Alternator shutting off?

[Busted_Knuckles]

I've noticed this winter, with my headlights and wipers on, that my alternator seems to shut down while at idle. This appears to be leading to damage to the battery, and I had to replace one this past summer, but got a pro-rated refund. Now the new one is showing signs of weakness.

As I'm warming up the engine, the voltmeter shows 13.5-14V, idle speed around 1,200 RPM. This is normal and healthy. But, as soon as the engine warms up, the RPMs at idle drop to 750-800 RPM and the voltmeter drops to 11.5 to 12V. Revving up to 1,200 RPM sometimes gets the volts to 12.5V, but I don't see 13V again until I'm driving with RPMs above 2,000.

I noticed this with my old alternator before it bit the dust a couple of years ago. The new one did the same, so I thought it was normal. But, because I spend a lot of time in commuter traffic, I've seen the voltmeter drop to as much as 11v while stuck in line at a stop light.

I'm wondering if anyone else has experienced this, is it normal, or do I have a problem?

 

[Grandavi]

Sounds like worn brushes.. simple fix. Supra's generally idle at 600 RPM (at least in my experience). Also, what are you running for a stereo? Big Amp & sub?

 

[Busted_Knuckles]

Not that big. Just a 300 watt amp and a 12-in subwoofer.

 

[hvyman]

Remove that out of equation and see what happens. Also cheap alternators from the parts store suck.

And alternators don't shut off.

 

[ilikebigbutts]

also make sure your alternator belt is tight, no belt squeal right?

 

[Grandavi]

I put a n inline cap before the amp because I noticed a drain when the amp was working. Smoothed that out. Another thing to watch for is the steel braided fuel line tapping the power post on the alt if you installed an AFPR.

My alt died slowly, though. I replaced it. On my 88, the mechanic who assembled my engine rebuilt the alt during a lunch break as a favor without me asking. He told me they are really simple to do.

 

[Busted_Knuckles]

Well, I had a chance to run without headlights on this morning. With the amp off, lights off, and radio head unit off, the alternator was running 13.5V at speed, but would still drop to 12.5V at idle. Turn on the headlights at idle, and the voltage dropped to less than 12V. I think the alternator is dying slowly. It could be that I'm running too much load above the original design, burning out the alternator. Are there options out there for higher amp alternators? I'm more interested in drop in solutions.

Thanks.

 

[3p141592654]

You need to diagnose this a bit better. First, measure the output at the alternator (B+ post to alt chassis). If that reads 14V, then your low voltage is due to voltage drop in your harness. You then need to do voltage drop tests across the harness to find out where the problem is.

 

[Grandavi]

Driftmotion sells a higher output alternator, but unless it's actually an alternator issue that might not solve anything.

 

[hvyman]

Even if it's alternator problem that still won't solve anything.

 

[3p141592654]

High output "rewound" alternators typically provide lower performance at low shaft speeds compared to the factory design. There was a discussion on this topic a few months back.

http://www.supramania.com/forums/showthread.php?187450-Alternator&highlight=high+current+alternator

 

[Busted_Knuckles]

Well, I did the B+ post to alt chassis that ' 3p141592654' suggested. At idle, the alternator output is 12.36V and the voltage at the battery posts (stock location) is 11.89V. Revved to 2,000 RPM, the alternator output goes to 13.8v at the B+, and the voltage at the battery only gets to 12.8v. This is lights off, minimal load. So, I guess I need to check the harness for voltage drop?

 

[Busted_Knuckles]

Ok, I had some time this weekend to really dig into it. I found an excellent video on Youtube for a voltage drop test. My first test confirmed a whopping 1 volt drop between the alternator and the battery with full load on (all lights, wipers, and stereo). All this loss was on the positive side. The negative side showed a 135 millivolt drop.

Still not happy with the running voltage, I pulled the alternator and had it bench-tested where I purchased it. It failed their bench test, so they gave me a replacement (lifetime warranty, baby!). With the new one, my running voltage climbed considerably, right at 14.5V at 2000 rpm at the B+ terminal.

My voltage drop went to 0.84V. I checked the voltage drop at the fusible link, and found another 0.23V. I found the 80A fuse from my late Toyota pick-up replaced the fusible link perfectly, and now my overall voltage drop went to 0.72V. Still not happy, I checked more wire connections. The wire from the fuse block to the battery was showing signs of fraying. I trimmed the wire and put on a fresh connector. Now, the voltage drop is 0.65V.

Running out of daylight and wanting to solve this riddle, I ran a jumper from the alternator straight to the battery, and the voltage drop when to 142 millivolts. On a test drive, I noticed the voltmeter on the dash was reading no more than 12.5V. It was matching my multimeter readings before the jumper. So, I believe I know which line is causing my voltage drop. The line between the fuse block and the battery. My next attempt will move the jumper between the battery and the fuse block. I'm hoping that fixes it.

 

[hvyman]

14.5v at 2k is more than enough. Even if you lose half a volt your still well in an a acceptable range.

 

[Dirgle]

High output "rewound" alternators typically provide lower performance at low shaft speeds compared to the factory design. There was a discussion on this topic a few months back.

http://www.supramania.com/forums/showthread.php?187450-Alternator&highlight=high+current+alternator

View attachment 71803

While this graph is good at showing the power curve of an alternator, one thing to note, for some of the less observant, is the rpms are shown for the generator shaft speed. And does not one-for-one apply to the crankshaft rpms indicated by the tach in the dash. The crank pulley is 140mm and the alternator pulley is 55mm. This overdrives the alternator by roughly 2.5x. So at the factory 7M idle of 650rpm the alternator shaft would be spinning at 1658rpm. At 5,500rpm, indicated by the tach, the alternator would be spinning at 13,750rpms. But your statement about rewound high output alternators remains non-the-less accurate

 

[Busted_Knuckles]

14.5v at 2k is more than enough. Even if you lose half a volt your still well in an a acceptable range.

I was originally losing 1 whole volt on the positive side. Now, without the jumper, I'm losing .65V on the positive and .135V on the negative side for a total of .785V loss. That is way above the .5V loss limit that all the references I could find say should be the limit. So, I feel the effort is worth it.

 

[3p141592654]

So at the factory 7M idle of 650rpm the alternator shaft would be spinning at 1658rpm. At 5,500rpm, indicated by the tach, the alternator would be spinning at 13,750rpms. But your statement about rewound high output alternators remains non-the-less accurate

Quite true. Note that the output current for this particular alternator (which is not the OEM Denso) is about 50% of full output at idle (1700rpm shaft speed). That is pretty typical of any alternator.

 

[Busted_Knuckles]

Well, I went on the hunt for the last of the voltage drop this weekend. I wound up re-wiring the wire between the fuse block and the battery. I found the fusible link wire for the battery connection was trimmed to about 2 inches long! Everything I've read is this wire should be no shorter than 9 inches. I picked up a 14 ga fusible link at the parts store and wired it in. I also re-loomed that part of the wire harness and really cleaned up the area.

After double checking all the connections, I re-ran the voltage drop test. Now I'm at .442V drop total! Today, we had cold and rain. I drove to and from work with headlights and wipers on. At the stops in traffic, I no longer am seeing a dimming of the headlights and a slowing of the wipers! I feeling good, now!