How to Test An Alternator - Page 2 - PeachParts Mercedes-Benz Forum

BillGrissom · #16 01-24-2017, 03:30 PM

Quote:

Originally Posted by nyko46

I tested the alternator with a good battery connected to it and got the same low voltage. Like i said in another post ...

Not exactly true.
You said, "I then connected a good 12 Volts battery in parallel".

Several posters then said you should connect just the known good battery, without the suspect battery, because the later could be drawing down the output. You should carefully read and reply to every response when asking a question.

vstech · #17 01-24-2017, 04:23 PM

Quote:

Originally Posted by nyko46

I tested the alternator with a good battery connected to it and got the same low voltage. Like i said in another post I got only 9.7 V

But is your bad battery connected also?

97 SL320 · #18 01-24-2017, 05:45 PM

A " bad " battery won't cause system voltage to drop while the engine is running.

If a cell is internally shorted, that specific cell will effectively be taken out of the circuit and you end up with a " 10 Volt " battery. Connect it to a properly working charging system and you will see 13.8 - 14.2 V when the engine is running.

If the battery was shorted across the + and - terminals, you would see 0 volts, same goes is the battery was an open circuit.

On the car in question the charging system isn't working. We need to find out if the engine computer has any control over the alternator, the ability to turn off the alternator occurs on some cars. If the engine computer does not control the alternator, tbe wires leading to the alternator need to be tested, if this checks out, pull the alternator and have it bench tested. I vote that the brushes have worn out and the slip rings where the brushes ride are worn as well.

BillGrissom · #19 01-25-2017, 02:23 PM

Quote:

Originally Posted by 97 SL320

A " bad " battery won't cause system voltage to drop while the engine is running.

Theoretical or do you have actual experience with this?
It would have been simple for the OP to try w/ just the known-good battery alone. But, he missed those suggestions and since started another thread on removing the alternator, which is proving very challenging in his Bluetec car. Sometimes "just replace" has a big "just".

vstech · #20 01-25-2017, 05:05 PM

Quote:

Originally Posted by 97 SL320

A " bad " battery won't cause system voltage to drop while the engine is running.

If a cell is internally shorted, that specific cell will effectively be taken out of the circuit and you end up with a " 10 Volt " battery. Connect it to a properly working charging system and you will see 13.8 - 14.2 V when the engine is running.

If the battery was shorted across the + and - terminals, you would see 0 volts, same goes is the battery was an open circuit.

On the car in question the charging system isn't working. We need to find out if the engine computer has any control over the alternator, the ability to turn off the alternator occurs on some cars. If the engine computer does not control the alternator, tbe wires leading to the alternator need to be tested, if this checks out, pull the alternator and have it bench tested. I vote that the brushes have worn out and the slip rings where the brushes ride are worn as well.

I have seen a bad battery cause a perfectly working alternator read 10v. I have seen a low battery have no effect on the alternator, and I have seen a bad battery show low voltage with a fully charged battery attached with jumper cables.

ALWAYS test an alternator in the vehicle after a full 12.6 or better voltage is shown on the static battery.

97 SL320 · #21 01-25-2017, 09:23 PM

Quote:

Theoretical or do you have actual experience with this?

I have 40+ years in and around the auto repair business and have seen lots of failures / odd behavior.

Quote:

I have seen a bad battery cause a perfectly working alternator read 10v.

Define specifically how the battery was " Bad ".

A very discharged battery can bring system voltage down for a bit if there are lots of electrical loads and the engine is idling but this isn't a sign of a " bad " battery. The system certainly won't drop to 10 V.

With a discharged and fully charged battery in parallel ( jumper cables ) , the system voltage should not be 10 V as the OP states. If the alternator is working properly, this combination should show high 12 / low 13 V with the engine running and rise to low / mid 13 after a few minutes.

nyko46 · #22 01-25-2017, 10:15 PM

Quote:

Originally Posted by vstech

But is your bad battery connected also?

Yes, my good battery was connected in PARALLEL to a bad battery and the alternator keep the same as before:not charging.
After all these problems I disconnected the alternator and was removing it from the car but there are no room to have it removed from the chassis.
I stopped by a very respected European shop repair today and talked with the mechanic about my problem and he said the same about a bad regulator on my alternator. I then asked him in how to get my alternator out of the car chassis and he said that one and basically the only way is to remove the swing arm which is in the way of the alternator. Haynes book say to remove the throttle body and then maneuver the alternator out the car. Hayness also talk about remove the fan unit from the radiator.
As I am doing this job on my hours after work I still where I was before:alternator waiting to be removed.
Any other option is welcomed.

nyko46 · #23 01-25-2017, 10:19 PM

Quote:

Originally Posted by BillGrissom

Not exactly true.
You said, "I then connected a good 12 Volts battery in parallel".

Several posters then said you should connect just the known good battery, without the suspect battery, because the later could be drawing down the output. You should carefully read and reply to every response when asking a question.

Yes, I should disconnect the bad one but when I was to do this the car was half the job done.
I just know that the car battery was reading 9.7 Volts wit the engine running.

nyko46 · #24 01-25-2017, 10:23 PM

Quote:

Originally Posted by 97 SL320

A " bad " battery won't cause system voltage to drop while the engine is running.

If a cell is internally shorted, that specific cell will effectively be taken out of the circuit and you end up with a " 10 Volt " battery. Connect it to a properly working charging system and you will see 13.8 - 14.2 V when the engine is running.

If the battery was shorted across the + and - terminals, you would see 0 volts, same goes is the battery was an open circuit.

On the car in question the charging system isn't working. We need to find out if the engine computer has any control over the alternator, the ability to turn off the alternator occurs on some cars. If the engine computer does not control the alternator, tbe wires leading to the alternator need to be tested, if this checks out, pull the alternator and have it bench tested. I vote that the brushes have worn out and the slip rings where the brushes ride are worn as well.

After all this head ache to remove this alternator I would prefer to have my time wasted than have to buy a new alternator because they are very expensive.

nyko46 · #25 01-25-2017, 10:26 PM

Quote:

Originally Posted by vstech

I have seen a bad battery cause a perfectly working alternator read 10v. I have seen a low battery have no effect on the alternator, and I have seen a bad battery show low voltage with a fully charged battery attached with jumper cables.

ALWAYS test an alternator in the vehicle after a full 12.6 or better voltage is shown on the static battery.

I thought remove the alternator from this car was easy like on those 300SD where removal and installation is 1 2 3.

BillGrissom · #26 01-26-2017, 02:49 PM

Quote:

Originally Posted by nyko46

... 300SD where removal and installation is 1 2 3.

More like 9,10,11 on my 1985 300D, especially the 1st time when I still had the EGR plumbing and AC HP tube in the way, and had to search for the pivot bolt & 2 clamping bolts, not to mention the quirky tensioner nut. 1985 CA also has a heat shield on the rear of the alternator which prevents easy wrench swinging. Today, it takes ~20 min, and I have to relearn exactly which wrenches and sockets barely fit. A 5 min job on my 60's U.S. cars.

vstech · #27 01-28-2017, 07:51 AM

An internally shorted battery can draw such high amps at 14v th not only prevent a good battery attached woth jumper cables from starting it can destroy the alternator.

Im not sure of a better way to say this, but you KNOW the battery is bad... remove it from the test so you can solve the issue...
No way i would install a new alternator on a bad baytery...

Quote:

Originally Posted by nyko46

Yes, my good battery was connected in PARALLEL to a bad battery and the alternator keep the same as before:not charging.
After all these problems I disconnected the alternator and was removing it from the car but there are no room to have it removed from the chassis.
I stopped by a very respected European shop repair today and talked with the mechanic about my problem and he said the same about a bad regulator on my alternator. I then asked him in how to get my alternator out of the car chassis and he said that one and basically the only way is to remove the swing arm which is in the way of the alternator. Haynes book say to remove the throttle body and then maneuver the alternator out the car. Hayness also talk about remove the fan unit from the radiator.
As I am doing this job on my hours after work I still where I was before:alternator waiting to be removed.
Any other option is welcomed.

nyko46 · #28 01-28-2017, 09:06 AM

Quote:

Originally Posted by vstech

An internally shorted battery can draw such high amps at 14v th not only prevent a good battery attached woth jumper cables from starting it can destroy the alternator.

Im not sure of a better way to say this, but you KNOW the battery is bad... remove it from the test so you can solve the issue...
No way i would install a new alternator on a bad baytery...

The same battery/alternator issue gave me these readings.
Battery connected to alternator with engine not running: 9.7 Volts.
Battery connected to alternator with engine running: 10 Volts.
Battery disconnected from alternator with engine not running: 11 Volts. (negative cable disconnected from chassis)
Another thing that I noticed right at the first time when all these things happened this past Sunday 01/22/17 was the trunk lid was not opening with the smart key. It was only possible to open with the mechanical key. Before this problem it was normal to open the trunk and all doors at the same time. Even selecting the trunk button of the smart key the trunk would not open.
Now after a week with the car in my garage, battery still have voltage sufficient to get all lights of the car working and the lock/unlock system (except trunk) working normally.
Can these information suggest some other reasons for the problem?
Thanks

97 SL320 · #29 01-28-2017, 10:10 AM

Quote:

Originally Posted by vstech

An internally shorted battery can draw such high amps at 14v th not only prevent a good battery attached woth jumper cables from starting it can destroy the alternator.

Im not sure of a better way to say this, but you KNOW the battery is bad... remove it from the test so you can solve the issue...
No way i would install a new alternator on a bad baytery...

Please define "internally shorted battery". Is there a problem where two plates from a single cell have a damaged separator plate allowing them to touch turning a 12V battery into a 10 V battery or a short across the 12V terminals?

If his battery really has a shorted / dead cell, that needs replaced regardless but system voltage will still rise with the engine running.

In order to drag a fully charged good jumper battery down to 10 V, there would be a massive spark when he attached the jumper cables. If the alternator was being pulled down by this same "bad" battery, either the main fuse would clear or the alternator power wire / harness would burn up.

Our guy is seeing barely any rise in voltage with the engine running, even with a massive load there will a greater rise in voltage. He really needs to check the volt meter on another car, I'm really doubting that his meter is accurate.

nyko46 · #30 01-28-2017, 11:12 AM

Quote:

Originally Posted by 97 SL320

Please define "internally shorted battery". Is there a problem where two plates from a single cell have a damaged separator plate allowing them to touch turning a 12V battery into a 10 V battery or a short across the 12V terminals?

If his battery really has a shorted / dead cell, that needs replaced regardless but system voltage will still rise with the engine running.

In order to drag a fully charged good jumper battery down to 10 V, there would be a massive spark when he attached the jumper cables. If the alternator was being pulled down by this same "bad" battery, either the main fuse would clear or the alternator power wire / harness would burn up.

Our guy is seeing barely any rise in voltage with the engine running, even with a massive load there will a greater rise in voltage. He really needs to check the volt meter on another car, I'm really doubting that his meter is accurate.

When connecting the jump cables to the car battery, there is a very minor spark. There is only a small spark almost not noticeable and car will start very quick with the help of the in parallel battery.
After it start, with the meter on the battery terminals I only read 10 Volts.
The alternator have no smell of burning parts. The only thing that keep me concerning about the alternator, other than my previous experience with bad alternators on previous Mercedes that I had, are all the information from the owner manual for this car related with this symptoms.
As I could not remove the alternator from the engine compartment, a mechanic will charge me $250.00 only to remove and install an alternator back. I will bring the alternator to test and buy another one or install the same one if it still good.
The only thing that I can't do now is make other testes since the alternator is completely removed from chassis. As I said before, I though that doing this job on this car was 1,2,3 like I did before on 300 SD.
My last shorted battery was on January 2017 on my 1984 MB 300SD where after tested by Autozone the guy told me that battery was in short and would never be charged because of the bad cell. In That case was just replace the battery the car ran good again. But these are the old Mercedes with NO computers
Thanks