78 gs400 fuse blowing while riding

[angrypants]

Just had to include my earlier post

Sounds a lot like the second issue I had where a faulty battery caused more than 15 amps charging voltage and was blowing the main fuse.

I notice you mentioned a low battery in a previous post. How old is the battery? Is it a lead acid type? Have you kept the acid levels topped up? That was my issue, the levels kept dropping due to where the exhaust runs on my 450 which I wasn't aware of at the time (battery since relocated). Funnily enough the battery was still capable of cranking the bike etc.

Just mentioning the above again in case you can't find an issue with the R/R and end up still scratching your head. The only way I could verify it was the battery was fortunately having a spare on hand as I had a brand new one ready for my Katana.

hi pete,
battery is new AGM , well it's 16 months old, used infrequently i.e starting weekly, and when wrenching and a few rides (not been on a ride of more than an hour or two at a time, but it's alway kicked first time, -- lol always after the carbs were dipped and the timing set correctly ). 2nd time it went flat was during testing recently, ( first time was a year ago when wrenching testing and not riding at all).
i have a fancy charger they is supposed to tell you when a battery needs recovery/ reconditioning charging etc and it all comes up good there if that means anything. i do have two batteries one slightly newer and will swap and see but with the rectifier having a diode ?? leg test to output fail, im hoping i might have found the issue.. i will hook up the sh775aa tonight and see how we go there ... i should also chase the red from the rec to the fuse to make sure it hasn't burned and created a short... its only 6 inches of harness to unwrap so easy.

[Chuck78]

Replace/upgrade the very-well-kniwn-to-be-garbage 40+ year old antiquated rectifier & seperate regulator, & find the space/rearrange brackets/make a custom bracket to mount the SH-775 regulator-rectifier...
Re-wire the fuse locations slightly with some new auto parts store water-tight inline fuse holders. I think this may likely solve your issue.

Running those original 1970's technology charging system components is never a good thing even if the bike is not blowing fuses, thats ALWAYS one of the first essential upgrades towards making a healthy & reliable Suzuki GS...

Add an ignition relay circuit as well, it will improve the spark quality drastically. Make sure the points/condensers are in great shape or upgrade to a Dyna-S for the GS twins. I believe some GS500 late model ignition coils may be the correct ohms/resistance to work with the Dyna-S, and can be had much cheaper than the Dyna single fire coils, and have replaceable wires vs the stockers which have the plug wires permanently soldered and glued to the coils.

Best luck, keep us up to date on how things are going!

[angrypants]

Update:
got the bike running hooray! -- Replacing the old rec and old regulator with a combined unit worked, i did not find any other burnt / melted wires, all connections and grounds now cleaned.

i haven't worked out where and how to fit the sh775 though, it's huge and the gs400 doesn't seem to have enough room under or behind the battery box, i'll have to pull things apart to check properly or i'll need to rearrange things at the usual area or put in a plate somewhere, until i work that out, i have installed a SH640D-12 in there (i know its a shunt, but it looks rather new, it is off the gs1000 i have in the garage).

i need fully charge the battery and do the tests, as is its charging at at 13.7 at 2500 rpm and 13.9 at 5000,the battery has started the bike some 30 times and not run for more than a minute so i'll charge that up and do a full set of tests, ( the stator already tested fine unloaded)

the step #2 and 3 of charging stet said no voltage drop at all ....if i did it correctly QUESTION (sorry to sound stupid) when doing this test Step #2 - phase one test charging ( i.e connect the black multimeter lead to the battery (+). Connect the red multimeter lead to the RED output wire of the RR (+). Leave the RR connected to the bike. INCREASE ENGINE SPEED To 5000 RPM Check the reading on the meter)

Q: When it says leave RR connect to the bike...am i s leaving the output connected to the harness and just jamming the lead in at the connection to get contact and a reading ?

thanks for all your help, i'll post the results for all tests in the next few days with this SH640D-12, and once i work out where the SH775 goes i'll report the results there too.

thanks again for all your help, i now feel i understand a little more about electrics and will try and tackle the SPG and other suggested elec mods


cheers
grant

[Guest]

Hey Grant, sounds like success, glad to hear it!

With your question, if you're trying to measure if there's any voltage drop between the battery and the R/R while charging then yeah that sounds right to me.

[Chuck78]

My buddy is coming over in a few for some 550 fork parts for his GS450S. I'll see if I can ask him to take a photo of where he fit an SH775 on his GS450, & post it on here.

Glad to hear your electrical system woes have been solved, & also glad that you were forced to go through the entire system very thoroughly.... All bikes could use that every 5 years or so. I just noticed on a motocamping trip last weekend at 10:30pm unpacking my bike & setting up my tent and watching a spider crawl behind my headlight bucket while weari g my headlamp, that my ignition switch connector, which is sitting outside of the heaight bucket because I used an L headlight mounted low, that the wires are all green and oxidized! Luckily I have an ignition relay setup to give full power tothe crucial co.ponents. My headlight voltage has probably been suffering though, without a direct relay switched battery power source... That will be a mod that I do over the winter on it, so I can repurpose this L headlight bucket onto my GS550 or GS425.

[Chuck78]

David said he had to cut off a few tabs (from the airbox or battery box), & then he had a perfect place to mount an SH775 on his GS450E. I think he said one hole may have been shared with the starter relay, or perhaps he had to relocate the starter relay?
Sounds like basically the same thing I have to do to fit them onto a GS550, which I have done three times over now on different bikes. Cut off a few mounting tabs for accessories on the side of the metal airbox, then use one of the L-shaped scrapped tabs with a welded nut on it to bolt through the air box to hang off the back side and give a second mounting point for the SH775 on a 550's remote located airbox.


I'll see if I can get David to find some time to take a photo and post.

[angrypants]

cheers Pete !

thanks for the info chuck , ill get the grinder out and do some cutting before i have to give my mates welder back ;-)

[angrypants]

Sorry forgot to post the results from testing with a the full battery with the SH640D R/R

numbers seem to be OK based on the test, maybe a little low charging at 5000rpm, and over charging at 1500 rpm?/ is that a serious concern . Ive been for a few rides and no issues ( as yet knock on wood). Battery is staying charged.

stator / charging system tests record
bike	78 gs400
date	8th sept/2019
R/R type	SH640D
BATTERY (meter to bat terminals red to red and blk to blk) DC volts	READINGS			GOOD	BAD
key off V	13			12.7-12.9
key on V	12.5
key on + lights	12.2			12.2-12.5
idle (1500rpm)	13.7			12.6-13.2
2500 rpm V	14.0			13.5+ - 14v
5000 rpm V	14.0			14-14.8v	low or drops
Off (again AFTER TEST)	13.2			higher than 1st 12.8-13v	less that test 1 key off
RR to BATTERY	drop	reading
volt drop pos V 5000rpm blk to bat +, red to RR output+	0	14.0		<0.25	>0.25
volt drop neg V 5000rpm red to bat +, blk to RR neg / grnd	0	14.0		<0.25	<0.25
STATOR	leg 1	2	3
ohms between each leg				0.5 - 2 Ohms	<0.5 or > 2.0
leg to engine case ohms	OL	OL	OL	zero / OL	0 ohms +
output 5000rpm leg to leg (not connected to RR) Vac	78	78	78	60Vac+ & similar	<60 not equal
5000rpm leg to battery NEG -	OL	OL	OL
R&R	leg 1	2	3
diode test disconnect RR, red lead to red out RR black to input leg	0.492	0.491	0.494	1.5V +	<1.00v