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Discussion Starter · #1 ·
The bike runs strong until i hit the high beams or activate the cooling fan, then it runs at half power as if its running on one cylinder. I got a new voltage regulator on order, which is a pain in the butt to get to on a ST4s.
Any other ideals on what's causing the bike to lose power under heavy electrical load?
 

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The bike runs fine if not switching on the fan or high beams, eg no stumbling, hesitation under roll-on acceleration once at speed, say 50-80 mph roll on?
 

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Do you have a volt meter installed? Perhaps your charging system is deficient and the added drain with the fan or lights is bringing your voltage to the ECU down to the limp home mode.
 

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Checked the grounds?
 

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Ducati Designs
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This sure sounds like a grounding issue. A weak ground will cause circuits to seek ground through odd paths, limiting voltage to that circuit. ST's are famous for bad connections and poor grounds.

Were it mine, I'd strip all the plastics off, locate every connector and disconnect (one at a time, clean, tug on wires, use some contact treatment (dielectric grease/gel, Deoxit, nose grease, whatever) and reconnect. The big connector between the front and back harness hales can get wet inside, corrode, crust over and die. Remove the back shell and look for blue/green creeping crud. Then check that the main ground at the back of the tranny is tight, clean, etc. Ditto battery leads. Also check that any grounds between the ECU and frame/battery holder are there and clean and tight. If the battery is more than 5 years old, I'd toss it and get a fresh new AGM battery.

A new headlight can help, too. ;)

pg

DD
 

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Having gone through that issue on a couple of old 911’s I have worked on I can tell you its most likely a bad ground. Start with the whole circuit for the lights and check all the grounds. As someone already said the circuit is not complete properly and therefore seeking and alternate ground zapping power from the entire system. You may also want to take the switch apart and clean all the contacts internally on the thumb switch.

Nice looking ST
 

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Another vote for a bad earth. Reminds me of a fault I had on the old Volvo. The engine stopped when the windscreen washers were used. That was a result of a dislodged connector in an obscure place. That meant the injection ECU had to earth via the washer motor!

Keith
 
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