Hi and welcome. I'm far from a /6 expert, but I will try to help.

• Power is delivered via Red wires (plural becasue it is multiple wires and connections) all the way up to the ignition switch. Connect your test lamp between the battery terminal Neg and you should see the light come on at the Red wire on the back of the switch AND at the Red section of the multi-color circuit board inside the HL shell. Since your lights work, this test should pass.

• With the ignition switch fully ON, power passes thru the switch and comes out on 2 Green wires. Both these wires feed the two 8A fuses on the multi-color circuit board. On the other side of the fuse, power comes out on wires with Black stripes (Grn/Blk and maybe Blu/Blk). So the next test would be to check BOTH sides of the fuses to look for the presence of power. This is an easy test with your test lamp still connected to the battery as before.

You didn't say, but I take it this bike has been sitting for some time. Due to the wind-blown water these bikes saw, corrosion often inhabits the connections... and about 80% of them are inside the HL on that multi-color circuit board. And when I say "connections", I also include the connection to the 2 fuses on that board.

Multi-color circuit board

Often times pulling the connector off the terminal or spinning the fuse will restore power, but it is only temporary once corrosion is present. For a more permanent solution, I highly recommend you get a very small tube of No-Ox-Id (Amazon) then disconnect each connector one at a time, add a dab of No-Ox to the female socket and carefully re-plug it to the same terminal. You can follow this same practice on the connectors under the fuel tank and any other connectors, relays, bulbs and terminals all over the bike. The only exception being the 3 prongs of the headlamp bulb due to the extreme heat.

You'll see. Lights will be brighter, horns will be louder, turn flashers will start to blink more evenly. And best of all, the condition of your battery will most probably improve too.

Other Notes
• All you need to crank the bike is power at the Green wire at the ignition coil. With the tank removed you can easily remove the Grn wire and make this test.
• You must have an operating bulb in the "Gen" indicator panel in order for the alternator to charge the battery. No light there may indicate the field is not receiving power to initiate battery charging.
• Clean battery terminals and battery cable connectors is a must. No-Ox can help at both ends of both battery cables.
• Also see this thread about updating the fuses: https://www.airheads.org/kunena/wrenching/392-home-mechanic-more-reliable-fuses#2849
• The starter interlock may involve the neutral lamp and a switch on the clutch lever. You can bypass all this by momentarily jumpering the Red wire on the starter relay (left side under the tank) to the Black.
• I think the diode you are looking for is built into the multi-color circuit board.
• Be very careful when working with the ground cable at the gearbox /speed drive cable. That bolt is hollow and will break if the wrong torque is applied.

Hope this helps.

Hi Wobbly,

Thank you for the info. This helps me further conceptualize this system and organize my approach to trouble shooting. The bike has been sitting for a while. I have replaced switches cleaned corrosion all over the bike. I am using de-ox-it that I normally use on my stereo gear. I have also been using dialectic grease when reassembling. Right now, my suspensions are of the circuit board. I assume that once power flows across the fuse the board is charged bridging the two sides of the board across the fuses. cleaning the fuse buss's may do the trick as I know the fuse itself is charged but I didn't have power at the terminal for the brown and black wire that goes to the neutral light switch.

I know little about the circuit board itself. I imagine it is similar to other printed circuit boards that can sometimes be damaged from over heating. Does it make sense to remove the circuit board and inspect the back side?

I did not want to do that this weekend for fear of confusing what goes where when I unplug everything. I will definitely take pictures if I start unplugging wires to remove and inspect the board. I was also hesitant to remove the screws holding the circuit board to the bucked and turn the board for fear that the old brittle wiring could be damaged.

I will start with your recommendations and see where it leads.

Thanks again for your help!

I know little about the circuit board itself. I imagine it is similar to other printed circuit boards that can sometimes be damaged from over heating. Does it make sense to remove the circuit board and inspect the back side?

This is not that type "circuit board" at all. In fact it's not a copper-clad "printed wiring board" like you may be accustomed to. This is nothing more than a non-conductive piece of fiberboard, which has 1/4" spade terminals riveted to it. So there is no "brittle wiring" anywhere on the board; it's quite robust.

For example, take the Yellow colored terminal section. As with many of the other colors, you'll see 2 terminals in that color section. That is a U-shaped piece of metal riveted to the fiberboard. So all the circuit board does is provide an easy way to electrically join 2 or more wires of the same color. Yellow happens to be HL Low Beam, and so power from the handlebar switch is thereby joined to the wire going to the actual HL bulb. The function of Low Beam is discrete and does not require connection to any other source, and so there is no connection between Yellow and any other color terminal. And that is how 99% of that circuit board is constructed.

► The one and only set of terminals that does have a connection to another color terminal section is the Brown/Black which is connected in the rear by the diode you spoke of. That diode connects the Brn/Blk to the Blue/Yellow terminal set. If there is power on Blu/Yel, but not on Brn/Blk then the diode is bad.

► There should be 2 Brn/Blk wires inside the HL shell. One goes to the neutral switch, the other goes to the neutral indicator lamp. If you have power on those terminals, but no lamp, then your neutral indicator lamp is simply burned out.

► If the connections on that circuit board are in good shape, then it is more likely that the neutral switch itself has gone bad. You might try unplugging Brn/Blk at the neutral switch and connecting it to the engine case. That should make the neutral indicator lamp light up in any gear.

I have also been using dialectic grease when reassembling.

Think you meant "dielectric", which is an insulator. It's not clear how adding an insulator to a bad connection is going to improve the electrical connection. Just saying.

Hope this helps.

This helps me immensely! First, It's good to be sorted out with the improper use of dielectric grease. I thought it was used to enhance electrical continuity between two surfaces and protect from terminals from corrosion.

Given this information about the circuit board I suspect the problem may be the diode. I work long hours and come home tired so Sunday is when I am likely to get back to it. I think the information you have shared will help me figure this out. I will write again on Sunday.

Thank you so much. Cheers!

I am checking the neutral light diode on the circuit board in the headlight bucket of my R90/6. It is not passing continuity in one direction but has continuity passing in the opposite direction as I would expect. I need to know what value of resistance is acceptable across this diode as my readings show high resistance across the diode. I am getting a reading of 4.6 K ohms. I am thinking this diode is no good. Any one know what specks are for this diode?

This diode appears to be a general purpose, about 1 amp, diode. You can test it with a ohmmeter, but the readings will depend on the type of meter you use. The first step is to isolate the diode from the rest of the wiring, disconnecting the wire from one end will do. With the meter negative or common lead (black) to the banded end and the positive lead to the other, a good diode will show some resistance. This resistance value will depend on the voltage that the meter uses to measure resistance. 4.6K ohms is probably a good reading if you are using inexpensive meter. Reverse the leads and the meter will read infinite or open or OL. You need to measure the diode both ways to assess it's health. These general purpose diodes very seldom short. Their usual failure is to open and therefore measure infinite resistance in both directions.
Expensive digital meters will have a diode test selection, and read the forward voltage drop of the diode, about 0.6 volts. Reversing the leads will show open or OL (overload).
I hope this helps.

Hi Murphyam53

I when I last wrote I had the diode board out of the bike. The soldering in the back is solid and at that time I tested the diode in both directions. This weekend I have been working on the bike again and I have allot of information to add to my ministry. I will be brief here and add info with my response to Wobbly.

I am using a Fluke multi-meter and don't have much experience using it. As you suggested the meter has a diode check function. I tested the diode in both directions with the Diode Function and found it passes .55 volts. I suppose that sounds good. I followed up on Wobbly's recomendations as well. I will write my findings in response to his posts.
Thanks for your help Murphyam53. I am in the weeds here and all help is welcome.

Hi Wobbly,
I just responded to Murphyam53 about values found with a diode check using a Fluke multi-meter. I think the diode checks out ok. I performed many of your recommended tests and here are my findings.

1) I have power to the ignition
2) I have power across both fuses
3) I have no power at the Blue and Yellow terminal on the circuit board
4) I have no power at the Brown and Black terminal on the circuit board
5) I have .2 volts coming in on the Brown and Black wire
6) I have 12.3 volts coming in on the Brown and Yellow wire
7) The diode passes .55 Volts in one direction only when tested with my diode test function on my Fluke multi-meter
8) The diode shows 2.5 M ohms in one direction only
9) If the Brown and Yellow terminal at the circuit board is grounded the starter motor activates
10) if the Brown and Yellow contact at the switch gear is grounded, the starter motor activates

11) If the Red and Black wires at the starter relay are jumped the whole bike has no power. No lights and no activation at the switch gear.
12) There is .5 volts at the neutral light switch at the transmission testing the wire at the coupling from the harness
13) There is 1 ohm resistance testing to ground at the transmission testing the wire at the coupling from the harness.

Other info:
This is a 1974 R90/6 with no kick start. The neutral switch was replaced with a switch for a 1994. This could have had the transmission changed.
The wiring harness had been cut and spliced where the headlight and the instrument cluster wire into the main harness.

Ideas:
Wiring harness spliced together wrong
Wrong neutral switch for the transmission
faulty right side switch gear
faulty starter relay

Any help is greatly appreciated!!!!!!
Thanks

Thanks 14724,
I will check over the headlight bucket to confirm the connections there and the splicing. I have gathered all the info I can, unless someone else has an idea. I think Murphyam53's information regarding the use of the diode check on my multi meter rules out the diode (thanks Murphyam53). Wobbly's recommended tests have given me allot of information to think about and I don't mind considering a puzzle with my wiring diagram in hand. Maybe Wobbly will make sense of the information gathered easily but for me it is more likely that I will spot a wire out of place than logic it out. I guess next weekend I will pour over that spaghetti bowl and look for that one noodle. Call me Marinara or call me Meat Sauce. I suppose I feel more like Meat Sauce. Marinara sounds too refined.
Thanks for your help. I will post more this week end.

Other info:
This is a 1974 R90/6 with no kick start. The neutral switch was replaced with a switch for a 1994. This could have had the transmission changed.
The wiring harness had been cut and spliced where the headlight and the instrument cluster wire into the main harness.

Ideas:
Wiring harness spliced together wrong
Wrong neutral switch for the transmission
faulty right side switch gear
faulty starter relay

So my next question was who did the cutting and slicing ? In my regular motorcycle work I see all levels of wiring from bad crimps to wires twisted together with electrical tape to the use of "wire nuts". Most of it would work in a home wiring situation (say for a table lamp), but it simply DOES NOT work on a motorcycle. The over-riding reason is the exposure to wild temperature changes along with water and oil/gasoline. People do what they are accustomed to doing, and then call it "good" not realizing that motorcycle wiring calls for a grade of wiring 100X better than what they have tools to do. And here I'm not picking on home mechanics, becasue a lot of the wiring I repair was originally completed by $70/hr professional mechanics. In short most people simply don't have the knowledge or correct tools. Basically, if you wouldn't find it under the hood of a new Toyota, then it shouldn't be on your motorcycle.

I came out of the military electronics industry. Most people can't tell my work from the OEM harness manufacturer. I'm not bragging, simply trying to describe the high level of craftsmanship it takes to get motorcycle wiring to hang together longer than 6 months.

I suggest that you start by opening the harness splice, which probably means unwrapping 10 miles of electrical tape. The tape was originally intended to keep water out, whereas every engineer in this audience knows it only keeps water in !! What you discover there will tell a story.

This suggestion is being made becasue harness replacement is easy... once the headlamp is open and the fuel tank is removed. If that statement is true, then how did your bike end up with a harness splice ? I highly suspect there was significant front end damage a long time ago, so a decision was made to put a complete front end on the bike. The scrap yard "gofer" unbolted the head post, but simply cut the harness. The shop mechanic, seeing the dandling harness on the replacement front end, decided to do the same and connect the 2 pieces of harness together. So he picked speed over quality, and I think you'll discover just how speedy he was when you open that splice.

Photos would be good for a laugh. (Well, at least at this end ! 👿 )

In response to your post from this lats Sunday:
I am working from a Clymer manual schematic for a 1974 R90/6, I hope the wire color code matches what you have on your bike.
You see to be getting a lot of comments, but not a lot of specific help. Lets try to find the problem.

1). 2). These are OK.
3). Press the start switch and see if you have power at the blue/yellow terminal. This tests the start switch and wiring.
4). You should have power on the brown/black terminal. It should be driven to 12v by the neutral lamp. I think we are
beginning to identify the problem.
5). There should be two brown/black wires at the terminal board, one goes to the neutral lamp and one goes to the
neutral switch. They may be crimped together into one wire terminal.
6). This is good. This says the starter wiring is good, as demonstrated by 9) and 10).
7). This is good.
11). I don't understand this one. That should spin the starter. But 9) and 10) show the starter wiring is OK. It may be a
wire color issue only.
12). Is this a black/brown wire? It should measure as grounded, 0 ohms when the gearbox is in neutral by the brown wire
on the neutral switch.
continued on the next post

13). Is this the brown wire from step 12)?

All of this points to the neutral switch and/or its wiring. As a quick test, pull in the clutch lever and hit the start switch. If the starter engages, you have narrowed the problem to the neutral switch or its wiring. Check the resistance across the switch terminals, there should be 0 ohms in neutral and open in gear. If not, the neutral switch is bad or the wrong switch for that transmission. The brown wire (disconnected from the switch) should read 0 ohms to ground, and the brown/black wire, disconnected from the switch, should read 12 volts. Connect the brown/black wire to ground and the neutral lamp should light, hit the start switch and the starter should engage.

It's a bummer only having Sundays to work on this problem. That means you can only worry about it the rest of the week. But rest assured we can find and fix this problem. Keep us posted.

Andy

Been away for a bit. Here are some pic's. I have ordered a new wiring harness and I have a new clutch switch for the clutch lever. I think it's best to replace the harness and I can afford to do so. The wiring insulation will only get more and more brittle so this seems to make sense.

The 1974 is a transition year and I find out that the wiring harness changed, possibly with the circuit board, in September of 1974. I have access to either but the later one is $300 less expensive than the pre-September.

As Woobly stated this bike may have been in an accident with a new front end. I have my fingers crossed for the health of the frame. So this brings me to some new questions.

1) How can I differentiate the early circuit board from the later board?
2) Has anyone matched the later harness to a pre September 1974?

When the harness arrives I will have the opportunity to return it but I will need to identify which harness will match the front of the bike (circuit board and instrument cluster)

Thanks for the help.