Ignition Warning Light Unit

I'm in the process of getting my 1975 MK III back after a 20 year hiatus. I tried pulling the air-box out to clean it out, had no luck but caused another problem of sorts. I now have a loose electrical component that I am unsure where to place and secure in the under seat area. It looks like the capacitor and it is enclosed in the spring like holder with the ring tab for securing somewhere. A check of the wiring diagram reveals it to be the Ignition Warning Light Unit of a different kind. There is no square type unit fitted to the frame up near the starter solenoid. There is not much info in the MK III supplement of my Haynes Workshop Manual. There is not a great deal of give in the wiring loom and I'm buggered if I know where it should sit. Any help would be appreciated.

Mobi Andy said:

now have a loose electrical component that I am unsure where to place and secure in the under seat area. It looks like the capacitor and it is enclosed in the spring like holder with the ring tab for securing somewhere. A check of the wiring diagram reveals it to be the Ignition Warning Light Unit of a different kind

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Is it blue? If it is, then it's the 2MC emergency starting capacitor, this is not the 3AW warning light 'assimilator'. The 2MC would normally have two brown/blue wires and one red connected to it.

On the MkIII, the 2MC spring bolts to the frame plate at the rear of the battery compartment, inboard of the starter solenoid.

Mobi Andy said:

A check of the wiring diagram reveals it to be the Ignition Warning Light Unit of a different kind

Click to expand...

Early MkIIIs did have the three-wire silver can 3AW assimilator, again suspended by a spring however, that unit would normally be positioned below the fuel tank.

Mobi Andy said:

There is not much info in the MK III supplement of my Haynes Workshop Manual.

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I would seriously suggest you use the MkIII factory manual in preference to the Haynes.
850 MkIII manual:
http://britmoto.com/manuals/Manuals/850_man.pdf

Hi, welcome, you've come to the right place.

Mobi,

MKIII 's had two types of warning light assimilators. The early ones had a can with three wires attached to it. That one is located under the tank. The later ones had a box with (I believe ) 5 wires attached to it, and is located under the seat/battery area. I think you are looking at the starting capacitor. It should have only two connection points. That was attached w/ a bolt to the plate near the rear fender near the battery.
Best bet is to get factory repair and parts books for accuracy. The manual is available online as a PDF. Old Britts parts listing is a copy of factory parts book. Be aware you will not find the can type assimilators in the parts breakdown. But I believe there is a mention of it in the repair manual. Also you might find the wiring color codes not quite right on the earlier bikes. Good luck
Pete

L.A.B.

Early MkIIIs did have the three-wire silver can 3AW assimilator, again suspended by a spring however, that unit would normally be positioned below the fuel tank.

Didn't see your post before I replied. I think we can make that three different locations now. In addition to the solid state module under the seat and your spring mounted one, there is mine. It is mounted under the tank, almost above the coils and has a tab on the can housing that gets bolted to the frame.

Pete

Deets55 said:

L.A.B.

Early MkIIIs did have the three-wire silver can 3AW assimilator, again suspended by a spring however, that unit would normally be positioned below the fuel tank.

Didn't see your post before I replied. I think we can make that three different locations now. In addition to the solid state module under the seat and your spring mounted one, there is mine. It is mounted under the tank, almost above the coils and has a tab on the can housing that gets bolted to the frame.

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Possibly even four locations? In section J of the MkIII manual, it states: "On early electric starter Mk III models, remove the Lucas type 3AW part number 38717 ignition warning light simulator [assimilator] complete with spring from the rear frame gusset plate."

L.A.B.

Yes, that must have been the notation I recalled reading about. I disassembled mine to see what makes it tick. It appears to be some sort of wire that must heat up when A.C. current is applied to it, causing it to break open the WL to earth contact.

Deets55 said:

I disassembled mine to see what makes it tick. It appears to be some sort of wire that must heat up when A.C. current is applied to it, causing it to break open the WL to earth contact.

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http://www.nortonownersclub.org/support ... similators

L.A.B.
Interesting read. I not sure if it 100% accurate.

"When you examine the way this unit is used on the bike you see that the circuit, shown in Figure 1, is a little naughty, in that it takes power directly from the alternator to earth and in fact across one of the diodes in the rectifier".

I just looked at the wiring diagram for my bike. The assimilator is directly connected to the alternator and does not go through the diode on the rectifier. I removed mine because I did not like the system or the current draw.

"When we come to the Mk.lll with the high output single phase (18OW) alternator, the 3AW would have burned out,"
Mine lasted 35 years before I replaced it with a CoolCat soild state unit.
Plus the can type unit is the same one used on old British cars, Jaguar being one of them.

I don't know how you keep track of all this info, but I'm glad you do. Helped me out a bunch of time, as did a bunch of other guys on this forum.

Pete

It is definitely the 3AW Assimulator Can as the Capacitor (blue can) is there suspended in the spring on the forward side of the gusseting on the frame just behind where the battery is situated. There is not enough loom for it to fit in the position that is shown in L.A.B.'s post but it may make it up to the are above the oil tank and bolt onto the fwd mounting bolt of the oil tank. The weather is a bit wet so I will have a go at it later today. Thanks for the help.

Deets55 said:

"When you examine the way this unit is used on the bike you see that the circuit, shown in Figure 1, is a little naughty, in that it takes power directly from the alternator to earth and in fact across one of the diodes in the rectifier".

I just looked at the wiring diagram for my bike. The assimilator is directly connected to the alternator and does not go through the diode on the rectifier. I removed mine because I did not like the system or the current draw.

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I seem to think he (Al Osborn - AO Services) is correct, although perhaps he didn't explain it too well, but I believe from previous discussions that it's possible for current to pass across one of the rectifier diodes.
http://www.aoservices.co.uk/index.htm

Deets55 said:

"When we come to the Mk.lll with the high output single phase (18OW) alternator, the 3AW would have burned out,"
Mine lasted 35 years before I replaced it with a CoolCat soild state unit.
Plus the can type unit is the same one used on old British cars, Jaguar being one of them.

Click to expand...

According to the parts books, the Commando 3AW assimilator was type 38717 (the type with the spring) however, I believe the 'car' 3AW assimilator is 38706 and this type appears to have a tab. Whether that particular car item has a higher rating than the 38717, couldn't say;- but as far as I can recall, I think it's the fist time a MkIII owner has mentioned anything about a tabbed 3AW.

[Edit: It appears the original electro-mechanical 3AW units are suitable for both +ve and -ve earth/ground]

It may be getting a bit off track but I think the reason that they take the output from the alternator directly is to monitor the alternator output and not the rectifier output. If the bridge rectifier decides to have a holiday then the alternator can punch out as much as it wants, but nothing will come through. Years ago I had an old Datsun car and coming home from work late one night the Brake light came on. Nothing seemed amiss in the braking department so I proceeded on and as I got closer to home things started to get decidedly dark. The cunning little Japanese electrical designers had put in a standby monitoring circuit. The ignition light cct looked at the Alternator output, no alternator output and on comes the light but a brake cct light could also mean the diode pack had failed which is what happened in my case. It did it once again on another Datsun (Nissan) I owned but I was wise to the fact then.

L.A.B.,

My bike was 15 yrs old when I bought it. That is plenty of time for sombody('s) to change a lot of stuff, as evident in my gearbox. So that assimilator could has easily been replaced. The wiring definitely places it under the tank in that location and I don't believe there is enough room for a spring and can. If the trigger voltage is AC or a rectified DC guess I'll never know because when I upgraded to a Lucas 3 phase and Podtronic I rewired everything instead of using existing wires. I just pig tailed off of one of the alternator leads to the CoolCat unit.
Kerinorton simply replaced his assimilator with a relay and it works for him.
Wow alot of time spent on turning off a red light isn't it :?