Dashboard woes

[Matt306]

I am moving on a pace with the TR7 the heater is back in and i connected power to the ancillaries before putting the whole dash back in.

Problems I have are fuel gauge;

I had a new tank and sender. I have measured Voltage at the sender unit its 12v and the earth is good. The rear of the gauge has a measurement of 12v on one 3v on another. The needle shows no movement even when the sender wire is earthed.

I have a spare Gauge so swapped it over and this time the gauge reads full the reading at the back is 12v and 6v this time. 

Now i think i have checked everything, is it time for another gauge? (the tank has about 15L of petrol in).

On the handbrake light front , there is a light for Hand brake but this doesnt illuminate but the BRAKE light does. This should be the light for the pressure switch? How do i get the HAND BRAKE light to illuminate

 

Heater;

I have only action on one setting on the heater blower, it this a broken resistor? if so can i repair ?

 

[Howard]

Matt

A usual problem with the heater is the contacts on the speed control lever. Unplug the wires and to the control lever and one at a time connect them to a suitable earth. If the fan only runs on fast the issue is with a failed/burnt out resistor. This located on top of the heater on the near side & can be replaced without removing the heater.

The handbrake switch should not illuminate the brake warning light, however looking at my circuit diagram there could be a problem. The brake fluid level warning light is shown as a black/purple cable however the handbrake light also has a black/purple cable which changes to black/white before it enters the instrument console! They could have been wrongly connected so suggest your recheck - you could initially simply earth the black/purple cable at the brake pressure switch and check which light comes on.

Cheers

H

[Matt306]

Took the resisitor out and had a look, the otter temp switch is well cruddy so did some research and the RoverSD1 club sell them for £16 delivered, I am going to replace and see how we go.

 

The handbrake switch is my stupidity i forgot to connect the plug in the harness. 

 

The Petrol guage problem still remains though.

[Howard]

Matt,

I was curious how the fuel gauge works and despite spending many hours tracing the TR7 circuits I had never investigated it, so did some digging. I hope the following helps

I have been doing some digging for my own information & have a few suggestions:-

=================================================

1) Check you have connected the wires correctly. The resistance between the terminals the green/black & black wires connect to should read between 10 & 320 Ohms. 10 Ohms is a full tank & 320 ohms is the reading with an empty tank. With 15 litres in the tank it should read about 200 ohms.

However if you have crossed the wiring, the resistance should be very low – either zero or open circuit. In this case swap the Green/Orange and Green/Black wires over.

 

The following was posted by Gareth on the TR8/TR7 forum in 2013:-

DC Power supply set at 13.9 Volts

Gauge reading full ...... 0.53 to 0.63 Volts

Gauge reading 3/4 ...... 2.71 to 2.75 Volts

Gauge reading 1/2 ...... 3.95 to 4.02 Volts

Gauge reading 1/4 ...... 4.88 to 4.97 Volts

Gauge reading Empty ..... 5.49 to 5.05 Volts

Variation read and recorded after many checks. hence the slight variations.

Full tank read 10 Ohms

Empty tank read 312 Ohms

Of course different gauges will vary slightly as will the variable resistor in the sender unit so figures for reference only.

All test carried out using calibrated test equipment so my figures can be trusted.

Just hope this will help someone in the future.

 

The following was posted by the late much missed J Clay in 2011 on the same forum

The green/orange wire is for the low fuel light.
Current comes from the light in the panel to the Delay unit (attached to the instrument pod) then through the green/orange wire to the fuel level sender. At the sender the circuit is completed to ground when the float ROD touches the ROD STOP TAB. Testing the circuit. Let the float rod rest on the stop tab for at least 30 seconds, then check the dash for the Low Fuel light. You can also remove and ground the green/orange wire, after 30 seconds the light should come on.

The Green/black is for the fuel gauge.
Power comes from the gauge in the pod directly to the fuel level sending unit. The potentiometer gives full current to ground when the arm is at it's high point As the arm drops, the resistance in the circuit changes and changes the the reading on the fuel gauge.

Testing the gauge. Disconnect the green/black wire and make a good contact to ground. If the gauge reads full, then the gauge is good. If it reads 3/4, install a new gauge.

Once a new gauge is installed and with the sender out of the tank, you can adjust the float rod stop tab to give a better reading. Holding the sender unit in the correct position as it would be mounted in the tank, take a reading off the new gauge with the float rod resting on the stop tab. If the gauge doesn't read empty, then carefully bend the rod stop TAB down in small increments until the gauge reads empty.

Lifting the float to it's highest level should give you a full reading. There is no adjustment at the sender to give a higher reading

 

He also added the following suggestion in another post:-

Put a potentiometer* in the middle of the positive side run. I would turn it down to 1/2 before putting power to the gauge. Now as you adjust the potentiometer, you should be able to see the gauge move. If it never goes to full, then replace the gauge.

Also, don't forget that our cars are old and there can be all kinds of things causing resistance in the systems.

================================================
* Note from Howard - A cheap linear potentiometer of about 1K Ohms would be suitable and should cost  about £1 each

Hope this help

Howard

[Beans]

Thanks for sharing that 😉

[Matt306]

Howard, thanks for the research. I have been puzzling too... I have two gauges one which reads full all the time and one which has doesn't move.

I took both out to examine them. The non moving one has corrosion on one of the terminals and one of the fine wires from the coil is attached to this , it appears the connection is rubbish as the electro conductive paint has come away. I plan to clean this and reattach. This unit has one less pin at the top rear to the second one.

The second gauge which reads full all the time has a duff resistor, I measured the resistance on both gauges, the broken one reads around the rated 560ohms the full reading gauge is around 300ohms. Both resistors are rated for 560ohms. I plan to replace resistor and see what effect that has.

I will report back soon!