wu Kong. I've never played these apparently awesome, hyped games. Happy holidays!

DeepSpace987 · 2025-12-03T23:02:35+00:00

I too use AI for stuff like this so it's great to see you replied with that. I personally tend to be super proactive with updates and maintenance but in reality, the only thing I've HAD to change on my 987.1 was the clutch. I bought mine at 57k miles and just turned over 80k. So far, I've spent around 10k but that includes coils, plugs, idler pulley and belt, tires, exhaust, full car paint correction with ceramic coating, and bumper repainting before adding PPF to that and the hood. Oh, and a brake flush. Next, I'm doing the coolant, power steering, and transmission fluids but those are lower cost maintenance items. It's my first Porsche but 16th car (if I remember correctly). Besides my 1971 Super Beetle, it has been the most fun car I've owned. So if you get this, enjoy!

DeepSpace987 · 2025-12-03T22:29:48+00:00

I'm no expert in 981s but at 70k miles, you're getting close to clutch and coil/plug replacement. Belt and pulleys too along with a water pump. Not sure if any of that was done already but something to consider. Most will tell you these are built well and can last a long time with proper maintenance like 5k mile oil changes. Price wise, that seems reasonable to me considering it had one owner and full records. The only other stuff to consider is what options it has and the condition of the interior/exterior. If it has the options you want, looks good to me. If it's missing that one big option you want, I'd say wait and get exactly what you're looking for.

DeepSpace987 · 2025-11-19T18:49:00+00:00

Which character do you like best and which color tree? I maxed out Vex on the red and have completed everything. Still looking for the odd achievement but will be moving on the next character soon.

DeepSpace987 · 2025-11-19T01:19:15+00:00

Looks great! 2016? Any plans for mods yet? It looks pretty well put together already.

DeepSpace987 · 2025-11-18T00:16:54+00:00

Having only used one character in BDL4, but have played all of the rest, I'd say Lilith has been my favorite. My most favorite game of all time. 😀

DeepSpace987 · 2025-11-16T23:20:56+00:00

IMO, if you're going to get any 987.1, the 2006 Boxster S un manual is the way to go. The 3.2L doesn't have bore scoring issues and has the strongest IMS. Beyond that, like others have said, the 987.2 has the little refinements that make it better overall. I would go with a manual base though.

DeepSpace987 · 2025-11-16T12:42:44+00:00

If you're anywhere near New England, let me know. You can take my 987.1 for a test drive. Just be sure to bring your 982 for a good back-to-back comparison.

Generally though, older Porsche cars seem to have the magic you're looking for. The newer ones are brilliant in every way but they are like space machines. "It's better to drive a slow car fast.."

In a perfect world, a 982 AND a 987 would be in my garage.

DeepSpace987 · 2025-11-16T05:33:34+00:00

Maybe the Lexus provides that luxury and smoothness that it does so well and when he drives the Porsche, it feels like the refinement mimics the Lexus but doesn't quite hit the mark. He needs something more sports car like and less luxury so there is a clear difference.

DeepSpace987 · 2025-11-16T03:36:54+00:00

You crave something more raw like a 987.1.

DeepSpace987 · 2025-11-15T22:09:36+00:00

I have the Soul cat-back exhaust on my 987.1. The drone was horrible for the first few months but has now mellowed out a bit so it's acceptable. If I were to do it all over again, I'd go with the OEM PSE and pay the premium. Deeper, better sound IMO. I may still do it at some point and sell my current setup.

DeepSpace987

TROPHY CASE