Fuel gauge shenanigans

[abri]

Hi all, I'm hoping someone has had a similar problem and was able to work out a solution.

The car is a late 66 1275 S. The fuel gauge reads normally when the ignition is switched on and normal (or very close) at idle. However as soon as the revs pick up, the needle moves to near empty and even over it if the revs are continuously higher, like when cruising on a motorway.

I have tried two different tank senders (including a brand new one), different voltage regulators, different voltage stabilizers, checked all earth points, two different fuel gauges. The temperature gauge which is connected to the exact same stabilizer works 100%.

The voltage stabilizers are BR1305/00A, same as on another car of the same era where it works fine.

The only thing not standard on this electrical system is that it's been converted to negative earth but again, the same parts on the other car work fine with negative earth.

I'm out of ideas.

[rich@minispares.com]

. However as soon as the revs pick up, the needle moves to near empty

all my cars do this, its called running out of petrol!




joking apart, its odd that it goes backwards, normally a voltage stabiliser fault makes the needle go up (with the temp needle)

could it be a slight short in the loom maybe?, or a battery / dynamo / regulator fault - have you looked at the voltage of the system when the car is running?

[minimans]

Make sure the Sending unit has a good earth/ground. In fact check all the grounds!

[abri]

Haha Rich, I opened myself up to that one, didn't I? I should have said ..."moves to empty a LOT faster than you'd expect"

I haven't tested for loom faults, but the battery is new and doesn't get drawn down when the car stands, sometimes for 2-3 weeks without driving.

I also haven't looked at the system voltage when the car is running. Do you suspect the regulator is letting too much current through?

[nick rogers]

Abri, How many spade terminals on the back of the gauge ? You may be trying with early pre-voltage stabiliser gauges. A picture might help.

[abri]

Hi Nick,

Here's a picture of the back of the speedo showing the voltage stabilizer. The little sticker on it says BR1305/00A



I'm pretty sure the fuel gauge is the right one for this type of gauge. I've tried another one off a 90mph speedometer that has a similar stabilizer on the back....same symptoms.

[gpmini]

Haha Rich, I opened myself up to that one, didn't I? I should have said ..."moves to empty a LOT faster than you'd expect"

I haven't tested for loom faults, but the battery is new and doesn't get drawn down when the car stands, sometimes for 2-3 weeks without driving.

I also haven't looked at the system voltage when the car is running. Do you suspect the regulator is letting too much current through?

That's your problem right there Abri, you don't drive it enough
Leon

[abri]

Haha Rich, I opened myself up to that one, didn't I? I should have said ..."moves to empty a LOT faster than you'd expect"

I haven't tested for loom faults, but the battery is new and doesn't get drawn down when the car stands, sometimes for 2-3 weeks without driving.

I also haven't looked at the system voltage when the car is running. Do you suspect the regulator is letting too much current through?

That's your problem right there Abri, you don't drive it enough
Leon

...and I only have three minis. You must have a team of drivers

[dklawson]

I know you said you had cleaned the grounds, that you had tried different combinations of components, and that the temperature gauge works OK. However, I would still re-clean all the grounds.

Start with the back of the gauge and the stabilizer. The stabilizer case has to have a good ground to the speedo housing. Remove the stabilizer and wire brush both the bracket on the stabilizer and the one on the speedo. While you have the stabilizer off the speedo, look carefully for where the thin metal foil strip is pinched between the stabilizer case and the stabilizer circuit board. That foil strip has to have a good connection to the stabilizer case as it provides the ground connection for the stabilizer's internal vibrator circuit. Make sure the black wire connected to the speedo has a good clean connection both on the ring terminal and the threaded stud. Make sure the other end of that black wire has a good ground connection to the car body.

If the above does not help, at the tank, remove the black wire from the sender. Run a temporary ground wire between the non-insulated sending unit spade terminal (where the black wire was) directly to the battery ground terminal. If the black wire has a bad connection to the car body the fuel gauge system will loose its ground and the gauge will drop to empty or below empty. If this temporary wire fixes your problem, run a new ground wire from the sending unit to the car body.

In general, if the fuel gauge starts to read low or empty (and you haven't run the car out of gas) then the circuit resistance has increased. This typically means a bad earth connection between the sender and car body. If the problem was with the stabilizer, you generally expect the temperature gauge to also read low. However, the temp gauge may not read "as low" as the fuel gauge during this problem so keep an eye on whether the temp gauge starts to drop even a little.

It is highly unlikely this has anything to do with the dynamo or its regulator. The whole purpose of the stabilizer is to keep a controlled, nominal 10V on the gauges so they won't fluctuate. HOWEVER, if the regulator is not working well and the car's system voltage drops to (or below) 10V, the output of the voltage stabilizer will also drop below 10V causing both the fuel and temperature gauges to read low. That said, once your car's charging system drops below 10V you won't be able to drive very much further before the ignition system stops working.

[abri]

Thanks a lot Doug. I will have a look at all the ground points again this weekend.

A test that I've previously done to check if the fuel gauge works is to connect both wires at the sender end together with a bridge. The gauge goes to full as expected when I do that which suggests the wires are ok, but that obviously doesn't say anything about what happens when the car is running.