TVR S1 Oil Pressure Gauge

[Bercilac]

I did have a search through old posts and utilised the info I found but I'm not sure if my current assumptions are correct.

I have never seen this oil pressure gauge working, nor have I tried to debug an electrical gauge before as all my MGs have had mechanical gauges, so I'm after advice!

Symptom: No oil pressure indicated on the gauge with engine running. Oil seeping from sender when engine running (indicating some pressure?).

Testing done

1. Removed gauge, earthed 'S' (sender) terminal and applied 12v to the '+' (ignition) terminal. Gauge shoots to 100psi.

2. Connected 'S' to 12v and '-' to earth on battery, then applied 12v to '+' - needle flickers from zero to 20psi.

My assumption here is gauge is working.

3. Replaced the sender with a two pin aftermarket same range for psi - 03- 180 ohms resistance (?). No oil pressure indicated. Earthed 'S' - no change.

Assumed also a problem with wiring (possible dodgy connector from engine loom to dash loom).

4. Ran a bypass from sender 'G' (Gauge) to the gauge. No oil pressure indicated.

5. Earthed the 'S' on the sender. Needle shoots to 100psi, drops to 80 psi on starting (so something is happening).

I suspect that I had a dodgy original sender and a wiring fault. I am guessing I now have a wiring fault and the wrong oil sender resistance.

Does anyone know the correct resistance for the sender in an S1 and is mine exhibiting the symptoms that you would expect for the wrong sender resistance?

[Blue 30]

Well done on working through the issue logically & one step at a time.
Getting an obvious thing out of the way... Have you tested the engine for a good earth , including the new sender casing, as I hope you haven't used PTFE tape, which would insulate the sender ! Yours has x2 terminals, one is a switch, to turn a lamp on/off. The other is a variable resistor, for your gauge operation.
Is the senders varying resistance output the right way round ?
IE. From high to low resistance as the oil pressure increases.
You'll need a ohmmeter to check this of course.

From memory, vdo gauges are 3-wire working.
That's, 12v, an earth, plus the senders resistive earth wire.
And if you loose the sender resistive earth, the gauge will read maximum (but I'll have to check that fact).
Keep going, you'll sort it soon.
TerryB.

Edited by Blue 30 on Thursday 18th July 11:11

Edited by Blue 30 on Thursday 18th July 11:15

[Bercilac]

Thanks for that, much appreciated. I'll work through those tonight and report back.

[Blue 30]

Your gauge should be marked.
+ = 12v supply
- = Gauge earth
S = from sender wire

So with a good gauge 12v and earth, if you earth the S tag, the gauge should read full deflection.
If that is all good using the cars wiring, then you can eliminate it.
With the new sender in your hand, and using a ohmmeter, you should get a high resistance reading between casing and the G tag.
Once fitted into the engine block, you should see 0 ohms between the sender casing and the engine block.
And you should read that same higher resistance between the G tag (disconnected) and the engine block.
Now connect the cars loom sender/gauge wire to the G tag on the sender.
And if all is good, the gauge should read that higher resistance as low pressure.
Once the engine runs, the sender resistance should go down towards 3ohms, and the gauge should go up to a higher pressure reading.
With the engine running and the senders G tag disconnected, you could do a ohmmeter reading on the sender G tag to earth to prove the senders resistance does go down from 180ohms towards 3ohms as the engine is revved.
TerryB

Edited by Blue 30 on Thursday 18th July 12:24

Edited by Blue 30 on Thursday 18th July 12:25

[Bercilac]

That's brilliant instruction, thanks, much appreciated.

[mrufcs1]

From memory the gauges are Stewart Warner sourced who seem to be mainly a US firm. The ohm range on the fuel sender is 240-33 ohms so it's reasonable to think that the oil pressure gauge will be the same. I think 240 is empty and 33 is full but don't quote me !

It'll probably be the same gauge, just a different face.

[NZDave]

mrufs1 you are correct. 240 ohms for 0 psi and 33 ohms for 100 psi. At least it is for my S1.

Dave

[Bercilac]

Thanks fellas - I can see that my resistance range is wrong on the new sender.

88S1 is lending me some bits from a sister car this weekend so I can eliminate various elements, but the wire from sender to gauge is definitely not playing ball so I guess there's a multi plug pin gone astray when I replaced the dashboard.

The original sender I can see is leaking near the small brass connector, bubbling up through what looks like a soldered joint and also from the rear seam, all of which is only visible after peeling off the outer black plastic.

Nearly there though, so hope to be taking it for an MOT in the next couple of weeks, work permitting.