G'day All,

I've persisted with my erratic fuel gauge long enough! So I decided to really apply my 30 odd years experience as an electronics technician and sort it out.
There are a number of areas where these go bad, 1 is the senders, 2 the connections at the tank,the interconnections in the loom, 3 the connection at the instrument cluster. 4 the dodgey crimp interconnections on the flex circuit board, 5 the gauge itself.

My 740 has the 80 litre dual tank arrangement with two senders, these are electrically arranged in a series resistance circuit.

I have previously replaced the connections and senders to no avail. I do know that my instrument cluster is suspect due to some "temperature" indicative faults. ie when its cold my auto od light will work, but occasionally the backlight won't.

My cluster is the Volvo/VDO cluster which is not interchangeable with the later (89+) Volvo/Yazaki cluster though there are similiar problems with both types.

Realising that I am dealing with a (IPC rating) class 1 circuit meant that anything I did would be an improvement. I'ĺl post some photos when I can get them off my phone.

I disassembled the cluster and removed the fuel gauge, I checked the board for dry joints, (it's the first thing I do for any electronic repair) I discovered that this assembly must have been designed by an accountant. The fundamental flaw is that the whole assembly relies on the tension of the mounting nut to ensure the electrical connection of the meter movement.
On closer inspection, after separating the circuit board from the meter movement , I discovered the "inspection ok" stamp covered some of the contact ring for the meter movement. This would be a likely cause of erratic operation, I cleaned the ink off the ring then tinned it with solder (common or garden variety lead, not pbfree). I made sure that the ring only had enough solder to make a good contact as its important not to increase the height of the assembly. If I had my druthers I'd put a star washer between the contact ring and the movement boss but it would cause fitting problems.
I also resoldered the board paying careful attention to the contact posts for the meter movement. As a precaution I put a strip of tinning on the posts as well, note I didn't tin the whole of the posts as that would make them too fat to fit in the meter movement sockets.

I reassembled the meter movement onto the circuit board,then the assembly into the cluster. As precaution, I soldered the "crimp" connections on the cluster.

So far its been successfull, I'm still chasing some other gremlins, I'm planning to replace the shoddy cluster/vehicle connections by wiring in some molex connectors instead.

I also have a Yazaki cluster from an 89 turbo, and I noticed that whilst it was a better quality instrument, it also has a potential for intermittant connections between the 2 circuit boards and the movement. tightening the assembly fixtures should help.

If they'ed stuck to the tried and true design used in the 200 series then none of these problems would have eventuated.