Standard Caterham oil pressure gauge

[StewartG]

Presumably this is nothing more sophisticated than a voltmeter. Does anyone know what voltage should make it read say 4 bar. With this infomation I will be able to determine if my gauge reads low and maybe narrow things down to either the sender unit or perhaps genuine low oil pressure.

[Petrolhead]

Can't answer you specific question but the std sender is not the best. A lot of people replace the std iol guage with either a racecraft or spa one

 

R500 no 65 😬

I am after a pair of half doors

[hugh]

the standard gauge is suspect don,t waste your time with it, replace it with a mechanical "Racetech" unit for £35 from rally design in Faversham Kent.

[mav]

the guage tends not to be suspect. It is the sender that continuously fails...

[WD]

I'll second Mav on that, having gone through two sender units in the last year!

WD

[hugh]

you can buy a a complete mechanical gauge kit for the price of a caterham sender unit for the electrical gauge, the choice is yours! regards hugh

[Bricol]

I too have gone through a couple of senders - but I like the matching Caterham guages so I either need to find a mechanicla one of the same pattern, or find another sender.

 

I think the problem is sender is not especially designed to be quite so out in the open that it is - it collects all the water, dirt, and general crap flying under the car - my second one lasted twice as long as the original as I plastered all seams and openings with sealant - still only lasted 2 yrs though. I'm used to senders lasting longer than the car!

 

So I have two thoughts - figure out if I can find a more industrial sender that is more robust - but it may well be an even more robust price.

 

Or mount it elsewhere in the engine bay and connect it to the exisiting mounting with a suitable hose - although I don't like having more hoses - something else to fail/leak - but a short hose will be cheaper than another sender.

 

Bri

[Casbar]

I fitted the Think kit, but I see no reason, why you couldn't use the remote pipe and T piece from Think, and then fit your current sender into the T piece thus retaining the original sender and gauge, but moving the sender away from the oil filter

 

The T piece has the space for two fitments, so one of the holes may need blocking off if you don't wish to fit a low pressure light switch, or I bet Think could just provide a T piece that would only have one hole

[Alex Wong1697456877]

I think it's a Ohmeter - if you put a resistance meter across the sender and heat/cool it, you can see the resistance change.

[Chris W]

Steward

 

I actually measured the volts at the sender against pressure and plotted the results. From this, I was able to make a comparator circuit which operates an oil pressure warning light that comes on at 2 bar.

 

In answer to your specific question, the voltage reading at the sender with 4 bar pressure was 3.1 volts and at 2 bar it was 4.3 volts. The shape of the graph is not very linear so you can't interpolate that well. If you need a copy of the graph, please email me and I'll be happy to send it to you.

 

Chris

 

2003 1.8K SV 140hp see it here

[StewartG]

That's very interesting Chris, an answer at last rather than the usual it must be a dodgy sender, which it may well be but I'm trying to analyse whether the oil ptessure is indeed low or if the sender is giving a low output or if the gauge is giving a low reading.

Just to clarify did you measure the volts at the sender when it was presented with a verified pressure of 4 bar etc or was this the voltage that caused your gauge to display a 4 bar reading.

In other words assuming your's is okay, does a genuine 4 bar oil pressure produce 3.1 volts which gives a reading of 4 bar on the gauge or does the sender read 3.1v when the gauge displays 4 bar but being uncalibrated we don't know if this is actually a real 4 bar or not.

Sorry to be so pedantic but I'm hoping that my actual oil pressure is ok but the system is inaccurate.

[Chris W]

Stewart

 

The 3.1v was the voltage measured when my gauge read 4 bar. I don't know whether that was a genuine 4 bar - it was just the indicated value.

 

Hope this helps.

 

Chris

 

2003 1.8K SV 140hp see it here

[StewartG]

Thanks Chris. I've just had a look at the back of the gauge and it seems things are not as simple as I thought. I was expecting the gauge to read the difference between the sender output and earth and so have an input terminal and an earth tag but there are 3 connections, no 3 is almost certainly an earth but I can only guess at this point what the other two are. Maybe the gauge acts as a voltage comparator reading the difference between a known voltage and the one produced by the sender unit. Who knows but my plan of seeing if my gauge indicated 4 bar when fed with 3.1v was scuppered.

[Chris W]

Stewart

 

The 3 wires on the back of the gauge are (with the normal colour code) +12v (green), earth (black and sender signal (green/black). You will also find 2 additional wires to illluminate the gauge (red/white) and its earth (black)but that's nothing to do with the measurement itself.

 

The sender doesn't actually send a voltage - it is in fact a variable resistor which decreases in value as pressure rises. This sender resistor is in parallel with part of a resistor chain inside the gauge, across which there is 12v. The current flowing through the resistor chain determines how far the gauge moves (the gauge is basically measuring current). As the pressure varies, so the sender resistance changes and so the overall gauge resistance varies thus moving the gauge.

 

I wrote a detailed article about gauges in one of my "Electrickery" series in Low Flyer last year.

 

Chris

 

2003 1.8K SV 140hp see it here

[StewartG]

So to calibrate the gauge you could connect a suitable pot between the green/black sender cable and earth, any idea what value would be needed? Then having checked the oil pressure at say 3000 rpm when hot with a mechanical, accurate external gauge you could adjust the cslibration of the dash gauge to read correctly.

[Chris W]

Stewart

 

You could do this in theory, but the issue would be that the gauge would only be accurate at that one point. Since current measuring devices work on what is known as a "square-law" curve (ie: not a straight line but an x^2 curve) for reasons beyond a simple description, the other readings may, or may not, be accurate.

 

That's why, principally, I made a comparator circuit that brings on the oil warning light if the oil pressure drops to idle levels when running normally.

 

I don't know the actual resistance value as I didn't need to know it for my comparator circuit (only the voltage was important). However, if you warm the engine up till the gauge is reading around 4 bar, then carefully (because it's hot) disconnect the sender wire, you can measure the sender resistance with an ohmmeter.

 

For completeness, here are the voltage readings I obtained:

 

gauge(bar) volts

6.5..............2.11

6.0..............2.66

5.5..............2.85

5.0..............2.83

4.0..............3.11

3.0..............3.85

2.0..............4.24

0.0..............4.54

 

I obtained the readings by inserting a variable resistor instead of the sender, although I don't appear to have made a note of the value 🙆🏻. My guess would be about 5Kohms max. A short circuit of the sender wire will give full scale (ie: max pressure) on the gauge. An open circuit will give a zero pressure reading.

 

Chris

 

2003 1.8K SV 140hp see it here

 

Edited by - Chris W on 6 Feb 2005 18:38:40