Too many volts! – 11 Mar 2017

I’ve mentioned before that, over the last few months, the voltage warning light on The Fire Bike has been “playing about”. I fitted this soon after getting the bike so that I will know if the charging system develops a fault and can do something before I get stranded with a flat battery. I have said that the light had flashed red on one occasion indicating low voltage. Well more recently it was flashing alternate green and red which indicates an over-voltage (more than 15.2V). It may be that this was what was happening the first time but I just couldn’t see the green colour in the sunlight. Anyway, an over-voltage is to be avoided because it will soon kill the battery.

My bike has a Magneti Marelli dynamo and mechanical control box. The original fitment. I consulted with my friends on the Guzziriders forum as there is a thread there about fitting a solid state control box/voltage regulator sold as a replacement for an old Fiat. Someone had bought one from Teo Lammers but they are currently out of stock. It’s here. I decided to see if I could find one at a classic Fiat specialist here in the UK and managed to get one from Motobambino. I thought the price was good and it arrived inside 24 hours.


There was some discussion on the forum about the current rating of the solid state regulator. The original mechanical box was rated at 25A and this replacement at 16A. The dynamo is also 25A. So far as I can tell, a 16A regulator with a 25A dynamo will work fine but, the output (current draw) is limited to the 16A of the regulator. I’ve done some searching and it looks like the Fiats had a 230W charging system as opposed to the Guzzi’s 300W. However the two systems seem to have had their ratings worked out differently.
– Fiat – 14.5V x 16A = 232W.
– Guzzi – 12V x 25A = 300W.

The lowered capacity of the system could cause issues on an original police bike running radio, siren, blue lights and other stuff but, should be fine for my “civilianised” one. I’ve used a “worst case scenario” of 12V x 16A which gives a maximum system capacity of 192W. I think my maximum constant draw is about;
– Lights (bike) – 75W
– Lights (trailer) – 10W
This leaves plenty for the ignition circuit. The intermittent draw from starter, indicators, horn and the like can be disregarded. I might have to think again if I add heated grips or stuff like that.

Changing the control box.

I removed the battery and, before I did anything else, photographed the original mechanical box in situ and made a note of all the connections.



I disconnected the wires and undid the nuts securing the box to its bracket but I couldn’t shift it! The two bolts were too tight in the mountings and appeared to need to be unscrewed from the back. To get to the first bolt I removed the nuts and bolts securing the left hand tool box which released the mounting bracket for the control box on that side.


I then realised that I didn’t actually need to take the tool boxes right off. I could just remove the top fixing.


This done you have access to the back of the bracket. Here the bracket is fully floating and the old Marelli box has been removed.


The mountings for the box weren’t threaded. Just tight and the screws had to be wound out.

Here are the two control boxes for comparison.


The mounting holes are at the correct centres but needed to be drilled out. There was also an earth/ground terminal on the original. I found this area could be drilled to accept a terminal while still clearing the mounting bracket.



Being a mechanical device, the old box had mountings to protect it from vibration. These are not removable.


I don’t think these are strictly necessary for the solid state replacement but I punched some bigger holes in some rubber washers and used them to mount the plate. You’ll notice that I filed all the corners off that plate. Twas flippin’ sharp.



You can see in the photos that the wiring connections were in the same order as on the original. I had to do some checking in my manuals to make sure this was the case as the terminology on the two boxes was (of course) not the same.
– Marelli box.              “D+ 61”  – “DF”      – “51 B”.
– Replacement box.   “51”        – “67”       – “30 +12V”.
– Guzzi manual says. “D+/51” – “DF/67” – “30/B+”.
This is the illustration from the V7 workshop manual which explains things.

Marelli Regulator Unit

With everything back together again the bike was restarted. All seemed good then it stalled. Once I’d turned the fuel on all was well!

Today I’ve been out for a test ride. Just the 20 miles up the A487 to Aberystwyth for coffee and cake on the sea front then back home. I have to say that the new regulator is an improvement. The strange messages given by my voltage light have stopped. Not only that, the light turns green (signifying normal charging) at lower engine revs than before. This means I don’t have to change down to keep it “in the green” and can potter about in a lower gear.

I have had a look inside the old control box which is clean with no obvious faults. Interesting.


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