Wiring fuel gauges

[sunquest]

Scenario. Two fuel gauges, one on flybridge one in saloon helm. Both have their individual positive and negative feeds, but the sender wire from the tank goes first to the saloon helm gauge S terminal and then continues up to the flybridge helm gauge to the S terminal. Is this correct, or should there be some form of resistance to compensate for linking two fuel gauges? Hope this makes sense. Geoff.

 

[VicS]

The arrangement you describe sounds sensible but what gauges (make and model) are they?
What does the gauge manufacturer advise?

What problems are you having? The one upstairs reads lower?

Any provision for adjustment/calibration

 

[sunquest]

The arrangement you describe sounds sensible but what gauges (make and model) are they?
What does the gauge manufacturer advise?

What problems are you having? The one upstairs reads lower?

Any provision for adjustment/calibration

Hi Vic.

The gauges were manufactured by Robertson/Stowe some 15 years ago and I do not have a wiring diagram nor are there any adjustments.
In normal conditions both gauges read full and only move towards 3/4 - 1/2 full when the fuel level is low.
When I disconnect the sender from the upper helm, the lower fuel gauge works normally. Currently the fuel tanks are full and the lower gauge reads almost full. When I reconnect the upper helm sender wire both fuel gauges move rapidly to a full full position.
Some tests I have done are firstly to measure the resistance between earth and the sender wire from the full tank which is 250 ohms and secondly to measure the resistance between earth and the sender wire connected to the upper helm gauge which is 30 ohms.
When both gauges are connected together via the sender wire, the resistance is altered and the problem results.

Geoff

 

[cimo]

hello sunquest
you have a 240-33 ohm sender by the sounds of it. Although I always had it in my mind that 33 was full and 240 empty - but I could have that mixed up.

your description doesn't cleary say if each guage works ok on its own or not. There is no reason i can think why you cant run two guages off the same sender.

checks you can make -
both guages are compatible for the sender type?
all cable connections, power, negatives and sender wires are sound?
verify each guage with the sender in turn independently of the other.

just a thought - you don't happen to have two fuel tanks also?
rgds
c

 

[VicS]

Sorry I cant quite get to grips with what you are saying it all does. (old and senile .... me that is)

You appear to be saying that the lower gauge on its own works OK

If you just connect the upper gauge on its own does that work OK

You are saying that when they are both connected they both read (more or less) the same as each other? Just wrong!

I could not dig up anything on Robertsons but Stowe are still alive and kicking maybe they could help

 

[claudio]

Have you checked the ground wire to the sender? If not, you could either measure the voltage drop across both ends, or just hook up another wire to 'bypass' the ground wire with a known good one.

 

[J*O*A*M*O*N]

Aaaahh... cheap and easy solution if using only one at a time results in a correct reading...switches next to each to isolate or activate when needed????
Sorry, just a thought...yea your probably better off trying to FIX it
Good Luck Ed

 

[rogerthebodger]

generally 2 types of gauges. Resistive or current loop (4-20mA).

If 4-20mA the gauges will be wired in series so the current that represents the fuel level passes through both gauges.

If resistive as said 33 - 240 or other way depends on UK or USA.

For duel gauges to work with resistive a special sender is needed some as when duel electric oil pressure gauges are used.

I have single VDO 4-20mA fuel but duel resistive oil and water gauges.

Have a look at the VDO site for details.

 

[sunquest]

Sorry I cant quite get to grips with what you are saying it all does. (old and senile .... me that is)....... Me too check my bio!

You appear to be saying that the lower gauge on its own works OK.....Correct

If you just connect the upper gauge on its own does that work OK......Yes

You are saying that when they are both connected they both read (more or less) the same as each other? Just wrong!....Yes, they always read full or 3/4 full depending on fuel level. This is a twin engine set up with two fuel tanks and four fuel gauges. All behave the same.
If you can imagine each gauge reads virtually full, when wired independantly and the resistance from the sender is 250 ohms. I believe the problem lies when the second gauge is connected, as a resistance of 30 ohms is linked in series or parallel. The 30 ohms is measured between the sender connection and the earth connection on a disconnected gauge.

Thanks to all you guys that are contributing. The boat is a 44ft Colvic Sunquest and was pre engined and wired by colvic/TCS in Southampton to a high standard,but whilst I do not have a wiring diagram all cables are colour/number coded.

I could not dig up anything on Robertsons but Stowe are still alive and kicking maybe they could help

.... Will do if all else fails.

 

[VicS]

Will do if all else fails.

Definitely take note of what Roger Shaw
says.

 

[cimo]

ah, okay i follow now. if your guages are are of the dual coil type, your will have paralleled the coils on the sensor side when daisy chaining the guages. You want to balance the guage currents again.

Couldn't find your guages on line, but you can place a resistor in series with the sender input.

On your guages, measure the resistance between the 12V terminal and the S terminal. Place a wire wound resistor approx 0.5 times this value inline with the sender lead (0.25 or 0.5watt (better) will suffice).
rgds
c

 

[sunquest]

Thanks Cimo, I think you have the answer. Since the boat was built, it would appear the gauges have never read accurately as there is no sign of a "balance" resistor. Geoff

 

[cimo]

hopefully so sunquest - you won't damage anything anyway.
I recommended the 1/2 watt resistor as their leads are a bit more robust and easier to work with.
let us know how you got on?
rgds
c

 

[sunquest]

hopefully so sunquest - you won't damage anything anyway.
I recommended the 1/2 watt resistor as their leads are a bit more robust and easier to work with.
let us know how you got on?
rgds
c

Thanks for your help and I will post the results in due course. Two things- did you mean a resistor for each gauge, as the resistance between 12v and S is 140ohms each, therefore I need a 70 ohms resistor on each? Also I cannot find through the usual Maplins etc a wirewound resistor of 0.25 or 0.5watts, they seem to start at 2watts upwards? Geoff

 

[cimo]

Thanks for your help and I will post the results in due course. Two things- did you mean a resistor for each gauge, as the resistance between 12v and S is 140ohms each, therefore I need a 70 ohms resistor on each? Also I cannot find through the usual Maplins etc a wirewound resistor of 0.25 or 0.5watts, they seem to start at 2watts upwards? Geoff

Only one resustor required. Connected inline between sender and both S terminals.
2watts is fine. (bulkier again).
rgds
c

(S1) (S2)
|____/
|
[70ohm]
|
|
[sender]

 

[rogerthebodger]

cimo

I don't quite get this as all the VDO resistive duel instrument setup require a special sender for duel gauges so if is so simple as adding a resistor inline with the sender I don't understand why VDO have these special senders.

Could you pls explain the theory of how this extra resistor works.

I have a simular setup on my rudder indicator and I measures a spair gauge I have as you advised which turned out to be 140 ohms as OP so I will try your setup next week when I am on my boat and report back. If it works it will save me lots of extra wiring and special costly oil pressure and water temp senders.

 

[cimo]

cimo

I don't quite get this as all the VDO resistive duel instrument setup require a special sender for duel gauges so if is so simple as adding a resistor inline with the sender I don't understand why VDO have these special senders.

I imagine the "special" senders are just this, incorporating a fixed resistor
in series with the variable sensing element. VDO have some great looking instruments and sensors but I find their website not to be overly informative.

Could you pls explain the theory of how this extra resistor works.

I’ll try (I’m no orator). when you daisy chain the gauges, you now have both sensing coils (internal to gauge) in parallel. that’s 140ohms in parallel with 140ohms. this give and equivalent resistance of 70ohms. (R1XR2)/(R1+R2). the sensing coil resistance is now reduced, the voltage remains the same +12V, therefore the current will increase. Increased current will pull the gauge needle further to the right giving the elevated readout - this is what the OP was observing.

In order to re-balance the current (between the sensing coil and the zeroing coil within each gauge), place a 70ohm resistor in series to bring total equivalent sensing coil resistance(s) back to 140ohm. this reduces the baseline operating current to normal & gauge needle falls back to correct readout position.

I have a simular setup on my rudder indicator and I measures a spair gauge I have as you advised which turned out to be 140 ohms as OP so I will try your setup next week when I am on my boat and report back. If it works it will save me lots of extra wiring and special costly oil pressure and water temp senders.

definitely. try to get as much technical data from the vendor when buying such items. Its often classified as "proprietary" information, but the more open manufactures will give you the data.

Note on wattage - I suggested a 0.25/0.5watt rated resistor. This is incorrect. The minimum you will need is 1.0Watt. I can't account for this error & wish to plead momentary insanity. The resistor manufacturers and maplins covered me though, not supplying a 70ohm of such low wattage. My apologies to the OP.

 

[rogerthebodger]

Still don't see have this can give a correct reading.

Page 6 on this http://www.samallen.com.au/vdo.pdf pdf shows the fuel senders with a resistance range 10-180 ohms and one 5-90 ohms marked dual station. The rudder indicator also shows single and dual station and if you look at the oil pressure and water temp senders all have single and dual stations which must have a different resistance range as indicated for the fuel senders.

In fact this is what the OP problem is IMHO he has 2 gauges with a single station sender where as he needs to change it to a dual station fuel sender

 

[sunquest]

Thanks for your help and I will post the results in due course. Two things- did you mean a resistor for each gauge, as the resistance between 12v and S is 140ohms each, therefore I need a 70 ohms resistor on each? Also I cannot find through the usual Maplins etc a wirewound resistor of 0.25 or 0.5watts, they seem to start at 2watts upwards? Geoff

The power of the forum succeeds again. With thanks to all contributors especially Cimo who provided the answer. A 70 watt resistor in series has solved the problem.

I believe the wrong sender was fitted as original equipment ie one for a single gauge, resulting the gauges to read between 3/4 and full depending on the fuel tank level. These are not VDO gauges but the principals are the same.
Rogershaw refers to page 6 in the VDO catalogue which demonstrates what this has been all about. You effectively need a sender with half the resistance on a dual gauge set up which is why Cimo came up with the resistor in series.
I am a very happy bunny

 

[rogerthebodger]

"You effectively need a sender with half the resistance on a dual gauge set up which is why Cimo came up with the resistor in series."

But a resistor in series will not half the resistance of the senders it in effect increases the resistance of the senders. So if it works I don't how it works but will have a play next week when I an on my boat.

Why I am interested is I have a similar issue with my oil pressure and temp gauges/senders so if it does work I will also be a happy bunny.