I don't have the original instrument cluster, But I do have the original sensor still in the fuel tank.

Can this sensor be used with a gauge, or if not can someone recommend me both please. Small independent gauge I can tuck out the way with a sensor to match.

If its the older 2 light type sensor in the front left of the tank, (likely on an 84 model), then no it can't be used with a gauge.

By far the easiest way is to retrofit a later tank with the sensor towards the back right. Even if it doesn't have a gauge, the sender has the output for it.
With age, sometimes the sensor needs removing and cleaning/fixing to get a reliable output.
There will need to be some wiring adjustment, but since you've already removed the original cluster, you can surely handle the relocation of the fuel pump wire and wiring the gauge. 
I've not done it, but I believe the sensor configuration gives a reverse reading (full when empty & vice versa) in some gauges. If so, further adjustment needed.

Bill

Thanks Bill,
The sensor is indeed at the front of the tank.

Unfortunately my electronic knowledge is very minimal, verses my knowledge of electrics as a whole. I'd like to assume that the OEM sensor omits a reading either periodically, or constantly. And this 1980s signal, whether it being varying resistance or voltage can be used to even create a crude ''low fuel'' light, as originally fitted.

If i can work out how the sensor works and what/when it outputs those values, then I'm sure I can fathom a way to make it light up an LED, worst case scenario!

It's a pair of thermistors set one above the other in a tube and my guess is that they're NTC type.

Dai wrote:It's a pair of thermistors set one above the other in a tube and my guess is that they're NTC type.

I'd never have guessed that in a million years! Cheers Dai!

So when the fuel level drops below the first one, the temp reading goes up, triggering light 1, and same for the second one when fuel level drops below that? (My electronics knowledge isn't amazing)

This is all on the assumption that petrol is colder than the air in the tank?

Dai wrote:It's a pair of thermistors set one above the other in a tube and my guess is that they're NTC type.

I don't think they are dai ...im sure that they are just standard silicon diodes  with a positive temperature  co-efficiant
as they  get warm - ie not sitting in fuel, yes they change value quite significantly , but with a constant current source the change is picked up by the comparitors (adjustable ) in the instrument cluster , turning on the relevant indicator globe

there were a few pictures in the forum of the "stick"  electronics
and past experience tells me they are just signal diodes

cheers


hope this adds to the understanding

Well, that serves me right for seeing two blobs in a tube and assuming they must be thermistors because the AFM has one! Thanks Charlie - any idea which diodes?

Jase - diodes or thermistors, your thoughts on the modus operandi are correct.

charlie99 wrote:

Dai wrote:It's a pair of thermistors set one above the other in a tube and my guess is that they're NTC type.

I don't think they are dai ...im sure that they are just standard silicon diodes  with a positive temperature  co-efficiant
as they  get warm - ie not sitting in fuel, yes they change value quite significantly , but with a constant current source the change is picked up by the comparitors (adjustable ) in the instrument cluster , turning on the relevant indicator globe

there were a few pictures in the forum of the "stick"  electronics
and past experience tells me they are just signal diodes

cheers


hope this adds to the understanding

Any knowledge of how i would wire one or two of these in to activate a warning light when it hits whatever the ambient air temperature is? Or is that a job for google and a lot of trial and error?

I found this circuit diagram ...somewhere ...and a while ago now

you will see that there is 3 diodes and a feed resistor ...followed by a capacitor to smooth out voltage changes  (likely inside the gauges  input circuits )

there is also a zenner voltage reference as the vcc feed  to the diodes

unfortunately the values as I have saved the file are hard to read ///sorry about that

charlie99 wrote:I found this circuit diagram ...somewhere ...and a while ago now

you will see that there is 3 diodes and a feed resistor ...followed by a capacitor to smooth out voltage changes  (likely inside the gauges  input circuits )

there is also a zenner voltage reference as the vcc feed  to the diodes

unfortunately the values as I have saved the file are hard to read ///sorry about that

I think I need someone with a greater knowledge of electronics than me to make me something that'll work. 

JaseYPK wrote:
This is all on the assumption that petrol is colder than the air in the tank?

Colder than the air can be a moot point!

On one occasion I received a burn on my thigh from the tank being so hot.

I've since installed a fuel cooler and no further burns. 

Just for my curiosity, as I have the later tanks on my 85 RTs, has anyone with an earlier tank noticed any difference in fuel light operation related to ambient or fuel temperature?

Bill

JaseYPK wrote:Any knowledge of how i would wire one or two of these in to activate a warning light when it hits whatever the ambient air temperature is?

Search the PCB for this use from a early model cluster with the 4 and 7 liter lamps.
Put it in a waterproof box, and hide it away somewhere on the bike.

yep good idea inge

wouldn't be hard to do at all

Just keep in mind that the two transistors in the middle of the photo needs cooling.

It also exist a later version where the 7 liter circuit is removed, then it have only
one of this transistors.

Inge's idea is the best.  The early fuel senders are able to detect 7L and 4L levels, and therefore will not have a continuous readout of the current level as what is possible for the later sender.  The circuit sends a 95mA bias current through the thermistors that warm them up.  When the thermistor is immersed in fuel, the liquid cools the sensor and changes its resistance.  When the thermistor is above the fuel level, it is free to heat up (due to the bias current) and that is what is sensed by the circuit card and results in the light on the dashboard.  This circuit will not work with any aftermarket gauge's fuel level input.

So they is thermistors and not diodes [head hurts]

that's right inge
the thermistors are used in the temperature compensation feedback to the opamp within the gauges ...not in the tank ..else there will be 2 x extra wires  to the tank connector

how ever the voltage references for the diodes in the tank have it dropping more than 1 volt ...more likely 2.4 volts ...which reminds me of multiple diodes in one package   ,,they are using the forward drop voltage so could be similar to what I have seen in sansui power amps and similar of the 70s and 80s   so ... likely to be a package of two or three diodes in one ...each silicon diode junction  usually drops about .6 volts each ... I did have a reference parts book that had those listings ...but too long ago now

That makes sense.

Looks like I'm screwed then until I buy an aftermarket sensor. I threw my old instrument cluster about a month ago after hoarding it for too long (it didn't work in the first place).

Thanks for the clarification, at least myself and others have an answer now!

If I take apart the black tube, fuel level sensor, what are the values of the thermistors, if I just want to solder in new ones?

I have checked that the 7L and 4L bulbs in the instrument cluster work.

schrocketeer wrote:If I take apart the black tube, fuel level sensor, what are the values of the thermistors, if I just want to solder in new ones?

DJ Ewen confronted this challenge in his Revive conversion. This post sums up his outcome in April of this year.