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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / December 2006Alternator ? (Excessive Voltage)
I've got a 19861/2 Nissan pu d21. The other night my son and I were driving home and discussing a voltage problem and doing a little fild experiment trying to figure it out. The home stretch he decides to run the RPMs up probably around 6,000 and waualla we blew both hig beams. Now this wasn't a spike voltage. The voltage (by the brightness of the lights and the rpms of the heater blower motor) seem to a parallelled, relative or proportional to the engine RPMs. When we pulled into the shop I turned off all accessories and checked the voltage at the battery between post at an idle. Then I ask my Son to slowly run the RPMs up. I was surprised to see the voltage (probably @ 3,000 rpms) go above 16 volts. I suspected the battery had shorted. We left the vehicle running and then I clipped a jumper battery (booster) onto the battery to confirm that I did have some weird battery problem (the truck had never failed to start even in cold weather) and to my surprise when we ran the rpms back up I got pretty much the same results although it seemed the jumper battery did have some effect on the rate of voltage clime initially. I pulled the alternator off a truck I hauled in from a Buddy's house (same model) and installed it and now everything is back to nominal. Just for future reference does anybody have the answer to this one. I suspect the alternator had issues with diodes and the excessive voltage was AC and eventually the battery would have gone down. But I was checking the voltage in DC mode and my meter shouldn't have been able to read the voltage out put in AC; only the DC output. i.e. If the alternator had bad or leaking diods the DC voltage should have read low? Still confused???? Hollis >I've got a 19861/2 Nissan pu d21. The other night my son and I were >driving home and discussing a voltage problem and doing a little fild [quoted text clipped - 26 lines] > >Hollis Replace the alternator before you destroy the battery and more. This is a fairly common problem with these. Don www.donsautomotive.com >Just for future reference does anybody have the answer to this one. I >suspect the alternator had issues with diodes and the excessive voltage [quoted text clipped - 3 lines] >alternator had bad or leaking diods the DC voltage should have read >low? Still confused???? It's not a diode, it's the regulator electronics..... it's cranking the field coil voltage way up, possibly due to a shorted pass transistor. This is a pretty common failure. Replace the regulator before you damage the battery and the rest of your electrical system. --scott  Signature"C'est un Nagra. C'est suisse, et tres, tres precis." I have managed to see a few that had bad diodes and when it happened, the DC voltage went way up to 15 and 16 volts. I believe the diodes drop about .8 of a volt for each working one so having 2 out can get you up close to 16 volts. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > I've got a 19861/2 Nissan pu d21. The other night my son and I were > driving home and discussing a voltage problem and doing a little fild [quoted text clipped - 26 lines] > > Hollis >I have managed to see a few that had bad diodes and when it happened, >the DC voltage went way up to 15 and 16 volts. I believe the diodes >drop about .8 of a volt for each working one so having 2 out can get you >up close to 16 volts. Not even wrong. If the diodes short out, you'll get AC out and a bunch of smoke from the insane level of current that'll soon blow them out. When the diodes open up, you'll get nothing. The voltage drop isn't addititive and a functioning diode will always have the same voltage drop. To get an idea of what is going on, draw a sine wave on a piece of paper. Draw another one shifted 1/3rd of the wave. Draw another one shifted 2/3rds from the first. Look at the tops of the waves; that is what is output from the alternator and what the diodes do by only conducting one way only. You can't get too much voltage out unless the regulator is bad. > >I have managed to see a few that had bad diodes and when it happened, > >the DC voltage went way up to 15 and 16 volts. I believe the diodes [quoted text clipped - 18 lines] > > You can't get too much voltage out unless the regulator is bad. The ones I am thinking of were Bosch alternators in Volvo 240s. When opened, there were 2 plates with 3 diodes on each plate. When I had the ones with 15 or 16 volts, one or more of these diodes when unsoldered tested as open. I tried to get the plates from Volvo and they wanted $90.00 each for them or more than a rebuilt would cost from them. That alternator had one diode on each plate gone and put out 16 V DC. These alternators also had the screw in brush and regulator pack so you could change the brushes with the alternator in place. Maybe it was just a coincidence that there were open diodes and high DC volts? Mike >The ones I am thinking of were Bosch alternators in Volvo 240s. When >opened, there were 2 plates with 3 diodes on each plate. When I had the >ones with 15 or 16 volts, one or more of these diodes when unsoldered >tested as open. >I tried to get the plates from Volvo and they wanted $90.00 each for >them or more than a rebuilt would cost from them. That alternator had >one diode on each plate gone and put out 16 V DC. >These alternators also had the screw in brush and regulator pack so you >could change the brushes with the alternator in place. >Maybe it was just a coincidence that there were open diodes and high DC >volts? Excessive current can damage both diodes and the regulator. Excessive Temperature will affect both as well. > >The ones I am thinking of were Bosch alternators in Volvo 240s. When > >opened, there were 2 plates with 3 diodes on each plate. When I had the [quoted text clipped - 13 lines] > Excessive current can damage both diodes and the regulator. > Excessive Temperature will affect both as well. I was thinking the 2 open diodes would let 1.6 more DC volts into the system as red from a digital multimeter set on DC while likely ramping up the AC part and or lowering the amps. Mike >I was thinking the 2 open diodes would let 1.6 more DC volts into the >system as red from a digital multimeter set on DC while likely ramping >up the AC part and or lowering the amps. Diodes don't work that way. If the diode had a zero volt drop, it would let .8 (I thought it was .7 for silicon, anyway?) more volts through and then the regulator would drop the field so that the output voltage would remain the same. If the diode had a 1.6 V drop: 1) it wouldn't be a silicon diode anymore; it would have to be made of something else. 2) the output voltage would be *reduced* another .8V. Because of the drop in output voltage, the regulator would kick up the field so that the output voltage would remain the same. If *two* diodes opened up, more current would have to flow through the remaining four and the output ripple would increase. At some point, the other diodes would start blowing as well and the whole alternator would fail. > If *two* diodes opened up, more current would have to flow through the > remaining four and the output ripple would increase. At some point, the > other diodes would start blowing as well and the whole alternator > would fail. That's what I was thinking was happening. The failing alternator with two popped open diodes was putting out a higher AC ripple which the digital meter was picking up as higher DC volts. Mike >> If *two* diodes opened up, more current would have to flow through the >> remaining four and the output ripple would increase. At some point, the >> other diodes would start blowing as well and the whole alternator >> would fail. >That's what I was thinking was happening. The failing alternator with >two popped open diodes was putting out a higher AC ripple which the >digital meter was picking up as higher DC volts. Higher ripple might show up as higher AC volts. Ripple on a digital DC volts range typically shows either bouncing around reading the voltage at the point the samples are taken or as the average. A higher voltage reading would either be a bad regulator, or a regulator having a hard time with the blown diodes -- regulating to the low side of the ripple. >Maybe it was just a coincidence that there were open diodes and high DC >volts? It's more likely that the open diodes were the result of the high voltage. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >>Maybe it was just a coincidence that there were open diodes and high DC >>volts? > It's more likely that the open diodes were the result of the high voltage. Often those diodes can stand a substantial overvoltage. The Delta V is fairly low for silicon diodes....around a volt IME, the diodes usually fail from heat, vibration, or heavy current draw like short circuits. The two normal modes of failure are (1) shorted , no rectifying power or (2) fully open, no current flow at all. > I have managed to see a few that had bad diodes and when it happened, > the DC voltage went way up to 15 and 16 volts. I believe the diodes > drop about .8 of a volt for each working one so having 2 out can get you > up close to 16 volts. Nope, doesn't work that way. When alternator diodes fail they either short out (which will add a ton of AC ripple, but won't raise the DC voltage) or go open (which will just reduce the output current significantly). i've had that done on 1 of my cars the guage was pegged at the highest then found out it had a short in battery and it was way over charging.. good luck SignatureScrapper http://www.automotiveforums.com > Just for future reference does anybody have the answer to this one. Most likely the regulator went batshit. It stick full voltage across the alternator field windings (the rotor) and the output voltage goes high. It only gets to 16V, not higher, because the magentic core in the rotor saturates, no matter how much extra current you push through it. Doing this deliberately is a "get you home" fix if your regulator dies. But keep the revs down! It's definitely not the output diodes. Regulator faults may put the output voltage up or down, but diode faults always put it down. It _might_ not be the regulator. If it's a battery-sensed alternator, then a broken sense wire from battery to regulator can cause this same fault. Some older cars (particularly with Lucas ACR alternators) can suffer this if the dashboard charge warning bulb has blown, is missing, or has been replaced by one of the wrong rating. It's also possible, on a machine-sensed regulator that the third set of diodes used to feed the regulator (paralleled to the positive output diodes) might have failed. This is rare though - the high-power output diodes generally fail long before these little ones. Andy Dingley <dingbat@codesmiths.com> wrote: >Most likely the regulator went batshit. It stick full voltage across >the alternator field windings (the rotor) and the output voltage goes >high. It only gets to 16V, not higher, because the magentic core in >the rotor saturates, no matter how much extra current you push through >it. Now, I remember some trickery out there that allowed you to get 200V or so off an automotive alternator, with some trickery to get a high voltage on the field coil, but without rewinding anything. What WAS that? I remember ham radio operators using it in the seventies to get plate voltage for radios off a second alternator installed in their vehicles. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." > >It only gets to 16V, not higher, because the magentic core in > >the rotor saturates, no matter how much extra current you push through [quoted text clipped - 3 lines] > or so off an automotive alternator, with some trickery to get a high > voltage on the field coil, but without rewinding anything. I've never heard of 200V, but you can certainly hit 50-ish. The trick is to use an old heavy alterrnator that simply has a bigger core, then to spin it faster by fitting a smaller pulley. > Now, I remember some trickery out there that allowed you to get 200V > or so off an automotive alternator, with some trickery to get a high > voltage on the field coil, but without rewinding anything. What WAS > that? There used to be a magic box you could install, normally under the hood, that allowed you to use many power tools like drills, saws,etc. The tool had to have a brush motor, which gives you the first hint of how they worked. I drilled one open to find out how they worked, and they just bypassed the voltage regulator allowing the alternator to run at full tilt. I didnt think they were a good idea, but my father in law used them in all his fleet cars without a failure. > I've got a 19861/2 Nissan pu d21. The other night my son and I were > driving home and discussing a voltage problem and doing a little fild [quoted text clipped - 3 lines] > lights and the rpms of the heater blower motor) seem to a parallelled, > relative or proportional to the engine RPMs. Dead voltage regulator. What's the big mystery? > I've got a 19861/2 Nissan pu d21. The other night my son and I were > driving home and discussing a voltage problem and doing a little fild [quoted text clipped - 26 lines] > > Hollis I appreciate all the info. There is an indicator of a monster of another color here (of sorts). I cant be sure how long this issue has been going on but I've discovered something interesting. Or maybe it's my imagination? It seems to me and all the evidence suggest my truck is running as smooth as it ever has and like new. I'm thinking that this issue also has been affecting the input signals readings and has been creating and erratic fuel, timing system as it seems this erratic voltage may play a bigger part in maintaining a sufficient charge. HUM?????????? any feedback?????? I've enjoyed the technical info and when I've had this investigated by my alternator "expert" I'll return with the verdict. Just for assurance I will pursue this as a quest to find the golden fleece of in other words; "exactly what he hell the problem is" (proof in the puddin) I think from what I see as I read all the post here it's The Regulator? Thanks for info and any further info on how this could effect the electronics as it relates to fuel delivery / firing / sensors etc I appreciate. Thanks again, Hollis |
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