Bench Testing 940 Fuel Gauge? Plus Lazy Tacho

[Exasperant]

So, while putting together a mental list, need to sort the fuel gauge.

It sits on empty, no warning light.

I was wondering if there's a way of bench testing it, maybe with variable power supply and/ or multimeter? It appears the screw marked in red is a shared regulated/ stabilised positive (based on it having continuity with the temp gauge), the black arrowed screw goes to the connector also arrowed in black, presumably that trundles off to the tank? And I guess that would leave the third, pink, for the low fuel light?

I can see, after removing a small cover, electrolytic and ceramic capacitors. What's the chances of me getting lucky, and it being those?



Second, the rev counter seems a bit hit and miss. Sometimes it sits at zero at tickover, which means it's either glitching or these cars are surprisingly loud when not running... What're the chances this is a known issue, and easily fixed with a soldering iron?

[Exasperant]

Popped the gauge out the cluster. It's got very handy marking on it, telling you pos, neg, and sender.

So a quick hook up to the bench supply and it seems to work.

Odd though, the warning light was on, which I'm 90% sure it wasn't when fitted in the car. Might have to go double check that.

For reference, I'd got the wiring (and means of operation!) totally wrong.

Red arrow is neg, black is sender (OK, I got it 66% wrong..), pink is pos. Found a schematic that suggests these run on 12v, but I set my bench PSU to 10 to have a bit of don't blow it up safety.

[Laird Scooby]

Originally Posted by Exasperant

Popped the gauge out the cluster. It's got very handy marking on it, telling you pos, neg, and sender.

So a quick hook up to the bench supply and it seems to work.

Odd though, the warning light was on, which I'm 90% sure it wasn't when fitted in the car. Might have to go double check that.

For reference, I'd got the wiring (and means of operation!) totally wrong.

Red arrow is neg, black is sender (OK, I got it 66% wrong..), pink is pos. Found a schematic that suggests these run on 12v, but I set my bench PSU to 10 to have a bit of don't blow it up safety.

Have a read through this thread :

https://www.volvoforums.org.uk/showt...l+gauge+repair

Also there is no separate switch for the "Feed Me" light in the fuel gauge, it's done by a simple comparator built into the gugae electronics and the output switches the LED on when the guage voltage drops below a certain level.

Before refitting the cluster, clean the connector strips in the multiplugs in the car and tease them out by using a thin screwdriver hooked underneath to increase the curve of the bow shape in the connector strips. Likewise clean the connectors on the back of the cluster.

[Exasperant]

Yeah, never looked inside one of these modern gauge things before!

Rested the cluster back in the car, and... Definitely no light. Had also traced the circuit back to a bit of a cruddy looking solder joint, might even be the same one as in those photos on that thread. Getting around 990 ohms across what I'm guessing ought to be a bridge. Do have to wonder why the track doesn't just branch off instead of being designed that way, but anyway...

Jumping the possibly dodgy solder joint with a bent paperclip, gauge shot into life, warning light and all. And then we began the dance... Remove the sender screw, gauge leaps up. Screw back in, gauge plummets down. Found the harness for the pump/ sender (after spending a little while trying to dismantle the C post....), disconnect and same as undoing/ doing the gauge screw.

Wrestled with the pump/ sender on the tank. Need to replace a couple of hose clips, and never did get all the pipework off. But I gave things a go anyway, and gauge seems to be sitting contentedly at about half. Which, going by the petrol being maybe a couple of cm below the sender/ pump aperture, may well be right.

I suspect the float had got stuck, and my banging and hammering around at the retaining ring then jogging the assembly free shook it loose.

[Laird Scooby]

There are a few 1k resistors dotted around the PCB on the back of the cluster, there is also a fusible link that feeds most of the instruments (but not the speedo) and if you follow the +ve round from the speedo connection, you'll find it goes to this fusible link. You'll also see the PCB is pre-printed with holes either side of this link and there is provision for two self-tappers in the cluster housing in the same place - this is so a repair kit from Volvo can be fitted or you can replace it with a bit of low amperage fusewire crimped into a couple of ring terminals.

Make sure you've done the locking ring up on the tank,they have a habit of settling after being disturbed and leaking all over the MoT testers head during the dreaded annual!

If after retightening it still leaks, the new seals are available from Volvo for ~£11, usually in stock. A right pig to have to remove the sender to change them so now if i have to remove the sender/in-tank pimp i make sure i have a new seal ready to fit, just in case. Learned that one the hard way with the only MoT failure on my 760 during my ownership.

[Exasperant]

Originally Posted by Laird Scooby

Make sure you've done the locking ring up on the tank,they have a habit of settling after being disturbed and leaking all over the MoT testers head during the dreaded annual!

Oh, it's tight!

Although an absolute *cough* to get back on, the thread just didn't want to start. So there was careful tapping, then turning. Then more turning. Then quite forceful turning, until it was snug against the sender all round. Then a bit more turn to be sure. Although I suppose I can't be absolutely certain until I've enough fuel in there to come above the bottom of it all.

I think I've been battling two problems - Stuck float, and dodgy PCB/ solder joint. Got everything back together, started up, gave it a few revs to check for leaks etc, and... Gauge died. Plummeted to empty, no low fuel light. Quick tap/ wiggle (instruments only rested in dash) and it sprung back into life. So I'm going to investigate that dodgy looking solder joint tomorrow, and might just flux and flow everything to be sure. Tempted to pull the rev counter and see if there's anything in there that could explain the intermittent under reading, but really as it's an auto it's not vitally important.