Mercedes-Benz Forum banner

Alternator/low voltage

9K views 47 replies 11 participants last post by  nobby 
#1 ·
1976 450slc
Normal voltage output when no accessories are on. Low voltage when any accessory is on.
Alternator is good. Ground good. Belt is tight.

This forum has been helpful in the past, so I'm hoping to get advise on my problem.
I have low voltage whenever I have an accessory turned on – headlights or auxiliary fan or blower fan.
Here is what happened:
This summer, I had a dead battery. After a couple weeks of driving around town, dead battery again. I could tell starting motor going slower towards the end. Test at autoparts store says battery fine, alternator only putting out 12.5 volts.
So, I had it rebuilt at the local electric rebuilder. Car seemed to work great. 3 weeks later, dead battery again.
Took to my indy mechanic and he checks things and says bad alternator. I take alternator to rebuilder and bench tests fine under load. 13.8 amps. He says it’s the car.
So back to indy who puts it back on and still low amps when an accessory is on (ok when no accessories). I take car to rebuilder who does some tests and thinks it’s the aux fan.
Back to indy and unhook fan, same issue with headlights on. So we decide to try another alternator from autoparts store. Same results as original alternator.
So to summarize:
13.8 volts at idle with no accessories. Checked at alternator and at battery.
Turn headlights on, 12.8 volts. Aux fan, 12.6 volts.
Belt is tight. Indy says idle is fine and all systems check out, including ground. Everything is stock. No energy leak when car is off. He’s stumped.
I have a voltmeter on my charger. I confirm the volt readings at idle with no accessories, and with either accessory on.

Any ideas where to start to diagnose? My first step is to buy a good voltmeter.
 
See less See more
#2 ·
Any ideas where to start to diagnose? My first step is to buy a good voltmeter.
And my first step would be to get a new (rebuilt) alternator and install it.

All your symptoms are classic alternator - that's why everyone (except the guy who rebuilt yours) keeps pointing you back to it.
 
#3 · (Edited)
This can be caused by poor connections at battery, and grounding points. While they may check "good" with a VOM, a VOM does not put the connection under load.

If you have not already, have the battery checked under load, to determine its condition.

Then I'd check is the Alternator under load. Shops should be able to do this for you. It will probably check out fine.

Next is battery connections at the cable end, not the terminal to battery (check where the cable attaches to the battery clamp). I have had experience where corrosion was under the insulation of the cable, and the cable was almost in twain, albeit invisible until thorough inspection. Only a few wires of the stranded cable were actually integral. The others were rotted in two. Low-load was fine, but the few wires were not able to carry current under load. Check ground cable as well. Don't forget to check the ends at the starter/selenoid...

Check vehicle/engine ground same way. A simple VOM reading here is not sufficient once again, because the VOM does not put circuit under load. Remove and inspect cable carefully for corrosion, etc at mounting points and cable itself.

If you employ a VOM, check the circuits for continuity, measuring the resistance.
 
#4 ·
Thanks for the reply scott. I really want to get this figured out before I try your Xenon conversion.
My mechanic says the same as you, so we did try another rebuilt alternator from the auto parts store. Exact same results. So, he is stumped now.

His software showed a similar problem with a 78 after they tried 3 different alternators. Their solution was to buy direct from MB to solve the issue.
So, I've tried 2 different alternators with the same results. I'd hate to buy one from MB just to try. Will do if I need to, but looking for opinions/options.
My choice is becoming clearer, though.
 
#5 ·
Thanks Ears.
Alternator was bench tested under load and I watched. Another alternator was tried and same results. I will probably dump the battery, 3 years old anyway. I will also inspect the grounds. Lots of Jeep forum guys recommend grounding direct from the alternator to the chasis.
 
#7 ·
Don't forget the battery cable terminals, as mentioned before. There can be corrosion under the insulating sheath that has eaten away a critical portion of your conductors. Look for bulges around the ends. If you see a greenish powder, that is cupric oxide. Clean with a baking soda solution and an old toothbrush, away from the battery. If you have too corroded cables, you will have to replace the terminals. You can go "expensive" and replace the whole cable from the dealership, or go "inexpensive" and just replace the end. If you coat the connections in dielectric grease before reassembly, you will save yourself a lot of problems in the future. You can get small packets of the stuff at most auto supply stores.

Scott
 
#17 ·
Maybe it's just the stuff I got from work, but they recommend doing it before assembly. If you use a light coating before, it will squeeze out of the way, and fill any voids to prevent future corrosion. At least that's how it was explained to me by the Western Electric installers.

Scott
 
#9 ·
Lots of you have chipped in with this, but I'm stubborn:

How would frayed/corroded battery cables cause a voltage drop on a running car when the headlights are turned on? At a minimum, this is rated at 55 amps for the alternator, which means somewhere between 600 and 700 watts available, at max output. Assuming there's no help at all from the battery, and the measurements are being taken at idle, the alternator should still be able to handle the ~130 watt draw of the headlights and marker lights without a voltage drop.

I'm sorry, but I'm still pointing squarely at the alternator. Bad windings, bad regulator, bad something.
 
#12 ·
yep...

And Scott, you'll just have to trust us. As the corrosion eats away the conductors, the current carrying capacity diminishes... Similar to using too small a conductor (wire) in a circuit. The voltage drop increases. Shouldn't be but about a tenth (0.10) volt drop for automotive connections, ie battery, battery to ground, across connections, etc.
 
#11 ·
I'd say that if (with accessories on) low volts go up with rpm, it is not the alternator.

A decent auto electrician should be able to measure current trough alternator-to-batt wire and make a decision based on measurements.
You may be surprised with measurements.
 
#16 ·
Perhaps I interpreted incorrectly and perhaps you are correct as to the alternator being the problem, but the battery is 3 years old and I'll still go with a replacement as the correct choice on my personal decision tree.:thumbsup:
 
#22 ·
When I used to wrench for a living many moons ago we had as many as 3 bad rebuilts in a row, starters, alternators etc. It's frustrating because some of the rebuild shops just did not do a good job. On the battery I read that they tested it but did they do a long load test on it? If there is a marginal cell it won't show up in the short tests.

On the alternator did they test the voltage regulator? It should be tested with no load and then varying loads all the way up to full load and you should be able to see the output "equalize" to balance the load. At the same time you should be reading the current from the alternator so you can see that it's keeping up. There could be some bad windings in the alternator that only show up under a load test but test fine with no load.

Lastly the wiring has to be solid, remember current must flow from positive to negative and if there is a "flow restriction" this creates excess resistance in the circuit which will impede flow. Flow restriction being corrosion, poor mechanical connections, wire too small, wire too small because strands have corroded away, bad ground ( very typical on these cars ). You can actually feel a restriction in a wire under load, they get warm to hot and can cause a fire over time because they begin to try and arc across just like a welder. As far as voltage drop it's all relative to the resistance, even good new wire has a drop, hence the reason for larger wire for longer runs because of the drop, less resistance. Check out Ohm's law and you can see what happens when you change resistance in a circuit. :teach:

If it can't flow it won't go :D

Oh yeah, I am sure you already checked to make sure nothing is drawing the battery down when the car is off right?
 
#23 ·
I spent a day in a cold March rain trying to get my 79 Park Avenue started.
Starter out, rebuild in.
Starter out again, bench tested and back in again.

That starter must weighed 50 lbs. And on your back under a big car with a bumper jack and cinder blocks soaking wet........no fun.


Turned out to be the ground from battery to block on the block side. Three starnds holding it all together.

5 minute fix.

4 days getting over a cold.
 
#24 ·
Wow. Lots of responses.
After the alternator rebuild and problems returned, i took it back to the rebuild shop. They hooked it up to their large machine and tested it without load and then with load. Watched them do it as they explained each step.
Just to make sure we put on another rebuild from the parts store. Same symptoms. There is no draw when car is off.

Here is what I've done so far:
1) Parts store says their machine will load test the battery and it is fine. Just to make sure, I put in a known good battery from my jeep.
2) ground cable looks fine, however I bought a new one just to make sure. I tried the new cable on the Jeep battery and the original battery. I do voltage tests from the negative battery cable and also from the bolt that connects the battery to the frame. No difference.
With the different battery and cable and lots of experimenting, not much changed.

I want to check the ground strap from the motor to the frame. What am I looking for and about where should it be?
 
#25 ·
Please put your car in your profile.
The engine ground strap is on the left rear of the engine. It attaches to a trans mounting bolt and bolts to the car near the underside of the brake power booster IIRC. This will get you in the right area.

My DJet also has a bunch of grounds attached to the intake on the right toward the rear.
 
#28 ·
Alright, I inspected the engine ground strap. Knocked off dirt, made sure connections were tight, Wires are still coper color, no corosion. Did my tests again, no improvement. I took my jumper cable from the alternator to the negative battery post with no avail.

So far:
2 different alternators. 1 was bench tested under load.
2 different batteries.
New battery ground cable.
Engine ground strap inspected.

13.8 with about 700 rpms with no accessories. I turn on headlights, blower fan, rear defrost, or Aux fan and volts drop immediately and slowly drop. Turn accessory off and volts climb back up.

Only thing I had done to the car this spring was new motor mounts and subframe bushings. Problem started this summer. Don't know if that is a clue.
 
#29 · (Edited)
Only thing I had done to the car this spring was new motor mounts and subframe bushings. Problem started this summer. Don't know if that is a clue.
This makes me even more suspicious of grounding, as you better insulated the engine from the frame. However, there is no arguing with your inspection of the ground strap.

I would have given odds...

Try your jumper cables from the alternator frame to a GOOD KNOWN GROUND on the car frame, not the battery ground, and use the other jumper lead to go from the battery ground to a GOOD KNOWN GROUND on the frame, and report back. Lets insure the alternator is grounded to the frame, as well as the battery.

If you have been so unfortunate as to have had bogus alternator testing and two bad alternators, I may owe ScottinSoCal a steak dinner...:)
 
#30 ·
Set your volt meter to measure AC volts, and check from the alternator terminals to ground. There may be a bad diode in there somewhere. If there is, you'll see an AC voltage greater than .5 volts.

This still "smells" like a ground problem to me, but I'm coming down off a nasty head cold, so my "sniffer" isn't working too well. Just as a preventative measure, go through and check as many connections for tightness as you can find, looking at the connections for corrosion and looseness.

Good Luck,
Scott
 
#34 ·
Batt under the hood - straps negative terminal to the firewall.

___________________________

I presume that two alternators tested were with their respective rectifiers.
If sy one diode is open circuit, alternator will have (~1/3) less capacity.
 
#36 ·
Pretty dissapointing last night and was about to give up and try another alternator, but your responses will keep me going cause I would HATE to do that.
Battery is in the trunk on the passenger side. Ground wire goes under the trunk mat and into the metal floor of the trunk and attaches with a bolt. I bought a new ground battery cable and attached that.

The engine ground connects from the Transmission to the boxy part of the frame. I did not remove this strab because the transmission bolt was very tight. After work, I will try my best to remove the strap and clean the ends. I will see if orielly's has a replacement. I will also try the jumper cable tests.

OK, after messing with that ground my non accessories volts went up from 13.8 to 14. Didn't mention because I thought maybe I was getting better at testing and mesurements. Turning on any accessory would drop volts immediately by .5 and then slowly drop. Turn off and volts would slowly climb. I will report back tonight.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top