non turbo 1990 244

my headlights started flickering. It isnt' just the headlights but all the lights, even dashboard lights.
at first I figured - alternator, so I swapped with the alternator from my 1990 740. nothing changed.
I checked with jumpers from battery ground , to case of alternator and to engine and I don't seem to have a bad ground.
I checked putting a hot cable from heavy red alternator cable to battery, no difference.
I cleaned headlight and taillight grounds and cleaned up the fuse near the distributor and the nearby hot wire terminal block, no difference.
I found a two wire connector terminal near the distributor, an oval shaped connector, cleaned that up. not sure what it's for,

I get about 12.5V DC when stopped and 14V when running.
there is some AC component in the DC and it causes all the lights to stay on but flicker with varying intensity.
If headlights are off, I can still see it on the dashboard lights. engine warning light is on. I probably need to reset the button on the back of the speedo, lamp sensor warning light is on I might have a tail light out but that's probably not causing the flickering.

when driving, the frequency of the flickering does not seem to tie in with engine speed, so I don't think its to do with alternator brushes.

so question. can someone explain the small red wire that comes off the distributor. does it only run the alternator indicator on the dash or is there another purpose? I have no tachometer.
is it safe to disconnect this wire or re-route it to isolate that out of the equation. I know the alternator wires sometimes go bad where they are near the front of the oil pan but I did fix those up some time ago.

could somethign be wrong with the fuel pump causing a disturbance in the electrical system?

I noticed that when I changed the alternator there what I think is the diode mounted to the case of the alternator near the wiring connections, I don't really assume I had two alternators with this exact same issue but I am confused at what to check next.

Could adding a capacitor smooth the ripple current out? it might mask the problem?

I wondered if it might be to do with the coil wires causing induction into the charging system but I think that would then cause an issue that would vary with engine speed.

there is a diode near the driver side headlights for the ignition system and I kind of suspected maybe that's bad but again the flickering doesn't seem to be dependent on engine revs. It seems constant as long as the engine is running.

I know something is wrong, the headlights flicker so much other cars are giving me strange looks and I'm questioning the reliability of the car because of it. any help of suggestions would sure be appreciated. I'm kind of stuck.

thanks for your time,
Phil

 

the small wire is connected to the battery light in the dashboard, and it supplies the D+ current that bootstraps the alternator field coils. without it the alternator won't work at all, or it will work poorly. that light should come on when you turn on the ignition, and go out when you start the engine.

the fact that you get about 14V with the engine on suggests your alternator is working fine.

a fully charged battery at room temperatures should be about 12.6-12.8 volts when the car has been shut off a couple hours.

if you have a volt meter, try reading the voltage form the engine block to the battery MINUS terminal (measure at the battery post, not the clamp) when the engine is running. this should be very close to zero V, if its more than a tiny amount off, you've got some marginal grounding.

 

Thank You Pierce, yes I think im reading normal voltages with the engine running. I don't think I have any engine grounding issues.

I did a simple experiment using a jumper cable to the engine and to the battery while watching the flicker , and no difference, I did the same to the case of the alternator to engine and alternator to battery , no change. also couldn't detect any appreciable resistance on my meter. battery ground clamps and cables seem ok.

the term "bootstrapping" is a bit foreign to me but I assume it is a control relating to a transistor where the control can vary the output. maybe that's a primitive or just wrong way to look at it but I am assuming it's basically a wire to control the output of the alternator. If this is true and it were somehow picking up some resonance , maybe through induction could it be causing this condition of fluctuating output causing the flickering lights? or am I grasping at straws with that? although my meter is reading 15V the output could actuallbe varying somethign like 14.75 to 15.25 and varying so quickly like about 30 cycles per second or so , so it is quite visible in any light bulb in the car, but the meter kind of averages out the reading.. the little bar graph thign on the meter shows a visual representation like needle would. so it might have 15 or 30 little LCD's and the last two or three are flickering, but it isn't dipping that far..

the result of the lights flickering isn't just slightly noticeable, its very obvious, somethign is wrong here, but I'm confused as to where it could be coming from. I could take the alternator I just pulled to the alternator shop and ask him if he could bench test it and see if it seemed funny to him. I can get it rebuilt for 100 bucks but if anythign is wrong withti , maybe jsut the diode? or is there a capacitor in there somewhere?

I noticed that the one I took out is Bosch but the one I put in I think is a Motorola. It looked pretty clean and the former owner of the car i just pulled it from said he replaced it so I think it's a rebuilt one. I also noticed that when they sprayed the case of it in the rebuild they painted the component too , it's just inside the end cap but visible through cooling holes in the case, near the wiring connections and I figured that meant maybe the rebuild shop didn't replace the diode.. which might well be fine.

If I test the one I pulled and it isn;t putting out a varying voltage it might double prove It isn't the alternator but it seemed unreasonable it is being caused by the alternator..

I thought maybe as a test I could just unhook the small red wire from both ends and run my own wire from the alternator straight to the alternator light on the dash and see if maybe that changed the flickering and if so run a new one more permanently. If no change then maybe that would shoot down my theory of it picking up some sort of interference.

I figured maybe a minute variance in this wire is getting basically amplified through the alternator?

my only other idea was that maybe bad brushes in the main fuel pump could be creating this electrical noise.. and not sure how to check for that, maybe if I could hard wire the fuel pump on and then check that temporary circuit to see if I can detect noise there maybe by putting a scope on one of the leads? but I don't know what would happen if I ran the fuel pump without the engine running.. it might just be trying to compress the gas rather than pump it and stall? I guess the same test could be performed with the car running and the fuel pump temporarily powered by an alternate battery so that circuit would be separate.. then if the lights didn't flicker I could suspect the pump. its not unusually noisy or anything but what else could cause this resonance?

 

no nothing as complex as that. battery + to ignition switch to batt light on dash to D+

D+ is on its own diodes in the alternator, and the current through the battery light powers the field coil so there's a magnetic field for the alternator to make juice against. once the alternator is making more volts than the battery, it doesn't need the D+ any more, and the current goes to zero, so the dashboard light goes out.

the diode board is inside the alternator, and almost never fails, its about 6 hefty power diodes (4 for the B+, 2 for the D+). the thing on the outside of the back of the alternator is the voltage regulator and brush assembly, this looks like a power transistor but is actually a power chip.

 

Pierce, thank you very much for taking the time to explain this. it helped me understand it better.

I still don't have a clue as to why it is flickering but I guess as long as it gets me to work I might just have to wait and see if anything bad happens.. sometimes these things surface in a part failure. I'm wondering how the lights and electronics like this "dirty power" most electronics seem to want clean 5 volts but i'm not sure if the electronics are just designed to work on the 12 V available.. It's probably possible to change the DC to lower voltage DC but just a transformer with windings wouldn't do anything with the DC.. I suppose they can break it into a pulse to transform it or use resistors to get a voltage drop and it could use that.. Maybe there are some capacitors in there to steady the voltage anyway. headlights I can afford I just hate to see it take out the ECU or ICU

Maybe it's an added volvo safety feature, people see me coming kind of like the bicycle riders with their flashing white headlights ;-)

 

car power is an unregulated 11-14 volts, so anything that's sensitive electronics has a voltage regulator in front of it, and probably runs on 5V internally. Being that this is all mid to late 80s designs, the voltage regulators are probably linear rather than switching (pulsed).

 

I did happen upon a note in the updates section of my haines manual that was referring to radio suppression
" Integral electronic voltage regulators do not normally generate much interference. But when encountered this in combination with alternator noise, A .1 microfarad or ,2 microfarad capacitor from the warning lamp (IND) terminal to earth for lucas ACRalternators and Femsa. Delco and Bosch equivalents should cure the problem,

funny wording and Im nto sure what year, but
I guess it wouldn't hurt to see if that makes a difference anyway.