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Old 10-08-2017, 03:01 AM   #1
Join Date: Oct 2017
Default 1990 244 -stumped on flickering headlights

my headlights and all the lights flicker, even instrument lights.

I figured it had to be an alternator problem, but swapped the alternator from my /90 740

it made no difference. voltage is 12.75 at battery. 14 V when running, so it's charging.

it seems to have DC with ripple current. I know with most power supples one would rectify and then smooth out the ripples with capacitors,and an alternator does ut out AC power but on cars I assume the battery acts to smooth the AC ripple from the alternator, and that it's rectified by a diode within the alternator,

I tried using jumper cables to see if connecting the ground terminal to the engine or alternator housing would prove a bad connection but the flickering continued.

i tried connecting a hot jumper tot he main output terminal on the alternator, still no difference..

the lights arent' going out they are flickering at a fairly high frequency but as far as I can tell with my meter the voltage is pretty constant.

I checked for bad grounds to the engine or to the headlights and taillights , cleaned the fuse near the distributor and the hot wire terminal strip, no difference..

does anyone know how to troubleshoot this further or has anyone seen similar issues?

it runs ok but hte flickering lights are annoying and I know something isn't right here. its kind of driving me nuts.

when driving, the flickering does not seem to change or coincide with engine speed, and since I replaced the alternator already and it hasn't changed it must be from somethign else, But what?

bad fuel pump causing ripple maybe? I'm stuck.

any help or suggestions would be appreciated,

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Old 10-08-2017, 03:05 AM   #2
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Clean your connector from headlight to the main harness... black connector

And your connections at headlight really

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Old 10-08-2017, 03:16 AM   #3
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1.5 fard capacitor about 1 foot away from your amp will help you
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Old 10-08-2017, 03:34 AM   #4
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Are you using an external voltage regulator? The 80 and 100 amp alternators seem not to play nice with the external regulators. Will cause the symptoms you're noticing.


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Old 10-08-2017, 03:59 AM   #5
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there is no buzz or anything from the radio. it's just an old school jensen tape deck radio..

Ill clean the plug for the headlights and relay connections if |I can locate them. I assume I can find the relay by listening for the click when I turn them on. but I think the flickering still happens with the lights off, I can see it most in the instrument panel lights during daylight but every single light in the car is flickering.

If it were alternator. brushes issues it would relate to engine speed and I swapped the alternator.

did you mean a 1.5 microfarad capacitor? where? what amp? a one farad capacitor is huge,, maybe a value used for radio suppression, but most power supplies might have a 22mfd , or somethign like that. I fix old tube radios so I have lots of capacitors.

Ive seen wiring from the alternator go south where it dips down near the oilpan at the front of the engine but I fixed up those wires a long time ago..sometimes they get kind of damaged there...
I ran a spare ground from the alternator body to engine because i did have issues with a ground at one point so I think the alternator ground point is ok.

this little red wire that goes to the alternator.. what does it do exactly? it goes to the charging idiot light in the dash right? can I check that circuit out more?

I checked it to ground with a meter and it seems to have power..

older 240's had a separate voltage regulator but I think by 1990 it is incorporated into the brush holder int he alternator and I swapped the whole alternator so I'm not suspecting that now.

but he little red exciter wire,, sould it be giving the alternator a flakey signal and affecting when it's charging? does it tell when to charge and when not to or does it just report to the idiot light so I know if it stops charging?

the engine warning light comes on but as far as I know that is only really a light that comes on every so many miles as a warning to change the timing belt.. I've changed the belt recently. it can can be reset if I take the speedo out and click the little button thing. or maybe by reaching behind if I can remember where it is exactly. I don;t think the engine warning light really does anything else does it?
and the bulf failure light is on but it's hard to keep those out.. the tail lights flaked out so often with the silly PCboard connector strip thing that i hard wired in standard bulb sockets for all the rear bulbs and did away with the flimsy plastic strip with copper traces on it.

Last edited by amazonphil; 10-08-2017 at 04:26 AM..
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Old 10-08-2017, 12:12 PM   #6
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Head Light Relay.....
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Old 10-08-2017, 01:50 PM   #7
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i can switch out the relay to make absolutely sure but it still flickers with the headlights in the parking lights position and I think it still does it with lights off too. the dash lights still flicker. the 14V has ripple. in other words the DC has some AC component. If I watch my meter the voltage fluctuates slightly at a failry high frequency. Its a fluke and has a bar graph to mimick an analogue meter. I can see the bar jumping around a little. If I hooked up an analoge meter I could probably see the needle at about 14V but not sitting steady.
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Old 10-08-2017, 05:06 PM   #8
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If the DC has some AC in there, I alway thought that was because of a failing diode.
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Old 10-08-2017, 05:34 PM   #9
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Bad alternator can cause flickering headlights. But you've already tried another. Check for slipping alternator belt or slipping crank pulley.
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Old 10-08-2017, 09:28 PM   #10
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my 240 does this too i can't figure it out it's 1991 it's like a steady flickering i too changed alternators cleaned all the connections and it won't stop
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Old 10-09-2017, 05:41 PM   #11
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When I had this issue it was due to mainly a bad regulator in the alternator. Also the ground connection from alternator to the block is really important.

If this is a problem caused by something else it is probably a circuit stuck cycling off/on. When the central locking fails sometimes it fails in a mode where it keeps trying to lock or unlock and cycles continuously. I have had this circuit draw ten amps so it isn't a low power circuit when they stick on. It is usually the drivers door switch for the key that does this. You can remove the fuse for that circuit and see if that helps.
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Old 10-12-2017, 08:44 PM   #12
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My Classic is doing this also.
Gonna have a look at the alternator brushes this weekend when I am changing the belts, hear that's a common fix along with the alternator ground strap.
What's funny is mine is down on power, and the same time my lights get really bright (the voltage raises at random) the power is restored.. interesting note on the door locks.. worth looking into!
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Old 10-12-2017, 09:01 PM   #13
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Maybe something is feeding back to the switch/relay. Alternator shouldn’t be making DC current. Check cleanliness of terminals in battery and big ground strap for body. I replaced an alternator switching from AC/DC because of a bad diode. Headlights would flicker pretty good.
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Old 10-12-2017, 09:13 PM   #14
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A few things you could try: disconnect the alternator for a little while and see if the lights stop flickering. Do a voltage drop test on the ground strap to the body. You could also try doubling up on the ground strap. Try checking voltage drop on a few key charging wires. If you have another good headlight switch, try that.
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Old 10-13-2017, 10:24 AM   #15
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Clean your grounds....

bad grounds cause all kinds of wonky issues...

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Old 10-13-2017, 11:06 AM   #16
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Originally Posted by Benjam83 View Post
Clean your grounds....

bad grounds cause all kinds of wonky issues...
This...Seriously. Remove the lugs and clean the surfaces and use a conductive compound.

I use copper based anti-seize compound.

The ground cable from the alternator, does it get hot?

I had this issue on my 780 and cleaned every ground mount and I augmented the alternator ground with an additional 6awg cable.

After that (and other things) no flicker, and charging is glorious.

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