I have a 1989 Cadillac Eldorado that has alternator problems (presumably). The output terminal voltage wanders all around, tripping the overvoltage sensor quite often. I figured either the regulator or other parts of the alternator were bad, so I replaced the alternator--twice. Same problem with each alternator that I used!
I can watch the voltage from the engine monitor display on the dash, and the A/C fan and lights flicker in accordance with what the alternator output is doing at the time. When the voltage wanders up past 16 volts, the car's computer shuts down, as does everything tied to it--headlights, dash, etc. VERY disconcerting when you are driving at night!!
Since the regulator is on-board with the alternator, what else might be causing the voltage to fluctuate? Could I have gotten two bad regulators in a row from the auto parts store?
That car came from GM with what's known as a "CS144" alternator. In my experience in rebuilding them, the only problems they ever suffer is voltage regulator failure. This can be solved by using a good quality regulator (very few exist outside OE) and taking the time to be sure it's right during the rebuild.
You didn't say where you got your alternators, but I have found that most large chain stores use rebuilders who do things as cheaply as possible ... they won't replace things that should be replaced because it's cheaper to not replace it and have you bring it back under warranty should it not work. Most customers are okay with this because they got another one for "free", but in reality, I get tired of installing them, only to find out they don't work and have to bring them back.
I am not of the opinion that this alternator is a crappy design. I am, though, of the opinion that those who rebuild them generally don't care what happens as long as they make a buck. My suggestion is to take it back wherever you got it and complain. I'm sure they'll give you another one ... but before they install it, ask them to test it in the store ... just to be sure.
If it works in the store but doesn't work in the car, I don't know what to tell you because I've never run into this problem. The alternator has a battery wire and either a sense or field wire (I've never seen one with both, though I'm sure one exists somewhere). Perhaps there is a problem with one of those wires, but get the alternator checked first.
I'd check all your battery and other main electric cables for looseness and corrosion. Check the ground straps from engine to frame to body, etc. Find any under hood fuse panels (don't know if GM uses them but a lot of Fords do) and pull all the fuses and see if the contacts look corroded.
The parts store I bought the alternators from is: Advance Auto Parts. They put the original replacement alternator on the dyno-checker or whatever they call that, and the voltage appeared to be constant. I explained to the clerk that on my car, the problem didn't kick in until about 5 to 10 minutes of driving. He just handed me a new replacement instead of holding his finger on the switch for that long.
On this unit, is the alternator completely self-contained with regard to current output? In other words, if the car's computer was setting its field current or what have you (computer was setting the ouput voltage and/or current), then I guess I mgiht have a problem with the computer. If not, why in the world would three straight alternators exibit the exact same problem?
I spoke with the mechanic at the dealership, who told me I couldn't buy a new one, because they aren't made anymore.
I have the same type of alternator in my Jeep and when my voltage gauge starts jumping all over the place like you describe, I need to clean all my main battery cable connections because one or more is bad. On my Jeep, when one connection is due a clean, they all can benefit from one.
One wire that can make the voltage really jump when bad is the ground strap from the bell housing to the body. This can make it charge high to keep up through smaller secondary grounds and really dive low when you hit the brakes and pulse with the signals.
Hope this helps,
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
It seems hard to imagine getting 3 bad alternators in a row, but stranger things have happened. There was a time when my Ford truck was burning starters up every week ... every Friday I was back under it putting another starter on it. It was burning voltage regulators up every 3-4 days, too. Changing from a 40 to a 60 amp alternator solved the problem.
In your case, I don't know where the car gets its field voltage from ... if it uses that terminal. Some GM products use the "PLIS" plug where others use the "PLFS" plug. I don't have the software anymore that would tell me which one you have, but if your car does indeed get its field voltage from the computer and you have a bad computer, that could very well be your problem. That said, though, your regulator should have a set point (usually not higher than 14.6 volts) and it shouldn't go higher than that no matter what the field voltage is (even if it's nonexistant).
Just as another check, though, I don't remember if you said you checked your battery or not. If you haven't, check it or get it checked. It could be cracked, have a shorted cell or many other problems. It could be the main problem in your charging system, too.
I looked at this possibility after reading your post. In fact, the brochure that came with the alternator said to use a voltmeter and measure the potential between the negative battery post and the frame of the alternator. I did try this, put the potential difference was tiny ( < .1V and steady). I took the side mount battery posts off anyway and cleaned them. Same problem exists. I'm starting to lean toward the idea that the car's computer is controlling the alternator, and the computer is to blame. I guess I need to get a shop manual at this point (if I can get one) in order to confirm this idea.
I did take the battery to the Wal Mart and it tested bad. I replaced it, but the same problem exists. I suspect that the intermittent high current hammer of the alternator going wild is damaging the battery, since this battery was recently replaced.
I guess if this is controlled by the car's computer, I'll have to try tp McGyver up something, since there is no way I"m replacing a computer on this old car. Maybe I can fix the field current with some sort of high wattage resistor divider. It may not track loads perfectly, but that may not matter. I just don't want the alternator to take itself out or to pop headlight bulbs or cook any more batteries
.1 volt difference between the battery negative and the alternator frame is a 'massive' amount that means you have a really bad ground connection in the system.
Since you have a meter I will outline how I check them.
I put the engine on and lights on and take a volt reading on the clamp bolts on the battery between plus and neg. I then take the meter and read the bolt on the main alternator output and alternator frame.
Then from the battery positive bolt to the body of the car.
These bracket and body readings must be exactly the same as the battery readings. Yours won't likely be.
I then put and keep the meter on the positive battery clamp bolt and then touch the alternator case with the other probe. Then on the alternator bracket. Then on the engine block. Then on the bolt that holds the negative cable to the engine block. Then on the clamp on the negative cable. Then on the negative cable clamp at the battery. Then on the bolt for that clamp.
When I find the spot where the voltage matches the battery clamps bolt to bolt voltage, I have found the bad connection.
You can also do this in reverse and walk the positive meter along the connections looking for the voltage drop.
Surprisingly enough, I have seen and heard of a whole pile of those old systems fail where the alternator bracket bolts to the engine block. This then requires a new ground cable from the alternator to the battery.
As far as I know, there is no way the field excite signal can cause the alternator to overcharge or be swingy on voltage. If the field shuts off, the voltage immediately drops to 12 volts or so and stays there.
When the fan and lights flicker, that 'normally' means the ground strap to the body is bad. This is the strap on the passenger side from the bell housing to the body. When this goes totally bad, the small black wire from the battery negative to the fender usually gets hot and the insulation on it melts. This also will only allow the battery to get half charged so it fails a load test.
FWIW, I had a CS144 alternator in my Olds 98. Replaced it with a unit form Advance Auto. A year later replaced it again. A year later replaced it AGAIN. Then Advance Auto told me that the "lifetime" warrantee is now 90 days.
I bought a rebuilt alternator from my CarQuest dealer. Never had a problem again.
Lesson learned- Advance Auto seems to sell low quality stuff, IMHO. I will never buy parts from them again.
Good to know this! I do have other parts dealers in the area, and should I have to try yet another alternator, I think I will try one from another parts store.
Awesome! Thanks for the detailed test strategy. I will perform these voltage node checks and report back. I assumed it was an active electonic component going bad, since the voltage started fluctuating after the car warmed up (and I assumed that a bad ground would produce essentially random fluctations with regard to time). In any case, I will check the ground wire in detail.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.