Dynamo test

[welshrat]

Hi all. I think that my dynamo has failed, red ignition light and zero on the amp meter, fan belt is fine. Anyone out there who can give me the idiots guide to testing the dynamo (have a good multimeter). Also been advised to check the controll box, again any pointers or direction to threads to read would be very welcome.

[mike.perry]

C40 Remove the end of the dynamo and check the brushes
C39 Slide back the inspction cover.............................

[bmcecosse]

Take it off - join the two terminals with a bit of wire - apply jump leads to the wire and the body. If it runs as a motor - it's fine. But as above - it IS likely to be needing brushes....

[silloyd]

Welshrat,

Somewhat spookily my Dynamo failed yesterday (after returning from the St. Fagans show) - red light and 0v/0A output.

Fortunately I have a spare and swapped it in. When I looked through the open section at the end of the failed one I found that one of the small springs that holds the brushes in place had failed. Easily fixed, just need a replacement.

[welshrat]

Take it off - join the two terminals with a bit of wire - apply jump leads to the wire and the body. If it runs as a motor - it's fine. But as above - it IS likely to be needing brushes....

Good tip, dynamo is fine. Regulator box has fried I think, crispy black coating on copper coil. Sods law though, found 3 spare boxes (one new), all have spade conectors rather than grub scews. Shopping for female spade conectors tomorrow, oh joy.

[welshrat]

Any new ideas for me, dynamo running as per bmcecosse test and brushes good, changed regulalator box but no difference, still have 0 on the amp meter and ignition light remains on.

[bmcecosse]

Must be connection problem..... Have you changed the polarity of the car?

[silloyd]

Where are you measuring 0v/0A? Try disconnecting the Dynamo and measure from the terminals to earth, noting polarity.

[welshrat]

Many years ago. Problem began on the M4, car has not done that sort of speed (only 60 mph) or distance for many years, my thought was that this change of use has created the problem.

[welshrat]

Where are you measuring 0v/0A? Try disconnecting the Dynamo and measure from the terminals to earth, noting polarity.

This takes me back to the original post, looking for a guide on how to check with a muti meter

[silloyd]

Measure the Voltage between the Large connector on the Dynamo (this is the one that delivers the charge current to the battery) and earth/battery negative.

It should read a decent charging voltage, i.e. not 0v (or when mine failed 0.99v). Clearly Dynamo large terminal should be positive relative to earth/battery negative, otherwise its polarity may be wrong.

Current will only flow if the path to the battery is complete, which it doesn't appear to be in your case.

If you have a voltage across the Dynamo, then reconnect the wire and measure again where the corresponding wire enters the regulator. If that is OK then measure again at the output of the regulator...and so on until the battery. Wherever the voltage drop ceases to exist is where your problem will be the component immediately before that measuring point.

Hope that makes sense.

PS Don't forget to swap the leads on your multimeter around when measuring volts and amps (different connection sockets on the meter, get them wrong and (a) it won't read properly and (b) it might damage the meter or the circuit)

[welshrat]

Thanx, will check it out tomorrow. Also the 'many years ago' above relates to it now having a NEG earth.

[bmcecosse]

You may have reversed the dynamo polarity when you tested it ..... Take a length of wire and connect one end to the battery +ve terminal and then 'flash' the other end against the small terminal on the dynamo a couple of times. Then reconnect the dynamo and start up - is it charging now ?

[welshrat]

Done the polarity trick but no change, checked the volts between large conector and battery earth = 0.39. Found an old dynamo that died while on my now gone 68 cooper, brushes had become stuck so springs could not push them down, cleaned up and a drop of oil applied. Fitted and now sorted, oh deep joy. Any thoughts why it would be running as per bmc trick but not doing the job it should do, it is a relatively new dynamo, wondering if worth tryng to fix or bin it. Also not sure now if the regulator box I removed was fried, the only way to check would be to refit but cant face it at the moment, just pleased to be back on the road. A Big Thank You for info and advice provided.

[bmcecosse]

A dynamo that 'motors' will always work as a generator... Something must have gone wrong in between times.......

[welshrat]

Not sure what you mean bmc, after temp fitting the mini dynamo that worked I doubted myself so refitted the original that was still not charging. Rechecked it with battery and jump leads before putting it away in the shed, still running as a motor.

[bmcecosse]

When it 'ran' as a motor - did it make good speed, or just stumble round |?

[welshrat]

Not a good speed to be fair, not stumbling but slower than I assumed it would run, the one from the mini was better and is giving a good charge. Thank you for all the good advice by the way.

[bmcecosse]

Well - I guess it is just marginal then. Maybe just needs a good clean to the commutator and a new set of brushes.