I have an '85K with the thermoresister and 4L and 7L fuel light arrangement.  I installed an Acewell speedo and its all working but the fuel gauge.  I know that to get the fuel gauge working I need a float-type sending unit like that which is on the 86-on models (or something similar).  I've read a number of other threads on the forum about this issue and I have some questions that I didn't find answers to.  I'm trying to figure out the best way to get there and would like the collective wisdom of the group on the following ideas and questions:

1.  Get an 86+ tank with the proper sending unit.  I know I'll need to weld on the mounting tabs to the frame do that (or figure something else out).  And I'm pretty sure I'll have to follow Robmac's instructions to modify the sending unit in his excellent article "messing with the in-tank sending unit".  Is there anything tricky about switching from the existing front of tank plug to one that wires through the float-type sending unit?  I assume that power to the fuel pump runs though there?  The other downside to this is expense of buying a new tank.

2.  Cut a hole in my existing tank and install an 86+ sending unit (or some other sending unit).  I'm extremely worried that I won't be able to get it tight and I'll have leaking.  Anyone ever try this?  

3.  Install some sort of aftermarket sending unit inside the tank (no additional holes cut in the tank).  I've seen some items--both float type and solid state--that might work.  I know I'll have to find a unit that sends the proper resistance for my Acewell.  I see two issues both related to wiring.  First, how do I get the wires out of the tank?  Would it be possible to open up the thermoresister unit and tap into the wires there?  I seem to recall reading somewhere that the wires inside that thing were very thin and delicate.  True?  Second, if that is a viable option, how concerned do I have to be about making the connections (both at the sending unit and at the thermoresister unit) gas/liquid proof.  Will contact with gas result in erroneous readings or, worse, other dangers?  

4.  I could abandon a fuel gauge idea and just wire up a low fuel light.  I'd rather have a gauge, but if I were to do this, I'd need the necessary circuit to light the light.  I saw a diagram to build that in a different thread, but it's beyond my technical capabilities.  I have a spare motometer cluster, so could I extract the necessary board from that to accomplish this result?  If so, can anyone tell me which board it is/how to do that?


I'm always open to other ideas and solutions as well.  Thanks in advance.

ithink the acewell will be geared up for most after market type sender units Justin

have a look around for one of those cork on a slide types ..they could - may fit to the mounts inside the tank bung ...with just a little bit silver soldering perhaps ?  usually on two wires required ...but avoid the capacitance type floats senders ...they will require an extra bit of electronics to interface

just some thoughts

My crazy idea:

You could use both the 7L and 4L signals to trigger whatever indicator on the Acewell. Have the 7L trigger the indicator to go solid initially and then wire in a generic turn signal relay triggered by the 4L signal to make it flash.

That way you'd get a solid indicator at 7L that would start to blink once it hits 4L. You'd get a two stage indicator with no tank mods required.

Not so crazy if you're a dapper hand at electronics.  Maru has published a circuit that adapts to the <85 fuel sender units.  There is also documentation about the OEM circuit on the download page on this site.  The early K-bike sender required a 95mA constant current to be driven through the NTC resistors in the unit, that would cause them to heat up in free air and change their resistance.

Fuel sending unit board looks like this:

Question about the 85 sensors: Don't they just ground when they get exposed to the air and heat up enough?

duck wrote:Question about the 85 sensors: Don't they just ground when they get exposed to the air and heat up enough?

No, they change resistance when exposed to the air and shift the sense voltage relative to the reference voltage on a comparator.  That causes the comparator to switch, turning on a transistor that grounds the circuit and illuminates the indicator light depending.  There are two such circuits on the card -- one for 7L and the other for 4L.

Thanks for all the comments.  So, it sounds like my idea #3 is feasible, but I'll have to use significant care in cutting into my socket that mounts on the tank to wire in a sender unit.  Can anyone confirm, in this case, is liquid tightness required on the connectors?  I'm thinking not given that the fuel pump has bare connectors that feed 12v+.  But given that we're talking gas and electric mixing, I'd like to be careful, for obvious reasons.  

Rob, if I were to harvest the board that you have identified and go with a warning light (I have an unused warning light in the Acewell) what do I plug in where on the board?  I'd have 1 wire coming from the sender unit, 1 wire running into the Acewell on the warning light and I assume that I'd have to feed the board with 12v+.  And I assume that I'd need to ground the board also?  Or would I do better to use a stand-alone light with +/- leads (and if so, where do I connect things in that case).

What did you end up doing here?  I have an Acewell speedo coming in the mail and was looking for solutions...

Cheapest to most expensive:

1) Figure out how to just have low fuel light active
2) Marulabs Bratbox 3.0 releases this week with full P&P instrument cluster functions ($156 + shipping).
3) M.Unit Blue ($360)

Supposedly the new bratbox 3.0 will have a conditioned speedo circuit, fuel level circuit, RPM input and self cancelling turn signals....

My work on this has stalled for a variety of reasons unrelated to the task at hand (mostly busy with work, family, etc.).  Plus its summertime and I'd rather ride than have the bike in pieces.  

I found a sender unit that is close in terms of necessary resistance and fabricated a float arm that I think will work with the K tank.  But I still need to mount it in the tank and connect the wiring to the existing socket.  I'll eventually get back to it.  I'll try to remember to post something about how it went when that happens.

good luck !!