Thursday, 19 June 2014

T-Max - further electrical mysteries (updated, maybe solved even)

I went to start my T-Max the other day and it was barely able to turn over the starter motor. Having only relatively recently had similar issues which led me to change the alternator stator (which was burnt out) I became instantly suspicious about what was going on.


I believe that I have now explained the issue, but still have not resolved this issue. I am sure that multiple short trips in the day will cause the battery to gradually be drained of charge as the discharging will be greater than the charging.

moving on

Anyway, going back to the other day, it started on the second go and so I gave the battery a quick charge (half an hour) and started it up again without trouble. So I did my trip and when I got home thought I'd check out the on battery charging circuit. As before it seemed to be "ok" but still a little under spec at 13.68V (which is 14V at 5000 rpm according to the manual).

So I then decided to pull the earth off and measure the current while the bike was turned off (and the ignition in the OFF position). I was a bit stunned to find that the bike was pulling 0.5 Amp

I started disconnecting things (by pulling fuses) and found fairly quickly (lucky) that the drain was brought to a halt by pulling out the (curiously named) Backup fuse.

Looking at the wiring diagram we can find the source of the issue.

So in the above diagram a few things are permanently connected to (and powered by) the battery irrespective of where the ignition switch is. One is the seat light, the other is the auto chokes.

On the left hand side we have the fuse (68) the seat light and switch (4 and 5) while over on the right we have a Thermo Switch and the powering of the Auto Chokes.

So I started doing some research and found the following:
Auto Chokes: the resistance across them, they should be 16-24ohms at 20 degrees C
Thermo Switch: switches OFF at at less than 55 ~ 60 degrees C and ON at greater than this.

So when you are sitting around cold (all things being normal) there should be no drain on the battery, but when you first come back from a ride there will be a drain on the battery from the chokes. This should stop when the bike cools down (which can be a while on a hot day after a long ride...).

Currently I'm still working this out, but for the interested you can measure easily if the battery is still being sucked dry by the following method. Be sure to have an amp meter that will cope with 10A and set it up right (plug the right lead into the right slot if needed).

  1. Lift the seat, and open the battery cover so you can get to the fuses. 
  2. Pull the Backup Fuse.
  3. Measure the amps flowing across that fuse terminals
If its zero (or less than 0.1Amps) then all is pretty good, the switch has indeed cut off (as it should) and all should be right with that circuit. Typically my bike has been sucking power when I get back in, and its up to 0.9Amps before it gets cut off by the thermo switch.

Please note: this circuit also powers the clock, so when you pull the fuse the clock will stop and need resetting.

If you wish to examine the setup on your bike and make measurements, the stuff can be found under the dress trims in front of the seat. So take off this cover.

You can just see them there in the middle. The brown bit there is the plug that connects the circuit (14 on the diagram).

In closer you can see the plugs to allow you to measure the resistance of the chokes, and you can just see the Thermo Switch and its bakelite (alike) plug at the bottom of the picture.

You can see it more clearly here (perched atop the housing for the engine thermostat). To disconnect the plug on the Thermo Switch (to then access the switch for testing) it disconnects with a press on a securing clip while pulling and can be seen easiest from the left hand side of the bike

However its bloody hard to get down in there and you'll need longer probe leads (and access it from the right hand side).

Note the hose? That takes water to the carbies to heat them ... gosh isn't this just wonderfully complex? Just keep the coolant changes up to this (and do not use plain water) because if you get corrosion from electrolytic reactions you just don't know what'll screw up.

The auto chokes (well where they plug into the carbi) will be visible now too.

This does not need to be removed unless you want to (I don't see why - and doing that may just make things worse).

So, whats the Fuzz

Right now, I don't know, I doubt its the battery being gumbie. So to sum up:
  1. things seem to be within tolerance but then there is the issue of why is my battery slowly winding down? Is it just because I've been making lots of short trips? (thus draining the battery while its cooling down but not operating for long enough to charge...)
  2. When the motor is hot (and the switch open) the charging voltage is 13.6, but 14 but when  motor is cold and the switch closed. This implies to me that the charging circuit isn't able to cope with this extra load (which is about 1 amp, nothing compared to the high beam) - why?

I'll post more when I have some better data ...

The Fuzz

Ok, so now I've done some more measurements and some unplugging and testing everything and thinking and I have come to the conclusion that this is the Fuzz.

If you do a number of short trips on a T-Max (well, at least my series) you'll really challenge the battery.


Well when its hanging around cooling its sucking power out of the battery to power the AutoChokes (which aren't actually doing anything) because they are solenoids and are powered by the independent supply circuit governed by the "Thermo Switch". This will continue until the temperature drops below 55°C

I took some measurements of the temperature and the battery when I got home to sus this out. I intended to have more measurements but I got a phone call and got distracted. So this is what I have.

Essentially the cooling rate of the bike at rest is relatively linear (and that's reasonably correct from a basic thermodynamics perspective until the bike gets closer to ambient temperature, but thankfully its not 60°C here) for the part of the range that's of interest to us (like 55°C when the thermo switch goes open circuit).

This graph suggests (and that's about my observation too) that the bike takes about 45 min to cool down enough to cut off the thermo switch (and stop draining the battery).

I pulled the Backup fuse out of the bike as soon as I got home so as to not cause the bike to suck the battery much more and to see what my battery stabilised to (to see if my battery was holing up to this). My battery was at 13.3V as soon as I got home (and shut the motor down) but at 30 minutes (without the drain) had stabilised to 12.68V which is pretty good.

However during the 45 or so minutes every time I sampled (see the graph for those 3 points) the battery needed to feed the chokes with about 700mA (actual values varied between 420 and 900mA - because as the copper cools its resistance changes and actually sucks more current [amps] as its resistance lowers as it gets closer to room temperature).

This means that if you do a number of short trips during the day the bike may spend more time sucking power from the battery while cooling down than it does getting charged. For instance you may reach operating temperature of 90°C in 15 min going to the supermaket but then spend 45 min cooling down and then ride 15 minutes home to spend another 45 minutes cooling down (sucking power out of the battery).

Depending on what you do with the bike this may not be a problem. Before this I used to ride it for 30min to work and then 30min back again. So I'd say that during that time the balance of recharge equaled the discharge of sitting around while it was cooling. Now however I'm using it differently, where trip times are about 15 min (just enough to get to full operational temp) and sit around and suck the battery times are still what they were (45min). Clearly this is resulting in more drain than charge and the battery is barely able to turn the bike over after two or three weeks.

So if your T-Max seems to run the battery down about once a month (so it starts sounding slow on the turn-over on starter) then the problem may be caused by the effect described above

But Wait, there's more

When I first started looking into this, the battery was charging at a rate slightly under the spec. I observed that it charged at spec when cold but under spec when warm. Interestingly this changed after I plugged and unplugged everything and its now charging at 14.2V all the time.

Why could that have done anything?

Well Ohms law (remember this is about electricity right) says that E (voltage) is the product of I (current) and R (resistance). So if by unplugging and re-plugging those connectors I essentially 'cleaned them' of corrosion then its possible that I altered the resistance of them a small amount.

from 13.68 to 14.25 V is actually a small amount too.

So it seems that (like so many things) the answer to this problem was unplug everything and plug it back in again. No matter how much I hate that as a solution its the only reason I can come up with as to why its now charging properly.

Of course this doesn't change the fact that the discharge to your battery will happen. T-Maxes it seems are demanding on batterys and more deeply cycle them than (say) cars. The charging issue seems to be almost unrelated to the "why is my battery getting flat over the week" issue.

So bottom line

If you use your T-Max for a number of short trips during the day you'll need to consider putting it onto a charger for a couple of hours every now and then (and you'll know because the starter motor will sound slow).

If your T-Max is not charging quite right, maybe unplug and re-plug the major items and see if that fixes it.

Best of Luck


I have now posted a solution to my discharging problem over here , this is of course done by inserting a relay into that auto choke circuit to cut it out when the ignition is off (and of course the bike is not running).


Yu-Lin Chan said...

From the title, I thought you were going to talk about the black & white film :)

obakesan said...

LOL, I never even thought of it from that angle ;-)