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S4s oil pressure caerbon gauge or sender or wiring issue


Giniw

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Hello Everyone,

I have recently bought a S4s and the oil pressure gauge needle stays (almost?) on 0. I don't think it's a real oil pressure problem in the engine because the oil tell tale doesn't light up even when the engine revs above 1600RPM.

So, I removed the oil gauge from the dash to test it. I now think the gauge is OK because I managed to make the needle move across all the range (please see the attached picture where I recorded the resistances tested and the corresponding displayed pressure at the gauge).

I also tested the wires in the dash:
 – The black one is a good earth.
 – The dark green one is a +12V when the key is turned in first position as expected
 – The resistance between a known good earth and the white/brown signal wire is 392 ohms with the engine not turning (0 bar at the sender). I don't know whether 392 ohm is the expected resistance?! I would say it's a bit high since I recorded that 0 bar at the gauge corresponds to roughly 220 ohms (please see my attached picture, again).
 – Strangely, with the battery disconnected and no key, I measured no resistance between the green and the black wire!? Does that sound OK to you?

The next step will probably be to remove the sender and test it with compressed air ... But in the meantime I was wondering whether anyone would have some advice about that? Especially if anyone has reference resistances values for both the sender and the Caerbon gauge, and about that apparent grounding of the green wire?!

Thanks in advance for your help!

PS: I have read here that the gauge resistance range is supposed to be from 27 to 275 ohms so I would say mine seems OK?

PPS: Some of you may already know me under the same nickname on lotuselancentral.com ^^

 

 

esprit oil pressure.png

Edited by Giniw
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Quote

– Strangely, with the battery disconnected and no key, I measured no resistance between the green and the black wire!? Does that sound OK to you?

 

Assuming it's like a normal oil gauge.... I would say you would get some resistance (coil). The diagram below is for the S4 (with a typical oil gauge in the upper left hand corner)

 

Cheers

Ian

S4OilGauge.gif

Edited by Qavion
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Thank you :)

 

1 hour ago, Qavion said:

Assuming it's like a normal oil gauge.... I would say you would get some resistance (coil).

Actually I referred to the wires from the dash without the gauge attached => I removed the gauge, disconnected the battery, removed the ignition key, and then measured the resistance between the earth (black wire) and the green wire (power supply but obviously 0V when I tested) and I found out that there are no resistance at all (something like 1ohm, similar to the resistance I measured between the black wire and an other known good earth).

I am not too sure, but that seems highly suspicious to me, isn't that a short circuit?! But if it is one, how is it possible that there is not a blown fuse somewhere?! (I haven't checked any fuse but the 12v arrives at the gauge when the key is turned, so ...?)

1 hour ago, Qavion said:

How did you get your gauge needle to move? Which wires did you attach voltage and resistance to?

I connected a 12v power supply to the gauge:

 – +12v where the green wire goes,
 – negative where the black wire goes
 – several resistances (from 10.7 to 330 ohms) between the signal prong and the earth prong of the gauge.

 

PS: You may be wondering why I didn't start the car to test the resistance of the pressure sender to check that its resistance varies. Actually I need to change the cambelt since it's rather oooooold (like 10 years) and I am a bit reluctant to start the engine for now ... ;) (I must admit it's a bit frustrating but I digress :D)

Edited by Giniw
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Quote

Actually I referred to the wires from the dash without the gauge attached => I removed the gauge, disconnected the battery, removed the ignition key, and then measured the resistance between the earth (black wire) and the green wire (power supply but obviously 0V when I tested) and I found out that there are no resistance at all (something like 1ohm, similar to the resistance I measured between the black wire and an other known good earth).

 

Ah... now I understand... I thought you were testing the internals of the gauge (but using the wire colours to indicate the terminals on the gauge).

 

With the battery disconnected, what you might be reading between the black wire and the green wire is the combined resistance of all the other gauges (hooked up to earth (which may or may not be 1 ohm).

Have you actually looked at the sender? Is the connector/s secure? I was just wondering why you were looking at the gauge first.

 

 

 

OilPressureGauge96a.jpg

Edited by Qavion
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"I have recently bought a S4s and the oil pressure gauge needle stays (almost?) on 0."

This is normal, it's because the gauge, which is calibrated in Bar, is of too large a scale to measure sensible engine oil pressure.

My old S4s, which was nearly new when I bought it, was the same.

I think Freescan displays the oil pressure on it's dashboard (if not maybe Espritmon) so you could download & install it to confirm what the pressure actually is.  :thumbup:

Edited by jonwat
update

Cheers,

John W

http://jonwatkins.co.uk

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  • Gold FFM
12 hours ago, Giniw said:

 – The resistance between a known good earth and the white/brown signal wire is 392 ohms with the engine not turning (0 bar at the sender). I don't know whether 392 ohm is the expected resistance?! I would say it's a bit high since I recorded that 0 bar at the gauge corresponds to roughly 220 ohms (please see my attached picture, again).

I agree 392 ohm is too high. Also based on your test of the gauge. It seems there is too much resistance in the wiring, so the gauge always sees a sender output that corresponds to 0 bar. I would measure directly at the sensor (with engine off) and also the wiring from sender to gauge.

The 0 ohm between the green ignition feed and earth with battery disconnected is indeed a short. But that could be by design, to avoid residual voltage when the ignition is turned off.

I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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8 hours ago, Escape said:

The 0 ohm between the green ignition feed and earth with battery disconnected is indeed a short. But that could be by design, to avoid residual voltage when the ignition is turned off.

12 hours ago, Qavion said:

With the battery disconnected, what you might be reading between the black wire and the green wire is the combined resistance of all the other gauges (hooked up to earth (which may or may not be 1 ohm).

Indeed, since the +12 is shared with the rest of the gauges and the earth is the same too. Thanks a lot!

8 hours ago, Escape said:

I would measure directly at the sensor (with engine off) and also the wiring from sender to gauge.

12 hours ago, Qavion said:

Have you actually looked at the sender? Is the connector/s secure? I was just wondering why you were looking at the gauge first.

No real reason to begin with the gauge, I just had to begin with an extremity and as I had already read the lotus esprit fact file about the binnacle, I just did that.
As for the sender, I haven't checked it at all yet. I have read somewhere the plenum should be removed? I hope it can be electrically tested without doing it!

8 hours ago, jonwat said:

This is normal, it's because the gauge, which is calibrated in Bar, is of too large a scale to measure sensible engine oil pressure.

My old S4s, which was nearly new when I bought it, was the same.

I have read that the oil pressure at tickover was very low at the gauge, but not when revving the engine, or is it?!

8 hours ago, jonwat said:

I think Freescan displays the oil pressure on it's dashboard (if not maybe Espritmon) so you could download & install it to confirm what the pressure actually is.  :thumbup:

Yes I will check it, but I can borrow a mechanical pressure gauge anyway :) (I am not that sure the oil pressure is read by the ECU though?)

8 hours ago, Escape said:

I agree 392 ohm is too high. Also based on your test of the gauge. It seems there is too much resistance in the wiring, so the gauge always sees a sender output that corresponds to 0 bar.

Yes, that is what I think, too.

 

 

 

PS : Qavion, where did you get that picture from if you don't mind me asking? I have a few workshop manuals but I don't remember that kind of nice electrical diagram!

12 hours ago, Qavion said:

OilPressureGauge96a.jpg

 

Edited by Giniw
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The picture is just a typical installation... Not a Lotus installation. Just something I found with Google. Actually, I realised I posted the wrong picture, but couldn't delete it from my message.

 

My memory is a little hazy at the moment, but I recall some of the Lotus sensors have spade connections which get loose over time. I had a few problems like that with my old SE. Before you go pulling off the plenum, just reach underneath the plenum and feel around for the sensor/sender and see if one of the two wires haven't fallen off. If it's that type of connector, just crimp and clean the spade connector and reconnect it. Then recheck your voltages. Here's an old message thread with a picture of the sender.

 

Hopefully you'll find something obvious.... and you can stop worrying )))

 

Cheers

Ian.

Edited by Qavion
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  • Gold FFM

A few more thoughts:

- a bad connection is most likely, even if everything still seems connected, resistance in the connectors can give false (low) readings. So next step should be to measure resistance in the wiring between gauge and sensor, before you try and take the sensor out. I had had a similar problem with a low reading for the coolant temperature, that turned out to be a loosely fitting spade connector on the sensor.

- I don't think there is a link from the sensor to the ECM, so no reading via Freescan or Espritmon. I can't find any mention of it in either the wiring of the ECM or the description of the parameters used by the ECM.

- the apparent short with ignition off/battery disconnected is unlikely to be through one of the other gauges, as they will all have some resistance. The same feed that powers the green dashboard wires also powers several relays and switches, one of those could be earthed with everything off.    

Good luck, I'm sure you'll find it soon enough, wiring on these cars is quite easy. ;-)

I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Well, the sender is a bit rusty indeed ...

I unplugged the wire and brushed the connector a little, then re-plugged it a couple of times and measured the resistance between the signal and the earth. It lowered a little but not much (around 360 ohms instead of 390 the before)

I then measured directly the resistance between the sender and the earth, and it's still around 360 ohms.

So, maybe it's the "earth side" of the sender that needs to be cleaned too, or the inside of the sender is rusty too, or it's totally broken!

I will need to remove it to test it in good conditions (air pressure to see if that change the resistance reading for instance).

Is it really impossible to remove it with the plenum still on the engine? I would say it seems to be accessible but ... I didn't try yet :ermm:

sender (1).png

sender (3).png

sender (4).png

sender (7).png

Edited by Giniw
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That is quite a lot of rust... did you use yours as a submarine to dive down to visit the Titanic? :P

I can measure the resistance of mine easily enough, if that'll help...

Mine looks like this (slightly different than the S4s version)

GrUwpkhb3-plM6m8HZh9oBOqjYsvEmK7xL8MRhT9

 

Travis

Vulcan Grey 89SE

My Lotus Photo and Projects Album

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15 minutes ago, Vulcan Grey said:

That is quite a lot of rust...

Yes, I was surprised too.

That is strange because the underneath (and the rest) of the car remains quite clean (except for the springs which are quite rusty too, I will probably change them)

15 minutes ago, Vulcan Grey said:

did you use yours as a submarine to dive down to visit the Titanic? :P

Well of course, isn't the Esprit supposed to be a good submarine? :D (:()

 

As for measuring yours, thank you for your kind offer but alas I think I have read that the dual prong version is different (even the resistance logic is inverted if I am not wrong — less pressure meaning less resistance whereas it's the other way round for the Caerbont gauge)

But that would be nice to measure it for other people reading that thread here (by the way, is there a wiki somewhere?)

 

 

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I then measured directly the resistance between the sender and the earth, and it's still around 360 ohms. 

Maybe you could scrape back the rust on the sender case and find a shiny bit of metal, then check the resistance between the case and the engine block?

When I changed the knock sensor on my car which is in that area, I removed the plenum and oil filter, but there was still not enough room to swing a large enough spanner. When I removed some of the rubber pipes as shown in that photo, it did help. But I wouldn't discount Travis' suggestion of removing the starter motor if you have to.

 

I drowned my knock sensor in WD40 (or something similar) for a few days, but in the end, I had to use a heat gun. But my old knock sensor was more like the one Travis' photo. I can't even imagine how hard it will be to remove yours. Those removed engine shots make the job look easy, but there isn't a lot of room in there.

With the sender in that condition, I wouldn't hesitate changing it (even if it isn't the problem) :P

 

Cheers

 

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46 minutes ago, Qavion said:

I can't even imagine how hard it will be to remove yours.

Riiiight ... OK, now that is reassuring :D

On the other hand the other side is in oil, maybe it will be easy to remove. OK, it probably won't.

 

Thank you to both of you, I will give it a try on the next weekend.

(Do you happen to know the spanner size? — 10 maybe?)

 

46 minutes ago, Qavion said:

With the sender in that condition, I wouldn't hesitate changing it (even if it isn't the problem) :P

Yes, I tend to agree with you.

Edited by Giniw
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FYI, I tried to unscrew the sender on last Sunday but indeed the oil filter or starter motor is in the way. If I were lucky, I could have put the spanner on the hexagonal bolt but of course it's exactly in a bad angle so the spanner won't fit without removing the oil filter or starter. As I will do an oil change soon when I change my cambelt, I will do all that at the same time.

I will keep you informed :)

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  • 2 weeks later...

I've been through this with my S4s - your sender is likely done.  The least expensive place to find a replacement is speedycables in the UK - you'll just need the part number off of the top of the sender - they are the supplier recommended by Caerbont. Lotus will likely want $200 USD...sjsportscars likely gets their's from Speedycables..

If there is any looseness to the sender's connector it is broken. After I replaced with the new sender the gauge always read above zero-not much but was above zero and read 60 psi when cold - it never read that prior to breaking and reading zero at idle.  Look for my post on the subject...

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