Ducati Owners Club of Victoria

Regulators & Charging Systems

The output from this coil(s) is an alternating electric current (AC) whose voltage increases as the motor revolutions increase. This AC can be used to power most items on a bike but cannot be stored in a battery. This may be fine on small machines or scramblers that have small electric loads but the brightness of the light varies with the motor revolutions and so at idle and low speeds the night lighting will be very poor.

For larger bikes with heavy electrical loads and/or electric starters we need a battery to store power to be used when the engine is not running or is spinning slowly at idle. To be able to store AC power in a battery it must first be 'rectified' to direct current (DC).

AC current flows alternately in one direction then the other in cycles that are dependant on the speed of the alternator rotor but typically could be hundreds of times per second. To 'rectify' this into direct current, which flows in only one direction, we use one or more electrical devices called diodes. These are like a non return valve that lets the current pass in one direction but blocks it in the other.

As you can see if we use just one diode then we will 'lose' 50% of the alternator's output. This may be OK on small bikes but on larger machines we use a 'bridge' rectifier that consists of four diodes wired in a special configuration that allows both directions of the AC current to be converted to direct current.

Now we have DC to charge the battery but the faster we rev the engine the higher the voltage that is put out by the alternator. This can be over 100 volts at high revs and that is why they tell you not to disconnect the battery whilst the engine is running as 100 volts will destroy your regulator!

A lead acid battery should not be allowed to charge over 14 volts otherwise it will boil and quickly destroy itself. We need another device called a 'regulator' to control the flow of current to the battery at all times. Ducati regulators are a combined bridge rectifier/regulator in one unit.

This 'regulator' must be able to 'sense' the state of charge of the battery. If the battery is low on charge then all of the alternator's output will be used to charge it up but if it is fully charged around 14 volts then the charging rated is reduced to a trickle.
Configurations

Ducati used a wide variety of alternator and alternator/magneto types to power their single cylinder machines. Some had batteries with DC coil ignition, and some had no battery and used AC to power ignition coils and lights. I am not going to discuss the single types in this article but will concentrate on the twin cylinder machines that have used only two different Alternator configurations from the 750 GT up to the latest 916's.

The first configuration is a centre tapped coil that has three wires, two yellow and one red. The second configuration is a straight coil that has two yellow wires. Both of these configurations have been used on various twins and with various different output ratings.

There are also two different configurations of regulators to suit these alternators and the correct combination should be used. A three wire alternator can use a two wire regulator by not connecting the red tap alternator wire but a three wire regulator will not work with a two wire alternator.

The following is a list of known combinations but knowing Ducati assembly methods do not take as gospel!

* 150 Watt 3 wire alternator and plain case 5 tag 12 amp regulator. This combination was used on all 750 round case models, all parallel twins and a few early 860's.
* 200 Watt 2 wire alternator and finned case 4 tag 18 amp regulator with black connection strip. This combination was first used on all 860 models and also on bevel 750/900 SS's, early 900 S2's, Darmahs, and 500/600 Pantahs.
* 200 Watt 3 wire alternator and finned case 6 tag 18 amp regulator with red connection strip. This combination was used on the later 900 S2's and 600/650 Pantahs.
* 300 Watt 2 wire alternator and finned case 4 tag 25 amp regulator with black connection strip. This was used on some later model 900 S2's, and on 900/1000 Hailwood Replica's.
* 300 Watt 3 wire alternator and finned case 6 tag 25 amp regulator with red connection strip. This was used on 750 F1's and later 900/1000 Hailwood Replica's.
* 300 Watt 2 wire alternator and finned case 6 wire 25 amp regulator with rubber connection piece. Used on '91 851, '92 851 SP, '89-90 900 SS, '91 750/900 SS, '93 851.
* 350 Watt 2 wire alternator and finned case 6 wire 28 amp regulator with rubber connection piece. Used on '89-90 851 Tri-colour?, 1992 851, 907 IE, 916's?

Regulator Problems.

Plain Case These are the most reliable of all probably due to the low power (150 Watts) that they have to handle. They rarely fail but can either half charge or fail to charge. Overcharging is rarely a problem as the bikes use most of the 150 Watt alternators output especially on around town rides at night with headlights on.

Finned Case The black strip type usually last well over 60,000 Km but have been known to overcharge, half charge and fail to charge.

Finned Case Some of the 25 amp Red strip type seem to have under rated components and I have seen several units short out and melt their resin, which is messy, smelly and potentially dangerous! They can also suffer from overcharging, half charging and failure to charge.

Finned Case The later type with wires instead of a connector strip can sometimes cause engine miss problems as they seem to 'spike' the computer upsetting the timing. I haven't yet seen a 'burnt out' unit so maybe they beefed up the components. I have only seen one failed unit although they are not very old as yet.

A second problem with these later models is that the regulators are rubber mounted and thus isolated from the frame. They must have a separate earth lead which attaches under one of the mounting bolts but this cannot be done up tight because of the rubber mounting. This lead is the main return path for the regulator and may have to carry 28 amps then it must be a good tight connection. The best way to fix this problem is to drill and tap a hole in one of the 'fins' on the regulator and connect the earth lead tightly to this.

A third problem with the latest models is the gauge of wire from the alternator. The two yellow wires are the same size (approx 1.25 square mm) as was used on the first 860's. This wire is rated at around 10 Amps max and was enough to handle the old 860/900 SS running load of approx 2-3 amps with a reserve for headlight running (5 amps) and charging of the battery. (Note: ignition on these models was self generating)

The new models run a high pressure fuel pump (5 Amps?), a radiator cooling fan (5 Amps?), hard wired headlight (5 Amps), electronic ignition (3-4 Amps), computer engine management (2 Amps?) and other instrument lights, gauges, sensors, etc. You can see these new models can have a running load over 20 amps without even allowing for battery charging! The newer alternators are rated at 300 or 350 Watts which equates to around 25-28 Amps!

The end result is that the alternator wire run too hot to touch and I have seen at least two examples where the insulation has melted and the wires have shorted together! This will ultimately burn out the alternator and/or cause a fire!

Some examples I have seen seem to have had an extra piece of cable added between the alternator and the regulator using crimp connectors. These connectors are another place where a bad connection can cause problems.

The solution is to replace this wire completely with something heavier ie 2 square mm (4 mm auto) which is rated at 18 amps or even 3 square mm (5 mm auto) which is rated at 27 amps! This should be connected directly to the alternator windings for maximum effect. If you own one of the later model dukes then check the alternator wiring regularly and if you find signs of overheating then either take the bike back under warranty and demand a fix with heavier wire or replace it yourself if the bike is out of warranty.
Regulator Tips

Make sure all connections fit tightly, they may have to carry up to 28 amps so they must have a perfect connection or they can heat up and melt covers and wires! Make sure the wires are well crimped into the connectors better still get them soldered if you can.

The body of the regulator must have a good contact to the bike's earth as this is the return path. Don't rely on mounting bolts, which may not break through the paint, but run an extra lead from a good earth point on the frame to an eye terminal under one of the bolts if this is not standard. For rubber mounted regulators drill and tap a hole in one of the 'fins' and connect the earth lead tightly to this.

The condition of the battery can also effect the regulator. If you let the electrolyte level get too low then the regulator 'senses' that the battery is flat and increases its output. This can result in the battery being overcharged and the regulator being overworked. A common failure with old motorcycle batteries is that one cell shorts internally. Once again the regulator is overworked with the impossible task of trying to bring this battery up to 12 volts!

Original article by Peter Shearman, 1988