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Post by Roverman on Apr 25, 2023 9:56:01 GMT 1
My 1956 Rover P4 90 petrol gauge stuck on half full, on checking the voltage at the gauge only getting a reading of 8 volts ? should it be 12, on checking the live feed wire at the sender unit same 8 volts, on removing the sender unit and check ohms when moving the float from full to empty ohms a bit hit and miss am I right in thinking new sender unit required ?
Thanks for all replies
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Post by chippie on Apr 25, 2023 10:41:12 GMT 1
If memory(?… ) serves me correctly, cars equipped with a temp/fuel gauge of that era had a basic voltage regulator that provided a fixed reference to prevent variation in readings…..so your 8v sounds credible….
Can you extend the wire from the tank area back to the front, connect to sender unit and watch the fuel gauge while you operate the arm?….other than that if the reading on your ohmmeter is erratic then it’s possible the wiper arm is dirty …..worth a clean ?
Otherwise time for another…
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Post by sorted on Apr 25, 2023 14:30:58 GMT 1
The voltage regulator Chippie mentions is usually closer to 10.5v output to the gauges.
The best quick and dirty test is to briefly earth out the sender unit wire at the tank end- the gauge should climb to full. And should of course read empty with the wire disconnected. Assuming the above checks out you know the gauge, regulator and wiring are good, so time for a sender.
Exactly the process I went through with the ‘72 MGB GT recently.
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Post by OldGit on Apr 25, 2023 16:02:37 GMT 1
10.5V sounds right, check to see what you're getting with the engine running... And of course the 10.5V is the supply to the gauge, you'll only see that at the sender with the sender unplugged. As above, if the resistance is fluctuating, it points to a bad sender. I've known people bend the moving arm a bit so it runs on another part of the resistive track to get more life out of them...
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Post by sorted on Apr 25, 2023 17:37:14 GMT 1
To be honest they’re not usually that expensive- the MGB one was around £20 inc a new seal- for that it’s not really worth sticking the old one back in!
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Post by OldGit on Apr 25, 2023 19:09:06 GMT 1
<best Yorkshire accent> Ow Much!!? </best Yorkshire accent> I didn't mean it was the best thing to do, just some fellow Yorkshireists would make it last a bit longer by doing that  especially if there was no other reason to go into town, wasting all that shoe leather! |
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Post by rhyds on Apr 25, 2023 21:35:58 GMT 1
I've had a nose around ebay and it seems a fair few of the senders for the P4/P5 can be disassembled. If yours can be it might be worth taking the covers off and seeing if you can clean up the resistor track
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Post by valhalla on Apr 25, 2023 23:31:16 GMT 1
rhyds is correct about disassembly - these are commonly just pulled apart to fix. It sounds like the rheostat track has got grimed-up - my P4 used to do this a lot when it was stood with old fuel. It didn't help (on my car) that the tank was lined with loose rust from being stood with sour-fuel for 15years before I had it, nor that I had tried to clean it all out and POR15-protect the interior - the gauge was out of the tank at the time, but the POR15 carried-on coating everything afterwards....
10.5volts ought to be the switched output of the voltage-regulator for the instruments, but......if memory serves me correctly (I may have to go and try this with a slave battery on the car) you can see around 8volts with some multimeters; the voltage regulation is not very good, being mechanical, and therefore it depends on the way that your multimeter "smooths" the waveform out. A moving-coil "old fashioned" meter will tend to be easier to read on the sawtooth that comes out of these things, and will tend to average-out the voltage much better than a modern digital multimeter.
From what you have found, it's more likely that your rheostat track just needs a light clean with some alcohol and a toothbrush, possibly a light polish with some Brasso if it's really grimy, than seeking a new-old-stock voltage regulator that will probably do the same thing. If you need one, just PM me, and I'll get a secondhand (good) one in the post to you.
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Post by Roverman on Apr 26, 2023 21:46:53 GMT 1
Thanks for the offer Valhalla but already ordered a new unit, tried cleaning mine but still not working the gauge, so I will let you know if the new unit works ok, I will be selling the car just want every thing working as it should.
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Post by Roverman on Apr 29, 2023 15:08:23 GMT 1
Fitted new (second hand) petrol sender unit today and gauge all working OK  so now P4 90 ready for the shows and possibly a new owner |
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Post by Roverman on May 5, 2023 11:16:37 GMT 1
Rover P4 90 sold the first person to view the 90 bought it, and now I am happy to be back to 2 cars (at 85 years of age) just have to stop looking on e bay & gumtree so now I can just concentrate on my P38a range rover for every day work and the P4 80 for the show's and trips out when the sun shines |
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Post by rhyds on May 5, 2023 11:19:14 GMT 1
First to see did buy!
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especially if there was no other reason to go into town, wasting all that shoe leather!
so now P4 90 ready for the shows and possibly a new owner