VFR charging issues only when hot

[VFRscrub]

Hey all, I have a 1993 VFR750. Had it for a month. Sorry for being a bit short but I don’t have much time to type. Already replaced stator with ricks unit, brand new battery, and new starter relay. Has an upgraded ricks rectifier in it, looks beefy. Owner says he installed a complete VFRness 4 years ago. Found bike dead a couple weeks ago and started it with shop battery charger, 350a 12v. Put the negative clamp on frame ground but voltage gauge on bike was still smacking the needle past 16v. Won’t charge when hot now.  Here’s what I’ve tested…

 

-1.3a at positive charge wire right at battery when cold. 0.8a when hot. When very hot, rad fans kicking in at idle, rev rpm slightly, climbs to 1.3a then drops back down to 0.8 and holds regardless of rpm. Voltage regulator could cook an egg at this point

-0.06v total voltage drop between rectifier output + and - terminals and ring stud terminals on battery when hot. Already replaced starter relay.

-9a AC at idle on one stator output cable next to regulator. Climbs very healthily when revving when hot.

-Aftermarket voltage gauge on bike shows major voltage drop, .5v when either brake is pressed, then seems to not charge nearly as well after. Brake light relay? Unsure.

 

I think I need to replace the rectifier. Planning to route the rectifier output wires directly to the battery as well. Thinking of getting a Polaris one, seen some people mention that being a good option. Please advise on any ideas, parts numbers, etc. kind of at the end of my rope here.

 

Thanks for taking the time to read!

[Terry]

There's a dandy fault finding guide on the Electrosport website that you should follow.

 

https://www.electrosport.com/pages/technical-resources?srsltid=AfmBOoqEM4-OgXUCxUIr6cWnVp6IJnRzyzFN3FoUqlm8EWLZPnixdWfD

 

Sounds like you already have a multimeter so have at it. I'd start at the AC wires first as if they aren't kicking out the voltage, nothing else works. 

[VFRscrub]

1 hour ago, Terry said:

There's a dandy fault finding guide on the Electrosport website that you should follow.

 

https://www.electrosport.com/pages/technical-resources?srsltid=AfmBOoqEM4-OgXUCxUIr6cWnVp6IJnRzyzFN3FoUqlm8EWLZPnixdWfD

 

Sounds like you already have a multimeter so have at it. I'd start at the AC wires first as if they aren't kicking out the voltage, nothing else works. 

Thanks! I’ll look into it. I’m a transit bus tech and capable of doing tests but I lack experience with these bikes. So I’ve basically been firing the parts cannon due to my limited time. 
 

I get 15V AC at idle and 70V AC at 5k rpm. Sounds right within spec. I meant that I got 9 amps at idle when hot, which I’m guessing translates to 3 amps DC for one stator output wire since it’s 3 phase. I’m not sure though 

[mello dude]

The goto for new R/Rs

www.roadstercycle.com

Also if your short on cash a CBR500 uses a Fh020aa if you can find a low mile used one. 

 

The current art on wiring R/Rs

 

If you do any of your own build a harness work.... marine quality wire, avoid the usuall auto parts stuff.. 

https://www.delcity.net/store/wire-cable/ancor-marine-grade-tinned-primary-wire/

 

Recommended fuse holders.. 

https://www.delcity.net/store/fuses-fuse-accessories/holders-circuit-taps/atc-ato-blade/inline-water-tight-black-with-red-leads/78308/

https://www.cycleterminal.com/fuse-boxes.html    MetriPack 630, 40amp

 

Furakawa connectors

https://www.cycleterminal.com/furukawa-qlw-250.html

 

Good luck

[VFRscrub]

19 minutes ago, mello dude said:

The goto for new R/Rs

www.roadstercycle.com

Also if your short on cash a CBR500 uses a Fh020aa if you can find a low mile used one. 

 

The current art on wiring R/Rs

 

If you do any of your own build a harness work.... marine quality wire, avoid the usuall auto parts stuff.. 

https://www.delcity.net/store/wire-cable/ancor-marine-grade-tinned-primary-wire/

 

Recommended fuse holders.. 

https://www.delcity.net/store/fuses-fuse-accessories/holders-circuit-taps/atc-ato-blade/inline-water-tight-black-with-red-leads/78308/

https://www.cycleterminal.com/fuse-boxes.html    MetriPack 630, 40amp

 

Furakawa connectors

https://www.cycleterminal.com/furukawa-qlw-250.html

 

Good luck

Thank you! I’ll follow that as close as I can. Just went through the whole fault tree that terry posted. I’ve already completed all those tests, except for testing the RR diodes while hot. Only tested them cold. As of now the entire assembly is based on wedge connectors, which could also expand when hot. So I’ll replace them and go down a gauge on the stator wires to be safe.
 

I’ll let you know what happens

[V4 Rosso]

Whatever you do, also consider relocating the RR to a cooler spot. I did some measurements and the frame by itself, where the RR is mounted, gets very hot because of the headers being very nearby. I' ve seen subframe temperatures well over 70C/160F which makes it very hard for the RR to dissipate heat.

[VFRscrub]

3 hours ago, V4 Rosso said:

Whatever you do, also consider relocating the RR to a cooler spot. I did some measurements and the frame by itself, where the RR is mounted, gets very hot because of the headers being very nearby. I' ve seen subframe temperatures well over 70C/160F which makes it very hard for the RR to dissipate heat.

Thanks, yeah the previous owner relocated it behind the rear master cylinder

[VFRscrub]

15 hours ago, mello dude said:

The goto for new R/Rs

www.roadstercycle.com

Also if your short on cash a CBR500 uses a Fh020aa if you can find a low mile used one. 

 

The current art on wiring R/Rs

 

If you do any of your own build a harness work.... marine quality wire, avoid the usuall auto parts stuff.. 

https://www.delcity.net/store/wire-cable/ancor-marine-grade-tinned-primary-wire/

 

Recommended fuse holders.. 

https://www.delcity.net/store/fuses-fuse-accessories/holders-circuit-taps/atc-ato-blade/inline-water-tight-black-with-red-leads/78308/

https://www.cycleterminal.com/fuse-boxes.html    MetriPack 630, 40amp

 

Furakawa connectors

https://www.cycleterminal.com/furukawa-qlw-250.html

 

Good luck

I'm planning on buying this one and getting some marine wire like you mentioned. I live in Canada and the USD conversion rate and shipping costs would be pretty high from roadstercycle. Do you think this one is good enough compared to the shindengen? Seems like a decent clone to me

 

Kimpex HD Mosfet Voltage Regulator Rectifier - 225843

[mello dude]

I am very reluctant to recommend any alternative to a Shindengen R/R. 

As demontrated by a lot of years hanging out on this forum, most alternatives die an early death. 

Maybe the best alternative is an OEM CBR500 you could get a Canadian dealer or a used on on Ebay

Get a FH020AA or some newer Yamahas have that too, I think a Yam FJR1300 is using that one too. 

 

Or, got a buddy in Buffalo?

 

Best wishes

[VFRscrub]

On 9/18/2025 at 8:28 PM, mello dude said:

The goto for new R/Rs

www.roadstercycle.com

Also if your short on cash a CBR500 uses a Fh020aa if you can find a low mile used one. 

 

The current art on wiring R/Rs

 

If you do any of your own build a harness work.... marine quality wire, avoid the usuall auto parts stuff.. 

https://www.delcity.net/store/wire-cable/ancor-marine-grade-tinned-primary-wire/

 

Recommended fuse holders.. 

https://www.delcity.net/store/fuses-fuse-accessories/holders-circuit-taps/atc-ato-blade/inline-water-tight-black-with-red-leads/78308/

https://www.cycleterminal.com/fuse-boxes.html    MetriPack 630, 40amp

 

Furakawa connectors

https://www.cycleterminal.com/furukawa-qlw-250.html

 

Good luck

Just so I’m getting this right… do you want me to take the red ig

 

On 9/18/2025 at 8:28 PM, mello dude said:

The goto for new R/Rs

www.roadstercycle.com

Also if your short on cash a CBR500 uses a Fh020aa if you can find a low mile used one. 

 

The current art on wiring R/Rs

 

If you do any of your own build a harness work.... marine quality wire, avoid the usuall auto parts stuff.. 

https://www.delcity.net/store/wire-cable/ancor-marine-grade-tinned-primary-wire/

 

Recommended fuse holders.. 

https://www.delcity.net/store/fuses-fuse-accessories/holders-circuit-taps/atc-ato-blade/inline-water-tight-black-with-red-leads/78308/

https://www.cycleterminal.com/fuse-boxes.html    MetriPack 630, 40amp

 

Furakawa connectors

https://www.cycleterminal.com/furukawa-qlw-250.html

 

Good luck

Ok I’m about to finish up the wiring, put in a OEM just one last hiccup. As I mentioned before the previous owner completely changed the harness. I believe all my issues are coming from a highly resistive charging cable to battery, causing the regulator to overheat as well as the cable, eventually sending a very limited current(due to heat) from the old diode style regulator through my starter relay, and trying to find the path of least resistance, which seems to be through my brake light circuit. 
 

The previous owner had spliced the regulator positive output wire to connect to both the starter relay and the battery ring terminal. It was a good soldered splice but with 16ga wire and sharing lots of current. The insulation near the wedge pin at the regulator was slightly melted. Hope I’m making sense so far.

Now I’m trying to match the wiring to the suggested schematic you attached. I’ll attach a photo of the back of the relay plug. The wire that was spliced to the B+ ring terminal is on the top right. He seems to have labelled the one on the top right R/W and top left R.

 

My question is this… am I OK to attach the top right wire with a ring terminal to B+? Does it double as a starter cut out signal wire and should I just delete it from the harness? I’m thinking no since it was spliced to B+ but I want to be sure. Both battery charging cables are separate and accounted for, 10ga with a 30a inline fuse. Please let me know as soon as you can. Thank you!!

[rc24dk]

Are you sure you dont have a short circuit in the brake light circuit ?

It also sounds like you used a 24V truck battery charger. Something labeled 350A shouldn't be connected to a motorcycle battery.

[mello dude]

@VFRscrub  ----- I cant really figure waht you got going on there. I am to the point of gotta be there..

That said, someone may chime in on this, but me, I dont wanna mix the starter relay harness with the R/R harness if I got that right of what you are doing. 

Also, any harness work, I prefer using 12 ga at a minimum.... 

 

Maybe someone in the peanut gallery can chime in to help.... 

[VFRscrub]

3 hours ago, mello dude said:

@VFRscrub  ----- I cant really figure waht you got going on there. I am to the point of gotta be there..

That said, someone may chime in on this, but me, I dont wanna mix the starter relay harness with the R/R harness if I got that right of what you are doing. 

Also, any harness work, I prefer using 12 ga at a minimum.... 

 

Maybe someone in the peanut gallery can chime in to help.... 

For sure. Thanks a million for the help so far. I definitely couldn’t have figured it out without your help. The voltage dropping with the brake light was a red herring. The volt gauge was plugged in to read voltage on the green return path. So any load caused that branch of the circuit to drop voltage. Verified this through tests. With the new CBR500 regulator and 10ga charging wires it’s handling loads when hot at idle. Like a champ, not dropping below 13.6v with rad fans kicking in. I ended up hooking the R/W wire from starter relay directly to B+. Did a lot of reading on the bike circuit diagram. From factory the R/W wire comes from RR positive output. Seems to work fine after a spirited ride, if this will make the bike spontaneously combust at some point please let me know haha.  But as I mentioned before, the previous owner had the RR positive output wire spliced to hook up to the R/W terminal at starter relay, and act as the charging cable at B+. Which is a LOT to be asking of a 16ga wire, basically charge at upwards of 25a if needed and send power to the fuse box for any loads.

 

In short, redoing the wiring with 10ga wire and the FH020AA fixed the issue.

[VFRscrub]

4 hours ago, rc24dk said:

Are you sure you dont have a short circuit in the brake light circuit ?

It also sounds like you used a 24V truck battery charger. Something labeled 350A shouldn't be connected to a motorcycle battery.

You’re right, it was a fleet charger, and I 100% agree in hindsight. I work afternoon shift and it was 1:00am so I really wanted to get home. I figured if I hook the charger negative cable to frame ground, the starter would only draw what it needed. But the aftermarket voltmeter on the bike that is wired to the harness, not the actual battery, was smacking the max voltage needle. So clearly something bad happened, I’m no scientist though. The lights were very bright and the bike immediately fired into high idle. I disconnected it after 5 seconds of it running. Seems to have cooked the regulator anyways though. It tested fine when cold but probably couldn’t handle much heat after what I did.

[JZH]

It's not clear to me what voltage the bike was charging at (at 5,000rpm, per the manual), but if it was a high voltage (anything over 16v), the only possible cause would have been a bad reg/rec--the one part that wasn't already replaced.  The reg/rec may have been already damaged before you zapped it--that's why your battery was dead in the first place.

 

In any case, a new MOSFET reg/rec should fix the issue (assuming the wiring is not damaged).

 

Ciao,

 

JZH

[mello dude]

12 hours ago, VFRscrub said:

For sure. Thanks a million for the help so far. I definitely couldn’t have figured it out without your help. The voltage dropping with the brake light was a red herring. The volt gauge was plugged in to read voltage on the green return path. So any load caused that branch of the circuit to drop voltage. Verified this through tests. With the new CBR500 regulator and 10ga charging wires it’s handling loads when hot at idle. Like a champ, not dropping below 13.6v with rad fans kicking in. I ended up hooking the R/W wire from starter relay directly to B+. Did a lot of reading on the bike circuit diagram. From factory the R/W wire comes from RR positive output. Seems to work fine after a spirited ride, if this will make the bike spontaneously combust at some point please let me know haha.  But as I mentioned before, the previous owner had the RR positive output wire spliced to hook up to the R/W terminal at starter relay, and act as the charging cable at B+. Which is a LOT to be asking of a 16ga wire, basically charge at upwards of 25a if needed and send power to the fuse box for any loads.

 

In short, redoing the wiring with 10ga wire and the FH020AA fixed the issue.

Glad you got it figured out. 

I would keep an eye on it anyway.....