ST1100 running/idle problem

Hi chaps, hope someone can help, this is the problem I mentioned in my introduction.
Just before xmas`14 coils packed up on my st1100 1999 model, both coils and caps replaced and restarted, however, before the coils packed up it always idled at 1000 or just below with no issues (6 years) now, at 1000 revs as soon as I put the lights on, revs drop to 7-800! When the fan kicks in drops to 500 and almost stalls. Have increased the revs to 1200 at idle, lights on drops to 900-1000, fan on 7-800! Have fitted a new battery (change every 2 years so was due) thinking that could be the problem, still the same. Riding above 1500 no problems at all, in all gears accelerating, cruising etc can sit at 90mph at 4500 revs all day, as soon as I am sat in traffic revs drop.
Battery holds the charge, have removed HID lights and reinstalled original bulbs, have heated grips but that is all accessory wise. Original alternator regulator/rectifier and 102,000 miles. Initial electrical checks have been done, starting charge, running charge, output from coils all good. I did notice a high pitched whistle coming from under the bike somewhere as soon as the lights go on??
Any ideas gratefully received. Many thanks for any input. andy

My guess is the alternator or regulator is starting to fail. The '99 model has the 40 amp alternator and should handle the load. When you mentioned the electrical checks indicated "starting charge", "running charge" OK, was that a voltage check while the bike was idling? I really think the reason for the rpm drop when the the lights are energized and further drops when the fan comes on is a faulty alternator working very hard to try to keep up. Take it to a shop specializing in auto electrical and have it load tested.

Consider going through all of the major power distribution points, such as the main fuse / relay area, fuse box and ignition switch. Also have a good look at the main bike ground right next to the seat key cylinder and battery ground at the block just above the oil filter. At 102,000 miles it's time to take these connections apart, clean and paste with dielectric anyways. Go through ALL of the connections FIRST. You may find something burnt, corroded, or loose.

thanks adam, haven`t checked those areas yet but will do over the weekend and let you know. andy

I'm gonna guess that you have corrosion in one or more points in your wiring harness. This causes an excessive voltage drop when each component you mention kicks in, which then taxes the alternator more than it should because the alternator interprets the excessive voltage drop as more load than it really is. Just a guess, but its free to trace out your voltage drops so you have nothing to lose by doing that and cleaning up any corroded connections you may find. As someone else mentioned, if your alternator is getting weak you could see the same thing with a normal wiring system, but its not very common for the 40A alternators to fail, especially if you don't load them much over the stock electrical load.

On US bikes the headlight is always on, so I don't know how that affects the idle, but its normal for the idle to drop 100-200 RPM when the fan kicks in (don't really look that close, but its a noticeable drop) Given that yours drops 200 RPM for the fan doesn't really sound all that unusual.

hi all, an update on my problem. the output voltage from the alternator checks ok (with lights on and off) battery voltage ok (no ignition) battery voltage rises when started and idles ok at 1200. main earth checked and cleaned (thanks Adam) problem starts as soon as lights are switched on, also, fuel pump starts before ignition? fuel pump relay needs to be checked and, unfortunately, the harness for shorts, corrosion,light switch etc and coils as the problems started after replacing them. the fella checking the harness is a honda specialist for st`s and goldwings (private job so not too many bucks!!! hopefully!!) everything seems to point to a short somewhere so will press on with the harness tomorrow and hopefully have an answer for everyone. many thanks for everyones input and noted for future reference.

Rigger:

I'm going through an electrical connection check on my bike too. On my bike the fuel pump starts as soon as you turn the switch to "on". If that's what you're saying above, then that fuel pump function is operating as it should.

hi all, after all the tests one of my coils (left) was a suzuki coil put on by the garage to get it going. the specialist was explaining about digital and analogue coils so this one, although working with correct voltage was not compatible with my bike and year. so, a replacement coil and leads are needed, will update once i find one.

rigger39 said:

hi all, after all the tests one of my coils (left) was a suzuki coil put on by the garage to get it going.

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What an odd culprit... but at least its circled down now...

so, a replacement coil and leads are needed, will update once i find one.

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This here seems like a reasonable priced set:
http://www.ebay.at/itm/HONDA-ST1100-PAN-EUROPEAN-IGNITION-COILS-LEADS-AND-CAPS-FULL-SET-/320844785210?pt=LH_DefaultDomain_3&hash=item4ab3d6e63a
This one looks like a NOS:
http://www.ebay.at/itm/Honda-ST1100-AN-AY-Pan-European-ABS-TSC-92-00-Ignition-Coil-/311350123778?pt=LH_DefaultDomain_3&hash=item487de9dd02
And another NOS, same seller
http://www.ebay.at/itm/New-Ignition-Coil-Honda-ST1100-Pan-European-90-02-/220747905032?pt=LH_DefaultDomain_3&hash=item336599b808

hope this helps

rigger39 said:

one of my coils (left) was a suzuki coil put on by the garage to get it going. the specialist was explaining about digital and analogue coils so this one, although working with correct voltage was not compatible with my bike.

Click to expand...

now there's something that nobody saw coming, hopefully a coil swap will cure your problems, good luck!

dwalby said:

now there's something that nobody saw coming, hopefully a coil swap will cure your problems, good luck!

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....and likely wrong ohm coil anyways......amazing the number of so called 'bike wrenches' think if coils 'look' plug'n'play.....must be 'good to go'.......good recipe for 'trashing' ignition modules unless ohms are first checked across the two coil connections.

afrymoyer said:

Consider going through all of the major power distribution points, such as the main fuse / relay area, fuse box and ignition switch. Also have a good look at the main bike ground right next to the seat key cylinder and battery ground at the block just above the oil filter. At 102,000 miles it's time to take these connections apart, clean and paste with dielectric anyways. Go through ALL of the connections FIRST. You may find something burnt, corroded, or loose.

Click to expand...

before you dielectric everything see this thread: Dielectric grease revisited