The saga continues - 1990 C2

RetiredRider · 10-25-2011, 12:57 PM

I purchased a reman Bosch alternator. Fresh battery. The alternator does not charge and the battery light on the dash is lit when I start the engine. I checked for a fuse but could not find one. Is there a fuse for the charging circuit? No indication of such on the fuse listing under the fuse box lid. I wanted to check here first before I removed the alternator again.
Steve

matt demaria · 10-25-2011, 03:16 PM

No fuse. There are only three wires on back of alt: big red to an 8mm post, small blue to a 5mm post (which goes directly to the dash light) and the brown ground which has a 5mm screw that grounds it to the case. If wired correctly then alt is dead.

RetiredRider · 10-25-2011, 03:43 PM

Thanks Matt. I figured that but I wanted to double check before going to the trouble of removing the alt again. My wiring was correct as you noted and I checked that there were 12 volts on the large terminal before I remove the alt.

After I installed the alt the first time and started the car, I checked the battery to see if all was well. It showed 13.5 volts, then immediately went down to about 12.25 - which is what a good battery will have right after starting and before the charging system has recharged the battery. The reading at this time should have been about 14.3 - 14.4 volts.

Steve

carsonmx114 · 10-28-2011, 06:02 PM

Steve,
A bad ground could cause this as well. Before removing the alternator try a voltage test at the alternator output using the engine block as ground instead of checking at the battery.

RetiredRider · 11-04-2011, 03:32 AM

The first alternator that I bought was defective. I returned it and before leaving the parts store with the new replacement, we checked it on their bench. Worked great ~ 14.3 volts output under a simulated load. Installed it yesterday - no output at all when checked at the battery - 12.2 volts, which is about right for a good battery just after starting the engine. The battery light on the dash was lit indicating a defect in the charging system. (The battery is 3 weeks old). So how do I check for a good ground with the alternator installed? Or perhaps I did not understand your response. When the alternator is installed, you cannot get to the back of the alternator to check output or ground. Can you explain a good method to do this? Do you have to remove the upper engine cooling shroud? Also, after I had removed the alternator, I reconnected the battery, then checked that I had 12 volts at the alternator output lead at the engine and I did. I had the same voltage at the battery as I did when measured at the engine. In fact, I measured the voltage between the output lead and the engine as the ground. So I think the ground circuit on the car is good.

Before I started this, the old alternator produced about 13.5 volts measured at the battery - not enough to sustain charging, hence the reason for replacing the alternator - that and I wanted to paint the engine fan red

So, I think I have damaged a wire or something else in the r/r process. I know this second alternator is good because it was checked prior to leaving the parts place.

Now for the good news - I am getting very good at removing and installing the alternator - not something I particularly want on my resume!!

This is a bit long winded, but hopefully if you have the tenacity to read this, perhaps you can shed some light for me. Any help, tips, insight is greatly appreciated.

Steve

Flat6pac · 11-04-2011, 08:34 AM

Not to hyjack the thread, but I need it absolutely clear. Wiring to the back of the alternator
red is the 8mm post
Ground, the brown isnt the post right next to the red?
Ths blue signal is the post next to the red witha 5mm nut and the ground is all the way around with the philips, by itself?
If this is true, I have to undo what I ve done.
Bruce

craig722 · 11-04-2011, 08:01 PM

An old school Audi tech told me that you could fry the diode trio in the
alternator if you didn't disconnect the battery before installing the new
alternator.

carsonmx114 · 11-05-2011, 11:47 AM

Steve,
I apologize, I have zero experience with the stock porsche 911 setup. I bought my 993 in boxes and went back with an LS1. I was mainly remembering a issue with my 951 that sounded exactly like what you're experiencing. My volt meter in the dash read 12v, I put my meter on the battery and it showed battery voltage, I moved the ground side of the meter over to the engine block and the meter read 14v. The test you did at the back of the car would be considered an unloaded test because there would've been no amperage flowing in the circuit you were testing. what I would try would be a voltage drop test. Set your meter to DCV and put one lead on the engine block then put the other on the chassis of the car, if the meter reads more than .2 volts you've got a bad ground strap on the engine. You can do this test across each leg of the circuit. Try the same test from the negative post of the battery to the body of the car as well. It would be nice to test the hot lead the same way but if you can't access it while it's loaded it won't do any good. I hope this helps.
Lee

RetiredRider · 11-06-2011, 03:25 PM

Bruce,
You are correct in the wiring connections.
Steve

RetiredRider · 11-11-2011, 09:11 AM

I took out the new alternator and installed the original. The original was putting out some voltage, but not maximum. It now does not work at all. So, something has come apart in the process. I can measure the voltage from the large alternator lug to the engine - same as the voltage measured at the battery. I have put it together so that I can check the output while the engine is running - nothing from either alternator.

Is there a relay or fuse or ?? that may have been blown during this process?

Any help is appreciated.

Steve

RetiredRider · 11-14-2011, 07:36 PM

There is a blue wire that is attached to the back of the alternator. It appears to be a ground wire, but the wiring diagram indicates it is not a ground. I cannot determine where this blue wire goes. Can someone indicate where this blue wire is attached, where it goes int the wiring harness, and what it is attached to on the other end?

All help is appreciated.

Steve

carsonmx114 · 11-15-2011, 04:23 AM

Steve,
I started to suggest the blue wire the other day, but I'm not sure about it. I think the blue wire is a field wire that basically regulates the charging output. I looked on my service manual but the wire diagrams are hard to see and I can't be sure from them. I'll try to talk to my Porsche connection today and ask about the blue wire. I suspect it energizes the rotor to produce to charging voltage but the question is how.
Lee

RetiredRider · 11-16-2011, 09:39 AM

As it turns out I had the blue wire and the brown ground wire reversed. This - even after I confirmed with "Flat6pac" that he had his wiring done correctly. So, needless to say this is a painful post to write. If you must throw barbs my way, please be gentle - my fragile ego can't handle too much dissing.
Thanks for your help,
Steve

carsonmx114 · 11-16-2011, 10:40 AM

All's well that ends well. I had my car die and turn over but not restart. I noticed my dome lights weren't working as well. I assumed my battery connections were good because the engine turned over fine. Turn's out when I first started wiring my car I wanted to be able to quickly disconnect the battery so I just pushed the terminals down on the post. The separate red wire on the positive terminal was loose and losing connection. I feel your pain, glad you figured it out.