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Discussion Starter #1
Hi everyone!

I'm new to this forum and my friend and I are in the process of making an old 1978 Suzuki GS750 run again. The previous owner before us tried to convert this bike into a cafe racer, however he left it unfinished. We were able to fix some of the stuffs like building a new seat, repainting the old exhaust, etc, but we haven't been able to fix the wiring yet.

My questions are:
1. Does anybody have a simplified wiring diagram for the a) ignition and b) lighting (both of them separate) for the 1978 GS750? I have tried to simplified the wiring diagram from the original wiring diagram that comes from the service manual and compared it to other GS750's wiring diagrams I found on the web. However, I found some things that are different with each other. For example, the original wiring diagram from the service manual (attached on this post/IMG__3066.JPG) doesn't have a fuse box (only a main fuse) while the diagrams I found on the web (attached on this post/750tz_wiring.jpg) have a fuse box. In addition, the original wiring diagram from the service manual doesn't have an ignitor/CDI on it while the diagram I found on the web does - I assume that the 1978 GS750 doesn't have one because the spark plugs firing are controlled by the contact breaker?
2. Speaking of fuse box, I read from a post in this forum that there are 5 fuses (including the main). Correct me if I'm wrong: fuse 1-lights, (10A), fuse 2-lights (10A), fuse 3-engine (10A), fuse 4-main (15A), fuse 5-direct from battery for extras (10A). Can anybody explain further which fuses I need for a) ignition and b) lighting? Like where each fuse connects and its position?
3. Is there a way to test the ignition wiring by just sparking the plugs without running the engine (without bolting the spark plugs into the engine)? We want to test if all the wiring works in order for the spark plug to spark in the engine. Do we need to run the electric motor in order to do this? If so, does it mean the electric motor tries to turn the shaft and piston (because if it does, then we need to lubricate it since it has old oil).
4. Is there a way to lubricate the whole engine with oil without starting the engine? We don't know when was the last time since the engine was running, so we want to make sure it is lubricated thoroughly with oil specific to the bike.

I'm sorry if those are a lot of questions to answer...I have been involved in a solar car team and PC boards before but not motorcycles. We're eager to learn something new everyday. Me and my friend's goal are just to start this bike running with bare bones wiring and lighting. Like we don't need tachometers (but need a speedometer), etc. Thank you all for your help! We look forward to hearing your advice... :)

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You are on the right track, you need fuses only on the things you are worried about frying, you have a charging circuit, an ignition circuit, starter circuit that usually needs a relay, and only fuses for the accessories that you intend to accommodate. Use the original only for a rough guide and try to make heads or tails out of what the previous mangler left you.
 

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Discussion Starter #3
You are on the right track, you need fuses only on the things you are worried about frying, you have a charging circuit, an ignition circuit, starter circuit that usually needs a relay, and only fuses for the accessories that you intend to accommodate. Use the original only for a rough guide and try to make heads or tails out of what the previous mangler left you.
Dear TrialsRider,

Thank you for the reply. We've been in much better progress than a few days ago. Labelled the wires with masking tape and sharpie so that we can know which cables go to which (because the previous owner confused us by using different colored wires). Drew a simplified wiring diagram for the lighting part. Been able to buy the necessary wiring connectors, fuse holder and fuses but haven't put in the bike yet. Also ordered new spark plugs (non resistance ones) that are the recommended one because when we tested the spark plug caps with a multimeter, three of them read OL under resistance test and one of them is over 5k ohms (9k ohms). Just want to make sure that the spark plug caps for GS750 are 5k ohms right?

Also, we are planning to install a GPS speedometer - are there any recommendations on a good analogue display one? How does the wiring work on speedometer?

Thank you for your help! Will update more pics as we make progress...

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Fuses are wrong you have a 10 amp fuse on the kill switch and nothing on the starter motor. Would think one 10 amp fuse will do all of your accessories. GPS speedometer isn't legal up here because it doesn't give you an odometer, you might want to check that where you live.

I would replace all the plug caps and wires you can, on a competition bike they replace the caps very frequently.
 

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Discussion Starter #5
Just want to clarify - so you are saying that there should be a 10 amp fuse between the starter motor and starter motor from the starter relay? Should there be a 10 amp fuse between the kill switch and the ignition switch?

Thank you :)
 

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Just want to clarify - so you are saying that there should be a 10 amp fuse between the starter motor and starter motor from the starter relay? Should there be a 10 amp fuse between the kill switch and the ignition switch?

Thank you :)
Well the top diagram only shows one fuse and that is on the DC positive output from the rectifier,
the second one has a fuse block with fuses for apparently all sorts of things that are not identified and we have no idea what amperage rating any of the fuses are, there must be another part to that schematic or section in the manual.
Starter will be your highest power draw, headlight and horn next. Once you fuse the output from the rectifier the only way you could fry anything down line is to attach an external power source, like jump start from something that is putting out way too much voltage, or experience a downline live short, that is the purpose of the additional fuses, to protect those devices from drawing too much power.

... starter motor is pretty safe because it draws from the battery and that physically can not output too much voltage, that's why they don't have a fuse on the big fat cable.
 

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Discussion Starter #7
Hmm...I figured that there are five fuses on the original bike based on the GS forum. These are the list:

Number 4 is the main fuse..
Power from battery to fuse 4 then up to ignition,then back to
fuse 1-2-3.

fuse 1-lights ++ (10A)
fuse 2-lights ++ (10A)
fuse 3-engine ++ (15A)
fuse 4-main (10A)
fuse 5-direct from battery for extras. (10A)

So where do you think I should put the fuse? Should fuse 4 (main 10A) be put before or after the rectifier/regulator? Should there be a fuse in between the battery + output to the starter relay?

Thanks!
 

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The fuse goes after the rectifier which may or may not contain a regulator circuit. Your alternator is a 3-phase alternator, it puts out a nominal oh say max 17 volts AC each on 3 separate wires, some or all of those wires go to the rectifier and voltage regulator where the output is filtered (by diodes) to output the appropriate 12.x volts the battery wants, the purpose of the main fuse is to prevent a problem between the charging circuit and the battery circuit. That could blow up your battery in theory, batteries tend to do that when you over-charge them.

So far it looks like that's all you have on the bike, everything else now can run off your battery and as with most things a fuse is put in there to limit damage to anything you care about, or prevent a short in the appliance from drawing all kinds of power from your battery all at once. That could in theory cause your device to blow up, like cell phones tend to do on occasion.

Here is the logic for calculating fuse requirements; say I want to fuse protect the expensive running lights, a conventional lightbulb is just like a fuse, it will burn out unless I put a fuse in line with that circuit that will over-heat and burn out (or throw a circuit breaker) before my lamp gets hot enough to burn out. Obviously I do not want to over-fuse something like this or my lamp will burn out before the fuse. We know the volts, we know the current dry of the appliance, now I can calculate a maximum amperage draw, which is the rating for the fuse.
 

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Discussion Starter #9
So to recap, the positive terminal from the battery splits to the starter relay, rectifier/regulator and ignition switch. I'm still kind of confused though; do I put the main fuse between the battery and rectifier/regulator, or do I put the main fuse between the rectifier/regulator and the ignition switch? Like in this picture below? Do I put a fuse between the battery and the starter relay since it draws out a large amount of current and voltage?

I'm sorry for the noob question, but is there a formula to calculate the rating of the fuse? What is the current dry?

Thank you!

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... looking at both schematics, 2 of your 3 coils are going to the battery charging circuit, one coil is dedicated to ignition.
Much of the motivation for change between the 70's and 80's electrics is in support of nanny features, old bikes didn't have shite like kick stand cutout switches we had a hunk of rubber on the end of the kickstand, if you hit the starter button with the bike in gear you crashed and then you didn't do that any more etc.
 

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Discussion Starter #11
Hahaha I didn't knew about that! So the main fuse still has to be wired in between the ignition switch and the rectifier/regulator or in between rectifier/regulator and the battery? Should I put a fuse between the battery and the starter relay?
 

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" Do I put a fuse between the battery and the starter relay since it draws out a large amount of current and voltage?"
no I covered that :I said they do Not put a fuse on the big fat cable going to the starter motor because it can only draw what the battery can output.

The fuse protects the battery from over-charge power if the alternator circuit goes crazy and suddenly starts outputting like 15 volts, which will blow up your battery :| Boom!
 

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Relay is just a remote controlled switch, so no, no fuse is required, it's a cheap periodically replaced item. It's 2 electrical contacts that get connected by a tiny electro magnet coil.
 

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Amp draw = total number of watts by the system's volts.
Example, 120-watt bulb in a 12-volt system will draw 10 amps.
Watts divided by volts = amps
(120w / 12v = 10a)
 

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Discussion Starter #16
We just got back last night from doing more stuffs to the bike. Realized that our lead acid battery is at 6v instead of 12v - I think we will get new batteries even though we can try to revive the old lead acid battery because we want a battery small enough to fit inside the compartment/box we made that is attached to the frame of the bike. We are looking at lithium batteries and wondering if there is anything we need to look for at lithium batteries other than the voltage rating (12v), amp hour rating and cold cranking amps (CCA)?

Also, can anyone explain how this fuse box work? I know it comes from one end but where does the other end exit?

Thank you!
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imho you would be crazy to put a lithium battery in that old bike, I don't even run lithium in my MV Agusta F3.
The battery shop that sells you a nice new cheap lead acid battery will want your old battery in exchange for a few bucks.

Fuse block is just like the power panel on your house, a bar runs down the middle and the fuses or circuit breaker all run off that, one in, many out.
 

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Discussion Starter #20
Would it be because the rectifier/regulator? The owner before us replaced the old regulator/rectifier with a new one. I'm wondering how much AC does the stator/alternator outputs for 1978 GS750?
 
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