AL129x Alternator Ripple - PeachParts Mercedes-Benz Forum

JamesDean · #1 10-14-2014, 08:59 AM

Hi All,

What kind of AC ripple are you guys getting off this alternator?

I've got the AL129x alternator in both my 420SEL and 300SD.

I measured my 420SEL at the radio connector and found that there was about 425-460 mV of AC coming out of the alternator.

I've not had a chance to measure the 300SD.

I did measure my 300D which has the 150A alternator (cant remember the number on it). It has less than 70mV of AC ripple.

I am wondering if one of the diodes is going out on the AL129x. I'll have to put a scope on the output and see when I get a chance..

Mxfrank · #2 10-14-2014, 10:34 AM

Should be under 90mv. Some meters have difficulty measuring ripple voltage, but if you're using the same meter in both cases, then the alternator is dying. Just make sure the meter is on the same scale in both cases. Best to read as close to the alternator as possible.

JamesDean · #3 10-14-2014, 10:42 AM

Thanks. I did use the same meter on both (Triplett 9045)

I'll re-take the measurements from the back of the alternator if I can. If not then I'll go direct to the battery terminal where I have it connected to the alt via 0 AWG wire.

It was not that long ago that I put this alternator in.

This ripple might have been what damaged my head unit. I found that a tiny fuse between the RCA shield and ground had blown out. I have a new head unit but I did not want to install it until I was sure a repeat would not occur.

dieseldan44 · #4 10-14-2014, 10:45 AM

for this type of system i dont think that is an unacceptable amount of ripple. also DMM type and setting (peak to peak, peak, RMS) is going to drastically affect the reading.

im currently having alternator issues as well with the charge light coming on.

is there a diode replacement diy somewhere? the bosch rebuilds seem to last no more than a year or two before needing to be replaced. this will be my third...and maybe 5 voltage regulators during that time. gahhh.

JamesDean · #5 10-14-2014, 10:48 AM

I will get my scope on it and see just how bad it is.

I've had okay luck. The one in my 190E is a Advance Auto Parts rebuilt from like 09? maybe earlier. Its still OK.

mannys9130 · #6 10-14-2014, 05:49 PM

I agree with Mxfrank. It should be below 90. Your ripple is WAY high. Most likely a bum diode.

JamesDean · #7 10-14-2014, 06:51 PM

Well now it gets interesting. When I measured at the battery the ripple was underneath 100mV or so.

These were taken measuring across the battery:

This was taken measuring at the radio's connector:

For comparison here is my 300D. I measured it at the dome light connector which was the easiest to access. Hmm probably not the best comparison, maybe I'll go out and get one off the cigar lighter I think thats on the same circuit as the radio.

So it looks like alternator is putting out okay. But there might be a poor connection or something...causing it to end up at 600mV of ripple at the radio's connector.

barry12345 · #8 10-14-2014, 09:03 PM

I may be off tonight but it looks more like straight noise to me rather than ac ripple. I noticed a strange noise component when I was playing around with glow plugs and a scope.

It was almost as if the car wiring was acting as a noise receptor. Even just across decent ground cables. I only eliminated it by grounding directly to the engine block when looking for any form of abnormality riding on the glow plugs. That got rid of it.

So you should be reading from the engine block to the alternator output to just check. I never bothered to locate or examine where the noise was originating from. Your peak to peak values are not that strange to me. Allow though that I might be getting old as they do not look like any rms type waveform I remember.

Your 300d posting looks more rms to me. Probably some action in the voltage regulator as well. I have no ideal where the extrainious noise was originated in my case. Brush arc in the alternator perhaps? There will always be a little. The amount indicated perhaps more than likely by whatever wiring is acting as an antenna for it.

For example if you take your meter on a very low range and place a probe on the engine and the negative terminal you might read something. This reading to me has to be radiated noise. I guess a scope would be better for the test. As you might see the meter reading any voltage drop across the cable with the engine running. Although there should be no ac component to read across it.

All I know for sure is I ran across some strange waveforms during my short period of use of my scope on the electrical simple 1984 300d. Your most interesting pattern was to me taken at the radio connector. I would have to think about it a little but suspect some action in the voltage regulator. The lack of general noise there is noticeable as well. As I said I would have to think about it for awhile.

Mxfrank · #9 10-15-2014, 07:16 AM

Tell us about the radio...are you using a supercap to power a woofer? Is the wattage very high?

It's possible for a device to pollute the power feed. Keep your scope as close to the alternator as you can, and unplug the fuses one by one.

JamesDean · #10 10-15-2014, 08:41 AM

The original setup was a Kenwood headunit (KDC-BT955HD) connected to a run of the mill 4-channel MTX amplifier. The amplifier drove the 4 front speakers and the 2 rear speakers. No extra subwoofers or anything.

Periodically the rear left channel would drop out. I found that if I went in the trunk and tapped or adjusted the gain or adjustment knobs it would come back in for a little while.

So rather than mess with taking the amp out and cleaning the pots I decided to just buy a slightly more powerful amplifier, a PPI (Precision Power) 4-channel amp.

During the installation I found that the wire going to the rear left speaker was damaged and partially exposed/disconnecting.

After installing the PPI amp, connecting up the RCA's and then turning it the car on I found that I had alternator whine and all kinds of noise in the speakers.

Initially I thought it was the amp, so I swapped back. No change.

After some experimenting in the trunk, I found like 30V when I measured from the RCA shield to ground. That struck me as extreme.

I was reading on a few websites about weak ground connections on the RCA shield at the head unit. Their solution was to directly ground the shield thus bypassing the head units internal connections. I tried this and found that the RCA wire got HOT. Red flag right there. Was not sure what was causing it but I know thats not good.

I ordered a new Kenwood unit (newer model, upper end). I decided to pop off the back cover on the old unit to see if anything looked a miss.

I found a blown fuse that went between the RCA shield and ground.

What I don't want to do is connect the new Kenwood unit and just repeat all this...

Photos:
Kenwood KDC-BT955HD - RCA Shield Ground Repairs - Imgur

So yeah, nothing complex, nothing fancy or supremely powerful.

SD Blue · #11 10-15-2014, 09:38 AM

Did you try increasing the rpm's to see if there was a major change in amplitude?

My thinking in this is if possible to detect slip ring/brush noise or rectifier break down. There appears to be a lot of noise in the negative side of the cycle at both the battery and especially the radio.

Consider that alternator circuits are a simple bridge rectifier with very little filtering. It is likely the radio head manufacturer has some filtering for the input power but offhand we usually don't know its limits. When the alternator rectifier has a problem, it can pass on a lot of nasty spikes because of the lack of filtering.

Have you done any additional grounding at the alternator, head unit and amplifiers? Do they have a common connection on the frame?

JamesDean · #12 10-15-2014, 11:30 AM

On the 300D I rev'd it. No notable change. On the 420 I did not yet.

I do have a direct ground line from the battery to the heat unit. I was considering just running a fused line direct from the battery (power+ground) to get a clean signal.

I have a ground from the amplifier to the chassis ground on the right side of the trunk (near the hinge).

spdrun · #13 10-15-2014, 05:17 PM

Radio probably has a small inverter to drive a VFD display. You're seeing noise from the inverter.

JamesDean · #14 10-15-2014, 06:33 PM

The radio was only connected in the case of the 300D. The 420SEL had no radio connected and the amplifier in the trunk had no power (fuse was removed from the line).