I've been forgetting to ask about this for awhile now. I noticed during deceleration when in "D" as the engine got around 1200 rpm, like when you are braking slightly, the tack would "jump" "down" or drop suddenly about 200-300 rpm and then rebound. It was occurring consistently. If I sped up, then slow down, and every time the tach needle acted like it was getting a very brief power cut then it'd rebound to where it was supposed to be. I could also "feel" the engine doing something I can't quite explain, sorta like when you have a worn engine mount on a transverse mount and the engine "lifts" slightly under load, then basically drops back. Sorta felt like a really mild version of that.

 

Ideas? Never experienced anything like it before and it only occurred in that narrow window around 1200 rpms or so. If I wasn't feeling something in conjunction with the drop, I'd suspect the actual tach gauge. Could it be a trans issue? It runs fine and shifts/downshifts like I think it's supposed to (never had a new one of these so who knows) and the trans fluid was done when I got the car. Last time I checked it had the correct amount.

I had this EXACT same thing happen on a buddy's early EJ22 and my EA82, only much more violently. And it happens intermittenly too, yeah?

 

Stop by a local parts house that offers free charging system diagnosis. Have them run the tests and if you must, make sure they check for AC Ripple. Should be near zero or tens of mV. Anything above a hundred or so mV is not a good thing and can start screwing with the ECU and instruments as they are designed for DC voltage, not AC.

Failing crank sensor? Ac ripple is possible not probable, but still wouldnt hurt to have the system checked.

AC ripple huh? Thought that was only for capacitors. Only thing I could think of causing that is a diode failing or a broken solder. Or the windings breaking off  where they are twisted together as there isn't much in there. The alt is a 130 amp out of an 11' Tribecca that had really low miles and been in a little over a year now. Hope it's not that as I'll need to mod up another unit to clear. Wonder if the AC setting on a multi-meter can catch it? Funny, I'd rather deal with a crank sensor failing as that should be original. 

 

Thanks for pointing the right direction. 

Only thing I could think of causing that is a diode failing

thats what is usually the cause

 

and yes a multi mter can pick this up , but Only if its not a cheap one.

your multimeter needs to block dc current in ac mode or it will give you a false reading. a good way to find out is to turn it to ac and see if it shows voltage off your batterie with the vehical off, if it shows ac voltage than it will give you a false reading

Mike, they just have a standard rectifier bridge in these, right? i.e. 4 diodes? Or do they use a specialized all-in-one rectifier? If just diodes, should be an easy fix. Still have the stock alt laying around somewhere, might be able to pilfer from that or try RS for just the diodes.

Alternators are typically 3 phase, so have 6 rectifier diodes, and they can usually have at least two more between the bridge and the regulator.

 

Most have the diode bridge soldered directly to the windings. Not sure on Subarus alts.

Depends on the maker. I know for a fact Hitachi (early EJ22 and some 253 A/T) can separate the stator (windings) and the diode assembly (4 screws), but I don't know about the Mitsubishi (mid 90s EJ22 and 25).

 

But no, the diodes themselves are encased in a basic heatsink block and cannot be torn apart.

 

But no, the diodes themselves are encased in a basic heatsink block and cannot be torn apart.

This is what my experience has been. I usually leave rebuilding them to the local automotive electrician, I have all my starters and alternators rebuilt by him.

It hasn't been doing it lately. Also, despite having a mid-range digital multi meter, the AC side has (what I think is) a blown diode and won't hold zero reliably. I shorted it many many years ago after accidentally running too much current through it, but everything else works and it'll still read AC and DC. I'll have to wait until the car acts up again and try and run down to AZ or finally tear the multi open and replace the bad part.

Edited January 28, 201511 yr by Bushwick