Topic: Rectifiertest on CB550F 78'

[BigBird]

Hi SOHC4s

Im breaking main fuse when i turn the key, except when i disconnect the rectifier (which is stock) on my 78 cb550f. I suspect that the rectifier is blown, because i rode with a near dead(shorted cell) battery before winter. It has now a new battery. Could anybody give a hand with the testing of the rectifier unit? I've gotten the following readings with a diodetester :

negative probe on green
1. I
2. I
3. 276

red probe on green
1. 538
2. 550
3. 275

neg probe on red/white
1. 552
2. 516
3. 538

red probe on red/white
1. I
2. I
3. I

And, seeing the results from diode 3, i test it with a ohmmeter (200Ω-mode) and it blinks momentarily a varying number before going to IO on both directions. What can this be indicating?

Cheers

[TwoTired]

Diode three is suspect, for sure, and likely diminishing your charging output.   Assuming you trust your test equipment.

But, the rectifier and charging system aren't in any fuse pathway on the stock wiring.  I think you'll find something else messing with your fuses.

Do they and or the fuse clips get hot?  Does the fuse blow in the center, or near the ends.  Over currents blow in the center of the fuse.  Fuse holder issue make the melt at the ends.

What is actual current draw for the bike.  With lighting on it is about 10 amps for stock bike.   Headlight or ignition changes from stock.

Are you positioning the fuses correctly?  A 7 amp in the main position will blow very often.  Needs to be 15 amp there.  Spares are at each end of the block.

Cheers,

[BigBird]

Thanks for the answer!

I do trust the measuring equipment. Wiring appears to be mostly stock.

I tried noticing any heating on the wiring as i broke the fuse, but couldn't sense any. Fuse blows in the middle, which then points to an over current. I have automatic fuses at correct amps (15A for the main).

Unfortunately i cant measure current draw until the fuse stops blowing. I worked it some more hours and tried pulling wires in the connectors and found that disconnecting green grounds from frame, the fuse stay on (please see marked area on diagram). Ground contacts are good.

Is it somehow possible to bypass the rectifier just to test the rest og the circuit? Im ordering a new unit.

Thanks again

[TwoTired]

You can unplug the rectifier and take it out of circuit.  Just don't expect any battery charging, as that is the output pathway for the alternator.

One method for finding shorts is to temporarily replace the fuse with an incandescent lamp using alligator clips.  This will limit any current, to that used by the lamp.  It will burn brightly, until you find and remove the short circuit, where it will dim significantly.  This saves on fuse replacement.


I must say, Honda never produced a 78 CB550F for the US market.  Last year model was 1977 for the F.  There was a 78 CB550k model, though.
The troubleshooting method for either model is largely the same, though.


It is the fuse clips that get hot if there are oxidation issues.  But, parting in the middle is far more likely a true over-current event.

Cheers,