Voltage unstable

During night driving I have noticed that the head lights vary in brightness. They brighten up at about 2500 to 3000 revs and dim when the revs drop. But when in say 4th or 5th gear at about 1500
revs(Diesel model) and I lift off the accrelator the lights brightenup?. Strange. This will voltage varation only happens AFTER the engine has warmed up. The brightness varation is also noted if you over-ride the interior lights. My garage has replaced the control unit but no change. Any suggestions. The battery and alternator have been FULLY checked out and found to be OK.
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Mike
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Accord 2.2 diesel 4 door saloon. June 2004.
"Michael Pardee" wrote: >
> > > During night driving I have noticed that the head lights > vary in > > brightness. They brighten up at about 2500 to 3000 revs and > dim when > > the revs drop. But when in say 4th or 5th gear at about 1500 > > revs(Diesel model) and I lift off the accrelator the lights > > brightenup?. Strange. This will voltage varation only > happens AFTER > > the engine has warmed up. The brightness varation is also > noted if you > > over-ride the interior lights. My garage has replaced the > control unit > > but no change. Any suggestions. The battery and alternator > have been > > FULLY checked out and found to be OK. > > > It would help to know what year and model. > > Mike
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OK, lets look at what causes a visible variation: change in voltage. Since you say the alternator and battery have tested OK, then its likely a mechanical rather than an electrical problem.
What causes an alternator to vary voltage? A slipping belt is the usual reason. A bad ground is another.

Have a good look at all the alternator mounting brackets. Is anything loose? eg the main pivot bolt on to the block. Check all the engine block to chassis ground links.
Is the crank pulley loose or is a two part balancer type? Could be the pulley is separating and moving the belt around causing slipage or lateral movement in the alternator.
SD
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I have thought of this route BUT this problem ONLY happens after the engine has warmed up. This leads me towards an electrical problem and your thoughts about a poor earth will be checked. If a poor earth should this not have shown up when the battery and alternator were checked? The unit that controls electrics has just been replaced with no change to the problem.
"Stewart DIBBS" wrote:
message > > > During night driving I have noticed that the head lights > vary in > > brightness. They brighten up at about 2500 to 3000 revs and > dim when > > the revs drop. > > OK, lets look at what causes a visible variation: change in > voltage. Since > you say the alternator and battery have tested OK, then its > likely a > mechanical rather than an electrical problem. > > What causes an alternator to vary voltage? A slipping belt is > the usual > reason. A bad ground is another. > > > But when in say 4th or 5th gear at about 1500 > > revs(Diesel model) and I lift off the accelerator the lights > > brighten up... > > Have a good look at all the alternator mounting brackets. Is > anything loose? > eg the main pivot bolt on to the block. Check all the engine > block to > chassis ground links. > > Is the crank pulley loose or is a two part balancer type? > Could be the > pulley is separating and moving the belt around causing > slipage or lateral > movement in the alternator. > > SD
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yourpete wrote:

The problem was the fact that the USDM car had an Electical Load Detector in the under-hood fuse box and the JDM ECU does not use one so it was confusing the JDM ECU and it was "telling" the alternator to stop charging when I would press on the accelerator. I remover the ELD and all is fine now. It could be that your ELD (if you have one) is bad, or the connection to it in the under- hood fuse box needs cleaned.
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