Fuel tank sender unit question

badrover

Member
Simply, what is the effect of fitting a 4 cyl sender unit in a V8 tank? Will it under read or over read or just not work?
 
If the tank is exactly the same shape, which I think it is, then the gauge will be calibrated to read fuel quantity in a similar way but will be set to match the resistance requirements of the gauge. If the V8 and 4 cyl gauge require the same input then there should be no issues. But if there is a different part number then you may need a V8 gauge for a v8 sender unit, and vica versa, as the resistance requirements will be different. I had this issue on an MG Midget years ago. I had an early gauge and a late sender. The gauge only ever read 3/4 full even when the tank was full.
 
Hi, Because of space constraints the tanks are the same height but the later 4 cyl and v8 tanks are fatter than the early 4 cyl. So the senders indicate level and are also not precision instruments as Harvey points out on numerous occasions.

Colin
 
As I recall, earlier tanks are 12 gallons and later ones are 15 gallons. I had a 1968 2000TC years ago which I seem to remember was 12 gallons.
 
I was under the impression that the 4 pot cars had 12 gallons and 8 pot had 15. I might have made that up :)
 
Hi, Because of space constraints the tanks are the same height but the later 4 cyl and v8 tanks are fatter than the early 4 cyl. So the senders indicate level and are also not precision instruments as Harvey points out on numerous occasions.

Colin
Not joking about precision. Mine stays at "full" for a long time then gradually falls to half way, then plummets to zero. I can only assume it becomes very non-linear due to wear. We really need the reserve tap to judge when to fuel.
 
The fuel gauge on my 75 hasn't worked for a couple of years now & because you have to go into the in-tank pump mechanism to measure resistances & fiddle with the apparently delicate & easily dislodged sender assembly, l haven't bothered with it.
The car does 400-420 miles per tank so l set the trip on fill-up & return to a pump at around 350 miles. So it's handy to know how far you can do on a full tank (l only ever fill the tanks).
 
Not joking about precision. Mine stays at "full" for a long time then gradually falls to half way, then plummets to zero. I can only assume it becomes very non-linear due to wear. We really need the reserve tap to judge when to fuel.

My 2000TC does the same. I think it’s a ‘feature’ of P6’s.
 
Hi, It's to do with the shape of the tank because there is less fuel in the bottom portion of the tank.

Colin
 
Thanks for responses so far. The reason I ask is that I’ve mistakenly fitted a NOS 2000 sender unit to my V8 tank. After installation, the gauge didn’t move and I assumed it was due to having very little fuel in. Now, after putting 4 gallons on, it’s still not moving and reading nothing (just as it appears with the ignition off). I need to get underneath and double check the connections but it it’s maybe pointing to needing a gauge off a 2000 as indicated by a previous post?
 
I doubt the the reason is the compatibility between the gauge and the sender. The gauge should at least register something. Also due to the same height between the V8 and 2000 tanks, the sender should be the same. I presume that you did not test the sender before fitting, right? Just to verify that there is no problem with the wiring between the gauge and the sender, or the actual gauge itself, you should earth the wire that connects to the sender. With the ignition on, if all is well with the wiring and the gauge, the gauge should read full. If it does, the problem is within the sender. If not, you have to trace the electrical fault.
 
It will show empty when you still have a full reserve, so may not be too far out. I'd test the gauge as Demetris says then if the electrics are ok, put another four gallons in and see what happens.
 
Ok, great, many thanks. That gives me something to go at, I’ll report back. And no, I didn’t test the sender first but at least I can check what’s what by earthing the wire. Thanks.
 
After some brief investigation, it seems that the problem was a simple one. The sender wasn’t properly earthed. The car has been stripped and restored and i hadn’t reattached the earth wires back to the body by the battery. Anyway, if anyone is interested, a 2000 fuel sender will work in a V8 tank. With around 3 gallons in the tank, it’s reading slightly under the quarter mark which I guess is about right. Hopefully it will remain accurate as I add more fuel. I guess the main thing is that it doesn’t over read. Thanks for your responses.
 
Hi, It's to do with the shape of the tank because there is less fuel in the bottom portion of the tank.

Colin
Here is a photo of the wire wound rheostat that is the basis of the sender unit for a 12 gal tank. The upper right terminal on the rheostat is connected to the instrument terminal and the lower left is connected to earth. The wiper terminal on the float arm basically shorts the wire windings to earth depending on the level of the float. With a full tank, the rheostat is almost completely earthed generating minimum resistance at the instrument terminal At minimum tank level, the full resistance of the rheostat windings is generated at the instrument connection. As you can see in the photo, the windings are much shorter when the tank is close to full, gradually getting longer as the tank empties and then reaching a constant length as the tank gets close to empty. A change in level close to full generates a much smaller change in resistance than the same change in level when the tank is near empty. Although far from being a precision instrument, the variation in the rheostat windings provides some compensation for the shape of the tank at various levels. I have never opened a 15 gal tank sender for comparison purposes but expect that the shape of the rheostat might be slightly different to reflect the difference in tank shape but having the same height.IMGP4650.JPG
 
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