757 Repower - Charging light

[KY Gun Geek]

Well, after the the 2nd messed up head on a 757 (seems to be a common problem) I decided to repower.

John Deere 757 Replacement Engines - Repower Specialists

Repower Specialists provided a Honda GX690 and the trimmings to do the conversion: decent instructions, mounting plate, throttle & choke cables (longer ones needed)

I'm still in the installation phase. Company says 6 - 8 hours, which is probably right for the 2nd kit (and if you can actually put that much time together uninterrupted).

So far my impression is that the kit is well done. I have run into only one issue - with the adapter plate between the new motor and the 757. The holes in the plate and the holes in the mower were off by about 1/16 of an inch. Plate holes were drilled and tapped so good alignment required to get bolts started. A little touch with a step drill on the 757 and all's well.

If you guys are interested I will do a little more write up/pics - or PM me.

There are some issues that they told me about that I'm working on: the seat mounting plate has to be trimmed a little, a shroud removed or trimmed, and the rear cover must be pretty substantially chopped to make room for the new muffler. Working on those tomorrow.

But there is an issue that has surprised me. The wiring/electronics on the honda don't seem to support proper operation of the charging light. Talked to the company's support, and they say you can hook it up so that it works the opposite of the way it works OEM (i.e. the light is on normally and goes out when there is a problem). Seems counter to the way most idiot lights work (they are on when there is a problem), and so may lead to confusion among operators (employees, wife). As well as a kinda short bulb life. Anybody have a perspective on this?

 

[Richard001]

Well that's interesting.

Did repower supply you with a schematic? If so, could you post it here.

Richard

 

[KY Gun Geek]

Well that's interesting.

Did repower supply you with a schematic? If so, could you post it here.

Richard

Repower supplied an owners manual and I did a little snipping - schematic below. They also supplied instructions that are bare bones and don't help - they are a distillation from this schematic. What is not in either diagram is a diode placed in the the wire coming from the rectifier/regulator going to the battery connection on the starter. This is the battery charging wire and the diode is set so that current can flow from the regulator to the battery (this is how it charges the battery). When the alternator isn't producing, the diode stops any discharge from happening back through the regulator.

I have a schematic from JD re the 757 wiring and it shows a yellow wire coming from the voltage regulator that passes through the harness connector and changes to brown as the lead that powers the light on the dash. This fits with my 757.



So my background (I mean what I went to school for, but haven't used much in 25 years) is electrical engineering. I went immediately into computers (I didn't invent the internet, but I was around as it was born and made some money from it :thumbsup. I was ok back then at electronics, not great, and that was long ago.

The best I can come up with is a voltage comparator to read across the diode. When diodes are conducting, they have some small voltage drop - like .5v. A comparator looks for a difference in voltage (say between battery voltage and the high side of the diode) and sends its output high. A comparator is typically an OPAMP and some resistors, so it is has a low current (milliamps, and the light will draw like 1/4 - 1/2 an amp) output, and so will require a drive transistor - we're talking 4 or 5 components on a little circuit board. Is there a simpler way?

Repower's solution is to tie the light in between the diode and the rectifier so that when the rectifier is outputting voltage, the light will be on. This will be anytime the engine is running assuming the alternator and rectifier are working.

 

[Richard001]

Repower supplied an owners manual and I did a little snipping - schematic below. They also supplied instructions that are bare bones and don't help - they are a distillation from this schematic. What is not in either diagram is a diode placed in the the wire coming from the rectifier/regulator going to the battery connection on the starter. This is the battery charging wire and the diode is set so that current can flow from the regulator to the battery (this is how it charges the battery). When the alternator isn't producing, the diode stops any discharge from happening back through the regulator.

I have a schematic from JD re the 757 wiring and it shows a yellow wire coming from the voltage regulator that passes through the harness connector and changes to brown as the lead that powers the light on the dash. This fits with my 757.

View attachment 461307



Repower's solution is to tie the light in between the diode and the rectifier so that when the rectifier is outputting voltage, the light will be on. This will be anytime the engine is running assuming the alternator and rectifier are working.

Geek,
Let me see if I am understanding this correctly. Are you saying that all the current that goes to charge the battery goes thru the dash light?

or is the dash light just tied from the regulator output to to ground?

If it is the second case, then the simplest way would just to use a relay to invert the signal. I have attached a drawing in jpg format to show what I mean.



In this configuration, the lamp will light when the engine is not running and thus there is no output from the regulator, once the engine starts, the output from the regulator will cause the relay to activate and turn off the lamp.

Richard

 

[KY Gun Geek]

Geek,
Let me see if I am understanding this correctly. Are you saying that all the current that goes to charge the battery goes thru the dash light?

or is the dash light just tied from the regulator output to to ground?
Richard

No, not saying the charging current goes through the dash light. I'm saying The OEM JD regulator/rectifier provided a specific output that would be sufficient to run the the lamp - (I dunno if was 12v or what - prolly should figure that out), and that the repower rectifier does not provide a similar output. The OEM would light the lamp when the the rectifier was not charging. Typically light was on when the engine was not turning, I never had a failure of the alternator to know if lamp would light if it failed (i.e. a winding problem). Since the logic was built into the rectifier/regulator, I'm guessing the light might not work if it failed.

Geek,
If it is the second case, then the simplest way would just to use a relay to invert the signal. I have attached a drawing in jpg format to show what I mean.

View attachment 461391

In this configuration, the lamp will light when the engine is not running and thus there is no output from the regulator, once the engine starts, the output from the regulator will cause the relay to activate and turn off the lamp.

Richard

I like the idea. There is already a diode in the circuit - same place as yours is, but it is turned the other way. Function is to keep current from flowing back into the rectifier from the battery when it is not supplying charge current.

Time for some exploration!

 

[Richard001]

Geek,

As you may have already figured out, the schematic above has a problem, it would allow the lamp to be on any time the engine is not running. Not good. Here is an improved version.



Just find a terminal on the switch that is hot when the engine is running. Connect the comm terminal to the hot when running terminal on the switch.

Richard

 

[KY Gun Geek]

Ya - I got that, but I like the idea - you certainly understood the issue and got me thinking down the right path. Shopping relay specs...

Your diode is backwards, too. As you have it, the battery would never charge. :shocked: I knew what you meant!

Thanks for the help. I'll report back when I move forward.

 

[KY Gun Geek]

Pictures are worth 1000's of words...