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.

[Laird Scooby]

Originally Posted by Exasperant

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.

Errrrrrrrr................ the lock ring isn't tight enough.

I use a length of 2x1" batten and a lump hammer. You can do it from underneath on a set of ramps, on the outboard side (nearest the left of the car), rest the batten against one of the ridges moulded into the lock ring and give it a few taps upwards with the lump hammer to tighten it. Leave it a month or two and repeat.

Loosening is similar but you tap upwards on the inboard side (nearest the propshaft). It's needs to be tight with a capital F to make sure it doesn't leak!

On that solder joint, i'd suggest reflowing with some fresh solder, use a solder-sucker on it to then remove all the solder then resolder it with fresh 60/40 tin/lead solder.

When you say the tacho is intermittently reading under, when and what/how much under is it? It could well be you have a lock-up box in the car in which case the revs will drop above a certain speed, also if on a trailing throttle, there's no direct drive so the revs drop again until you get below a certain road speed.

A sticky float/sender is usually cured with a slap with the palm of the hand on the bottom of the fuel tank at the back end of it.

[Forrest]

Fair play to you, grappling with the fuel sender within days of acquiring a 940. It took me ten years from first diagnosing a lazy in-tank pump to finally plucking up the courage to tackle that job.

Here’s my write-up of getting a completely removed one back in should you need it.

https://www.volvoforums.org.uk/showp...9&postcount=15

I replaced all the jubilee clips with stainless steel equivalents.

My “secret” to getting the large plastic ring to tighten up effectively was to soak it in very hot water before trying to refit it. This causes it to expand and makes it easy to get the thread started. It also enables you to tighten it more than otherwise. I just gave it a final, gentle tap with a large screwdriver and rubber mallet to nip it up. I reused the original seal and ring (although I had replacements of both on standby) and I have had no leaks despite generally keeping the tank fairly full.

[Exasperant]

Originally Posted by Laird Scooby

Errrrrrrrr................ the lock ring isn't tight enough.

Loosening is similar but you tap upwards on the inboard side (nearest the propshaft). It's needs to be tight with a capital F to make sure it doesn't leak!

Ahh, I should've mentioned, by "turned" I meant percussively so...

When you say the tacho is intermittently reading under, when and what/how much under is it? It could well be you have a lock-up box in the car in which case the revs will drop above a certain speed, also if on a trailing throttle, there's no direct drive so the revs drop again until you get below a certain road speed.

Oh, this is at tickover - I've not actually driven the car yet, wanted to be sure it had fuel in it first! When things are behaving it seems to sit around maybe 750 - 800. When not, the needle is barely off zero, although it does rise in line with the engine when blipping he throttle - Just presumably lagging a few hundred behind.

A sticky float/sender is usually cured with a slap with the palm of the hand on the bottom of the fuel tank at the back end of it.

Worth knowing, but would that still work with the float stuck at the top? I'm wondering if the previous owner brimmed it and then left it standing, or never drove it enthusiastically, causing it to stick at full.

Originally Posted by Forrest

Fair play to you, grappling with the fuel sender within days of acquiring a 940. It took me ten years from first diagnosing a lazy in-tank pump to finally plucking up the courage to tackle that job.

It was the stress of tackling this on day one, or every day being concerned I might be about to run dry in the middle of nowhere

Here’s my write-up of getting a completely removed one back in should you need it.

Ah, great! I had been a little nervous about wiggling the thing back in if I'd had to remove it entirely.

My “secret” to getting the large plastic ring to tighten up effectively was to soak it in very hot water before trying to refit it.

I did curse the design a bit putting the thing back on. It seems not quite big enough, while wanting to flex in exactly the way that's least helpful. If I have to do it again I'll try the hot water soak idea. In the end "This flexes, the thread's really coarse, let's try tapping it on then turning it so it pulls itself straight" seems to have worked. But felt a bit of a bodge around.

[Laird Scooby]

Originally Posted by Exasperant

Ahh, I should've mentioned, by "turned" I meant percussively so...




Oh, this is at tickover - I've not actually driven the car yet, wanted to be sure it had fuel in it first! When things are behaving it seems to sit around maybe 750 - 800. When not, the needle is barely off zero, although it does rise in line with the engine when blipping he throttle - Just presumably lagging a few hundred behind.



Worth knowing, but would that still work with the float stuck at the top? I'm wondering if the previous owner brimmed it and then left it standing, or never drove it enthusiastically, causing it to stick at full.



It was the stress of tackling this on day one, or every day being concerned I might be about to run dry in the middle of nowhere



Ah, great! I had been a little nervous about wiggling the thing back in if I'd had to remove it entirely.



I did curse the design a bit putting the thing back on. It seems not quite big enough, while wanting to flex in exactly the way that's least helpful. If I have to do it again I'll try the hot water soak idea. In the end "This flexes, the thread's really coarse, let's try tapping it on then turning it so it pulls itself straight" seems to have worked. But felt a bit of a bodge around.

Percussive rotation is fine then!


"They all do that sir!" - joking aside, i've had 3 740GLEs, all 1987/88 and often the tacho would go "lazy" as you put it but restore itself to the proper level soon after. It never bothered me as it always seemed to work when i was actually driving and usually only occurred after a period of inactivity. One of those intermittent faults i could never be bothered chasing down, especially as all of mine did it at some point with no other ill effects.

Yes, slapping the bottom of the tank would work if the float is at the top because as you may have noticed, the float slides all the way from the top to the bottom on its "slide" so any "percussive persuasion" on the bottom travels all along the slide. Try adding a couple of eggcups full (~100ml) to the next full tank of petrol you give it, it will work wonders in many areas including lubing the slides, cleaning the injectors and several other things!

If your guage is still playing up, it's worth investing in a petrol can and keeping it in the car until you know it's doing what it should and you get a feel for the economy. While i was repairing my guage, i think (but not sure) i knocked one of the calibration pots on the fuel guage PCB as now when i get down to about 3-4 gallons, it's on the top of the red. Normally i'd fill up at that point but in times when there's still too much month left at the end of the money, i have to wait and carry the can just in case - even if it's only half a gallon to get me to the nnearest station it saves a long walk!

Your certificate for Volvo Gynacology First Class should be in the post - think you've earned that tackling the in-tank pump/sender unit so soon into 7/9xx ownership!