Electrical help needed

This may seem a bit wierd, given that I have been up to my elbows in bits of Italian wiring harness since the days of the singles, but I’ve got a problem that leaves me mystified.
The ST4 has two seperate headlight bulbs, one dip and one main.
Dip works fine, but I’ve got no main beam. It isn’t the bulb, as connecting that straight accross the battery terminals it lights up OK.
It isn’t the fuse, as this serves both bulbs and the other one works.
It isn’t power supply, as a circuit tester from the power wire to earth shows connectivity. The main beam light comes on and all appears well.
However, when I plug the bulb back in, it doesn’t work, and the main beam warning light stays off. It’s only on the right hand (offside - H1 bulb) light.
Any ideas? I’m stumped at the moment.

Its a loose connection or damaged wire, that is just about making connection. It could be on the earth or the feed. The smaller the wire the higher the resistance and the less current it can take. The loose or partialy broken connection wire is the same as having a very small wire, a meter will show it is OK, but put it under load and the bad connection can not handle the load. Run a wire from the bulb holder neg side to earth and plug in your bulb. If it now works then your problem is on the earth side. If it is still the same you need to trace the wiring through to the switch or where ever the feed comes from.

Although be carefull and check the wiring diagram as I have come accross bikes that switch the earth not the feed!


Have a good look at the bulb holder. Had a similar problem with one of the indicators, the spring that holds the bulb and powers it had moved causing it to slide upwards instesd of attaching to the base. Good luck, it is a Ducati after all

I’m thinking it may be a dodgy wire. The bulb is seated ok and there does not appear to be any issue with any of the bulb assembly or seating in the reflector etc. Well, not as far as I can see anyway. When the bulb is removed and the live wire connected to it, then the bulb holder is earthed directly, the bulb lights. Well, it did on Monday anyway!
I didn’t try it the other way round by earthing out the bulb through the housing, then improvising a new live connection, which could have identified the earth wire as the culprit. I’ll have a look at that tomorrow evening to see if it works.
If the worst comes to the worst I’ll bring it to the Severn Valley Rally along with some spare wire and some insulating tape so Frank can fix it for me! I won’t need main beam in the daylight.
Had a similar problem on the Paso a few years ago when the ignition would unexpectedly cut out. Turned out to be a wire that was broken inside the plastic sheathing, and it couldn’t be seen until the connector was pulled off the end. Give me the '70s wiring on the old GT any day!

+1 On what Frank’s said,
I’ve just sorted out the rear light on my '92 851/916, by bending the connector in the bulb holder to make a more positive contact…
After ringing through all of the related wiring. :unamused:

It’s not the 1st time that the 851 rear light has given problems, I had one rear light unit that blew the bulb every time I went for a ride (I even bought a large box of Lucas bulbs!) and I couldn’t see/find anything wrong with the unit?
I tried all of the usual dodges to no avail so I put another one on and the problem went away.
I’ve still got a large stash of the Lucas bulbs, as the bike hasn’t blown another one in over 8 years and thousands of miles now.

Steve R

This just gets worse and worse, confusinger and confusinger!
I’ve spent hours on the bloody thing over the last weekend, and I’m getting nowhere. It’s got to the point where I’ve even checked the stupid things that common sense says shouldn’t be the problem anyway. This is where I am so far.

I checked the fuse, which I assumed was OK because it covers the dip beam as well, and it is fine. Rule that out.
With the bulb in place, I ran a positive wire direct from the battery to the bulb, and it lit, so both the bulb and earth are OK.
I’ve pulled the switchgear apart and checked the positive feed through that, which is OK. Rule that out too.
I’ve followed the wiring from the switchgear down to the junction box left of the headlight, and out the other side, and all read OK for power.
That took me on to the headlight/flasher relay. Again, the relay works OK, and all wires in and out read positive for power.
From there the wiring goes (I think) straight to the bulb, and the spade connector at the end of the wire still lights up my circuit tester when powered up. The main beam warning light comes on as well.

Attach it to the bulb though, and nothing works…aaaaaaaaargghhhhhhh! Cue much gnashing of teeth and screams!

I’ve tried to rig up a new feed wire bypassing the existing last part of the circuitry but it’s pretty much impossible to do without destroying the multiple junction blocks in the process, as Ducati must measure the length of the wires in the harness to the last milimetre. There’s no room to splice into the existing wires, and the connectors look they will only come apart by destroying them. I daresay it saves them about five euros on every thousand bikes they make, but it makes them a pig to work on and trace through. It means I can’t get a “spare” connection on to the relay as I can’t even see any way to connect to it. In going through this process I also discovered where the indicator relay and main lighting relay (?) live, and they were both OK too.

Something a bit strange I notice though, and wonder if this is the same on all ST4s, or whether it’s indicative of the problem I’ve got. When on dip or side lights, the main beam flashes (as you would expect). Well, the warning light does, the bulb does nothing. But when switched to main beam the dip flashes. Is this normal?

Right now I’m running out of ideas. Anybody got any more suggestions? Apart from using a bigger hammer…

Hi Spaggy. The flashing information lights could be part of your problem as mine don’t flash but read as a steady glow. Just fit a daylights running lamp!!!

Hi Spaggy,
You can get special tools CHEAP from RS or Maplins for taking the multipin plugs apart.
A mates just rewired the Power Commander on my bike and he borrowed a dinky little set from an auto electrician.
I’d done over 50% of it myself, but 75% of the wires left were as thick as a human hair and most of them were the same colour.

Just a thought here but the dash displays can play funny tricks when the battery charge is low.

Steve R