User 4221

Anyone modified their excitation circuit to enable charging at a lower RPM?

My car has trouble charging below 750RPM. Things are typically fine until the A/C is running, the revs drop and the voltmeter reads 11V.

Question 1: What is the resistor value of the factory installed resistor? Ohms, watts and tolerance.

Question 2: What value resistor was used as a replacement and what side effects have been observed? Over charging?

Thanks in advance.

Scott

ViribusUnits

Scott, your engine shouldn't drop below 750, should it?

You might want to make sure the a/c by pass opens up when you turn it on. When I turn my a/c on, the idle rises slightly.

The a/c bypass is the silver cylinder bolted to the cross brace. It's got to fairly large vacume hoses, and two fairly small wires. One wire should get power only when the a/c clutch is engaged. When it gets power, the valve should open up.

Good luck.

Lorenfb

You can't modify the alt. w/o rewinding the
stator and/or the rotor. The regulator always
forces max. charge below reg. voltage.

Loren
Systemsc.com

Weissach

I have the very same problem. When the rpm drops below 750, everything turns dim. I have my idle at 650 right now because it's so SMOOOTH at that speed.

dr bob

The value of the resistot in the charging circuit helps determine the RPM at which charging begins after you start the engine. Once charging is established, the resistor does nothing, as excitation is supplied by the alternator itself.

Sorry there isn't a simple way to craete energy at the low RPM's.

Cheers!

John Struthers

The next time I have to swap out my alternator expect to see something in a small block Chev or Ford moving in. Cost, output, and availability being the main concern.
If you buy a rebuilt outside of the Big Three you never know which of the 3 model/manufacturer you are going to get until they arrive. let alone if the cooling/protective shroud is going to fit the alt housing.
FWIW
The Alt change - when I got the last of the three- was well worth it as it eliminated a host of electrical glitches.

Greg86andahalf

I agree about not swapping your factory alternator as a core on an unknown rebuilt.

I had the original alternator in my car rebuilt by a local shop for $80.00 with a three year warranty. I saw no need to trade it for a rebuilt one.

WallyP

Some of these problems seem to be due to bad connections and high resistance in the battery circuit. If you have a fully charged, high-capacity battery, the voltage should not drop to 11 vdc at low idle speeds.

I would suggest that the first thing that you need to do is to go completely thru the power supply wiring, including both positive and ground wiring, and clean and check every wire and every connection.

The ground wire on the bottom right of the engine, the cluster of wires at the jump start terminal, the red wires at the starter and alternator, and the main power feeds to the central power panel are the usual suspects.

You can improve the charging at low RPM levels - all it takes is a larger pulley on the alternator. Of course, you then can't rev the engine to redline without risking an alternator explosion, and there will be a loss of power and fuel economy - but you won't have to fix the real problem.

John Struthers

But wait, Thats not all!
I mooched a throw away secton of heavy insulated cable from our district shop/garage.
It's heavier than the stock Battery/alt.feed cable. We use it for wiring emergency boards ,flashers and strobes to our dumptruck bed tops. Since the bed raises the cable has to run the length of the frame -aft- then all the way to the front and up to the the bed cap. This was a piece cut about two feet short.
Lucky me. Anyhow, once Dennis Wilson explained how simple it was to loosen/remove the battery retaining blocks the job took less than an hour.
If you do swap the large cable you might as well swap the small lead, too. Make sure you have the right length and right connectors. To much means slack hanging under the floor pan -tacky-or to little cable means you won't have enough to connect to the battery or alt.
I had in the past polished all the end connectors but could still see a little bit of corrosion -resistance- at the ends of the leads.
This by the way was my last 'fix' before changing the alt.
Problem was the alt., of course! :^(

User 4221

Wally,
Good points. The overall health of my connections are definitely in question and were on the to-do list this weekend along with chasing down why the battery discharges in 3 weeks.

Luckily, I'm better with electrics than clutches I'll post results

Scott

John Struthers

Scott,
FWIW
Even the owners manual states that if a vehicle is laid up for any lenght of time the battery should be charged every six weeks.
And wether its true or not I tend to believe the manual which states disconnecting the battery while the engine is running destroys the Alt. it also warns that this applies to vehicles with disconnects. The later, if you figure on conserving juice with a Batt. disconnect.
Since you are up in the air, so to speak, check out that multi-strand ground strap aft of the right front tire -seek and ye shall find-.

User 4221

John,
Thanks for the tips.

I replaced the battery about 3 months ago and did a thorough cleaning of all connections including the battery strap.

Yesterday I performed a test and connected a 1 Ohm/30 watt resistor in series with the ground strap. I measured 304mV across the resistor with the car off and all interior lights turned off. That equates to a draw of about 304mA........ Far too high in my opinion.

I then set out to find the culprit of the excess current drain by pulling fuses one at a time until the current dropped.

Low and behold.... When I pulled the interior lighting fuse (#23 I think), the current dropped to 14mA. 14mA is about right for the leakage current in the car taking into consideration the clock, and older electronics such as L-jet and EZK.

After some inveistigation, I found that the pin switch in the rear hatch was making a connection and turning on the rear hatch light. This was intermittant, but enough to provide a substantial draw on the battery.

For now, I disconnected the switch and will clean it up with contact cleaner today.

I hope this helps.

Scott

Randy V

I have the same problem - lower voltage at idle engine speed.

Wally, to increase charging rate, wouldn't it require a smaller pulley on the alternator, rather than a larger one?

I've thought about doing this, having cleaned all the main connections without seeing an improvement. I did not clean the main leads to the power panel however. Maybe that's the weak link I've missed. Thanks for the tip.

WallyP

Good catch, Randy - Yep.

Normy

Now maybe I'm getting into negative transfer or something, but how does the voltage regulator figure into all this?

For some reason, I've always thought that the voltage regulator modified alternator field current with respect to system voltage in order to make the later the desired value. That is, if the VR sensed that system voltage was low, it increased current through the core [making the electromagnetic field stronger], and therefore increasing alternator amperage output...which of course in turn led to voltage rising to the required value. The value of this system is that it allowed the alternator to produce the same output of amperes no matter what speed it was turning, within reason.

It seems that if your system is not charging at 750 rpm, then the logical place to start would be the voltage regulator, which I think is a component mounted on the back of the alternator.

-My alternator [Bosch unit] needs to be "spiked" to 1800 rpm sometimes after I start the car in order for it to begin charging- there was a post on here a while back about modifying some resistor in order to prevent this from happening, but I cannot remember the name of the thread-

Normy!
'85 S2 5 Speed