fix or replace the sender?

[Mike_Bouse]

Fuel gauge in the Spit reads 100% full all the time, regardless of fuel supply. a little disconcerting when you realize that i have zero driving experience with this vehicle, meaning no concept on range, mpg, or capacity.

The gauge isn't stuck. it pegs full when the key is on, and returns to zero when the key is off.

Should i think about repairing the sender, or is there little or no hope in making this repair myself?

is there a test i can perform to assure myself that it is the sender that is at fault, before i order a new unit? I have an old fiat sender that i could externally hook up to the leads...would that prove the issue?

 

[TR3driver]

is there a test i can perform to assure myself that it is the sender that is at fault

You can start by disconnecting the wire at the sender, and observing that the gauge goes to empty. If not, there is a short somewhere in the wiring (or at the gauge). If so, the problem is most likely inside the sender.

Worth commenting that there is supposed to be a device called a "voltage stabilizer" that controls power to the fuel & temp gauges. If it is failed or not grounded, both gauges may read high by 1/4 scale or more. You can check it by connecting a test light (or voltmeter) from the disconnected wire at the sender, to ground. Within a few seconds of turning on the key, you should see the light starts to flash on and off. If it stays on, you may have a VS problem.

Should i think about repairing the sender, or is there little or no hope in making this repair myself?

Can't hurt to try. I've not had a Spit sender apart, but usually the internals are very simple. Just a coil of resistance wire, and a wiper that moves across it. But the usual failure is reading too low, because the resistance wire wears thin and eventually breaks. So I think there is a short somewhere. If the short is inside the sender, it may be easy to fix.

I have an old fiat sender that i could externally hook up to the leads...would that prove the issue?

IMO, no. There is a lot of difference in fuel senders; I don't think connecting an unknown sender will tell you any more than just disconnecting the one you have and seeing what the gauge does.
Someone probably has information on what the resistance of the sender should be (meaning you can test it with a DVM, which IMO every Triumph owner should have), but I don't have it handy.

 

[Mike_Bouse]

both the fuel and temp gauge are hooked to the same VS. the temp gauge seems to act normally, reading roughly 2/3 at normal operating conditions; but the fuel gauge ALWAYS reads 4/4 when the key is on.

ignoring my other post, i think this rules out the VS for this issue.

i will try the disconnect sender test and the test light at the disconnected leads.

 

[martx-5]

I have an old fiat sender that i could externally hook up to the leads...would that prove the issue?

I don't have a wiring diagram for the Spits, but all of the other Triumph diagrams I've seen have show a variable resistor in series to ground in order to operate the fuel gauge. The Fiat sender may not be the same resistor rating as the Triumphs, but I see no reason why it at least shouldn't be able to give you an indication of whether or not the gauge is good.

You might want to ohm out the Fiat unit to see what the range is. On the TR's, the range is 0-75 ohms or there about. The Spits may be different, but it shouldn't really matter. Hook the wire going to the sender unit to the Fiat sender, and ground the other connector, or if the Fiat unit has only one connector, ground the body of the unit. Then slowly move the float back and forth and observe the gauge. If it swings in relation to movement, then you have a bad sender. We're not looking for absolutes here, just to see if the gauge acts like it's supposed to.

 

[NutmegCT]

Mike - Barney Gaylord has a great diagnostic and calibration guide for those resistance type fuel gauges. I got mine working in about 30 mins using two 68 ohm resistors, a paperclip and a 12v battery.

https://mgaguru.com/mgtech/electric/fg_06.htm

Tom

 

[TR3driver]

Mike - Barney Gaylord has a great diagnostic and calibration guide for those resistance type fuel gauges.

True, except it's for the "balanced" type gauge found on MGAs (and TR2-3). Totally different inside than the "hot wire" gauges found on later Triumphs (like a 69 Spit).

 

[NutmegCT]

Randall - thanks. I didn't know that (obviously).

T.

 

[dklawson]

As Randall pointed out the gauges without the stabilizer (pre mid-1960s) were different, didn't use the stabilizer and were magnetic. The later gauges (which this probably should be) were bimetallic, required the stabilizer, and used different sending units. My '67 GT6 has the stabilizer so if this is your '69 it should also have the stabilizer.

The following info is generic Smiths stuff and the exact numbers may not be right for your specific cars. The non-stabilized sending units generally run from 0 Ohms = empty to about 90 Ohms = Full. The gauge and the sender operate at full battery voltage. After the mid-1960s Smiths started selling the bimetallic gauge system that required the stabilizer. The stabilizer is a form of voltage regulator that delivers an "average" output of 10V by switching on and off rapidly. It can be impossible to measure the voltage accurately with a meter because of this rapid switching. The fuel sending units for the bimetallic system generally run from about 240 Ohms = Empty to 30 Ohms = Full.

As mentioned, if you remove the green/black wire from the sending unit and ground it with the ignition on you can identify if the gauge is working properly.
Non-Stabilized cars: Grounding green/black = Gauge shows Empty.
Stabilized cars: Grounding green/black = Gauge shows full.

So how do you know which gauge type you have? The quick answer (apart from knowing the year your car was built) is by how quickly the gauge can respond. If you are used to seeing a working fuel gauge needle "jump" off empty the instant you turn the ignition key on, that is an early magnetic gauge without the stabilizer. If you turn on the ignition and the fuel gauge needle creeps up slowly, that is a later bimetallic type gauge for the system requiring the voltage stabilizer.

Since the sending units work in the opposite directions and cover very different resistance ranges, you cannot mix the components from one system to another.

As for the Fiat sender... it may work fine as a test tool. If you have an ohm meter, connect it to the Fiat sender and move the float arm through its range to see how it compares to the numbers I mentioned above. As mentioned, for this to work in testing your fuel gauge you will need to connect the insulated terminal on the Fiat sender to the green/black wire and make sure the second terminal is grounded... or the frame of the sending unit is grounded. Some Smiths senders have a third terminal that is used for a low fuel indication. If the Fiat has three terminals you'll need a meter to determine which terminal is which.

You said that your temp gauge reads 2/3. I guess you have to take that with a grain of salt. If you have a way to accurately measure your coolant temperature when the gauge is showing 2/3 you'll have more confidence in what that means. Smiths bimetallic temp gauges typically are calibrated for mid-scale to be about 185oF. The beginning of the "white" zone around "H" (on a gauge marked C-N-H ) is about 210oF. As mentioned, if the stabilizer isn't properly grounded or its internal points are welded shut it will pass full charging system voltage to the gauges (about 14V while driving). That can actually make the electric gauges read as much as 40% too high. Reading 2/3 on the temperature scale could be accurate, or it could be an indication that the stabilizer isn't working or isn't grounded.

Sorry for the long message.

 

[BOXoROCKS]

Have you checked to see if the float is working,and not stuck.

 

[TR3driver]

The stabilizer is a form of voltage regulator that delivers an "average" output of 10V by switching on and off rapidly. It can be impossible to measure the voltage accurately with a meter because of this rapid switching.

I've checked several originals, and they switch at about 1 Hz. The important point here is that even though they are "10V stabilizers", you will never see 10V. The output is always either full battery voltage or zero.

Reading 2/3 on the temperature scale could be accurate, or it could be an indication that the stabilizer isn't working or isn't grounded.

I don't know how many other cars this applies to; but on the Stag, the correct temperature sender is no longer available. The sender sold by almost all Triumph suppliers (when they sell one at all) makes the gauge read too high.

FWIW the voltmeter in my Stag had such loose adjustments that just setting the meter down on the bench would alter the calibration.

 

[Mike_Bouse]

thanks guys. you gave me a lot to digest. I will letcha know in a couple of days what did and did not benefit this situation.

if i were to be a wagering man, i'd be betting that i'm ordering a new sender.

and, for the record, it is hooked up to a suspect VS, but so is the seemingly functioning Temp gauge.

lots of suggestions, i will incorporate as many as i can into the solution.

 

[dklawson]

If you already question the stabilizer, you may want to download and read through this:
https://home.mindspring.com/~purlawson/files/SmithsVoltageStabilizer.pdf

On both the GT6 and the Mini I run a u7810 chip in place of the standard Smiths stabilizer. They are inexpensive and easy to adapt to negative ground cars.

Randall, I think the sending unit issue has hurt a lot of LBCs. For the Mini there are at least three different temperature sending units out there and no one knows which goes with which gauge.

I never knew the switching frequency of the stabilizer, however, I knew that I never got an accurate reading even with a good analog meter. I have re-calibrated a handful of stabilizers but I never used a volt meter for that job. The method I have used is in the PDF above. However, it's a lot easier to replace the stabilizer with a chip than to repair and recalibrate one.

 

[BOXoROCKS]

one more option, and it works every time. Go to the hardware store and buy a wooden yardstick, thats what I use for my TR3. Its fuel gauge is off.

 

[Geo Hahn]

1" = 1 gallon... very approximately.

 

[dklawson]

A time honored method.