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morgan 4/4 petrol tank sender

Posted By: bruce

morgan 4/4 petrol tank sender - 21/11/20 09:15 PM

[align:center][/align]have a problem with fuel gauge car had been sitting over winter now gauge shows empty when full , have measured resistance to chassis shows 70 ohms was wondering if the logic should be the other way round eg zero resistance on full tank ? checked and cleaned the sender wiring also the earth to chassis , looks like the regulator is connected to the fuel and temperature circuits and should be ok as the temperature gauge works ok hoping someone has come across this , car is 1978 thanks bruce
Posted By: Dave Collis

Re: morgan 4/4 petrol tank sender - 21/11/20 09:43 PM

Pull the sender out of the tank and make sure the float has not filled up with gas, usually happens with the old brass floats but I have seen the plastic ones do it also.

When you have the sender out, move the float arm...does the gauge position change?

I had one where the float arm had seized in the body of the sender.
Posted By: RichardV6

Re: morgan 4/4 petrol tank sender - 22/11/20 05:06 PM

Another quick check is to remove sender connection and touch to chassis. With ignition on gauge should rise slowly towards full assuming you have bi-metallic gauge. 70 ohms does sound high for full tank.
Posted By: IvorMog

Re: morgan 4/4 petrol tank sender - 22/11/20 05:50 PM

Richard, I might be barking up the wrong tree here but the VDO lever type sender registers empty with minimum resistance (actually states 3 ohms) and full at maximum resistance (states 180 ohms).

So on that theory wouldn't grounding the wire give zero (or very low) resistance and therefore read empty.

Carrying on from that theory, if you simply disconnected the sender connection it would give a very high resistance and hence a full scale deflection on the instrument i.e. full.

Caveat 1. That's assuming a VDO Lever type sender. The VDO tube type sender is the other way round and I have no idea if Smiths senders are the same but it does suggest that we need to know what type of sender and instrument is fitted here.

Caveat 2. I've mentioned before that electrics are not my strong point.
Posted By: SimonH

Re: morgan 4/4 petrol tank sender - 22/11/20 06:50 PM

Originally Posted by IvorMog
Richard, I might be barking up the wrong tree here but the VDO lever type sender registers empty with minimum resistance (actually states 3 ohms) and full at maximum resistance (states 180 ohms).

So on that theory wouldn't grounding the wire give zero (or very low) resistance and therefore read empty.

Carrying on from that theory, if you simply disconnected the sender connection it would give a very high resistance and hence a full scale deflection on the instrument i.e. full.

Caveat 1. That's assuming a VDO Lever type sender. The VDO tube type sender is the other way round and I have no idea if Smiths senders are the same but it does suggest that we need to know what type of sender and instrument is fitted here.

Caveat 2. I've mentioned before that electrics are not my strong point.




Vdo senders work the opposite way around to the smith’s senders...
Posted By: RichardV6

Re: morgan 4/4 petrol tank sender - 22/11/20 08:47 PM

Originally Posted by IvorMog
Richard, I might be barking up the wrong tree here but the VDO lever type sender registers empty with minimum resistance (actually states 3 ohms) and full at maximum resistance (states 180 ohms).

So on that theory wouldn't grounding the wire give zero (or very low) resistance and therefore read empty.

Carrying on from that theory, if you simply disconnected the sender connection it would give a very high resistance and hence a full scale deflection on the instrument i.e. full.

Caveat 1. That's assuming a VDO Lever type sender. The VDO tube type sender is the other way round and I have no idea if Smiths senders are the same but it does suggest that we need to know what type of sender and instrument is fitted here.

Caveat 2. I've mentioned before that electrics are not my strong point.



I assumed a '78 car would have a bi-metallic gauge Bob given the age and mention of an instrument voltage regulator. If so these require max current flow to actuate needle towards full hence minimum resistance. Apologies if I have made the wrong assumption.
Posted By: IvorMog

Re: morgan 4/4 petrol tank sender - 23/11/20 08:25 AM

Richard, I think you probably made the right assumption as I didn't notice the date.

Might be worth double checking but I suspect you are right.
Posted By: RichardV6

Re: morgan 4/4 petrol tank sender - 23/11/20 08:45 AM

If you go back further Bob vehicles had moving iron fuel gauges with two coils. These relied on max resistance when full and had no need for voltage regulator. Easy to identify because you got an instant needle reading when ignition switched on. Both types were used on Series Land Rovers with the change to bi-metallic in mid 1967, to add some chronology.

As you can imagine there are numerous LR forum posts with owners querying why their gauges are working backwards innocent
Posted By: bruce

Re: morgan 4/4 petrol tank sender - 28/11/20 01:52 AM

just figured out messaging thanks to all see post in maintenance
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