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K Series, temp/press sensor details??

Grant Dryburgh

By Grant Dryburgh
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Grant Dryburgh

Grant Dryburgh

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Hi all, 

I'm installing an Aim MXS dash in my seven and need to calibrate it for the Water temp, Oil press and Fuel level. 

Fuel is easy will just get the values empty and full from the sender however water temp and oil pressure I'm unsure of. I have options on the software to add custom sensors but I need to input values in either mv or ohms which correspond to either the temp degree c or pressure bar/psi ? 

Don't suppose anyone has any information regarding these values for the standard sensors?

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Grant Dryburgh

Grant Dryburgh

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I can't make out any part numbers on the sensors but they appear to be the standard Caterham items. EU2 1.6K SS engine.

Water temp is located on the rail beside the brown plug ecu sensor and oil pressure sender branching off the underside of the oil filter. 

caterham parts site shows oil sender : 71216 and water sender : 71167 (slightly different connection but listed as replacement for original sender. 

I'm assuming these both give resistance values for the readings so i just need to know the ranges of pressure and temp in relation to the ohms ? 

 

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Grant Dryburgh

Grant Dryburgh

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yeah I did come across that thread but it refers to a 2 pin sensor which i assume is more likely the ECU sender not the single connection sender for the gauge ? 

I'm considering just replacing the senders with new VDO units 40-120 degree for water and a 10 bar pressure unit for oil as the Aim software has these listed in sensors so may save the hassle of calibrating the existing sensors ? 

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Jonathan Kay

Jonathan Kay

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Hmmm... hoped that someone would give a definitive answer... here's a brain dump:

  1. Have you got a wiring diagram that matches your Seven? Let me know if not.
  2. We need the real part numbers, not just those that Caterham use.
  3. TTBOMK all the factory K gauges are simple resistance devices.
  4. There aren't many ranges of analogue senders and gauges around. Some oil pressure senders go the opposite way. See for example the VDO catalogue.
  5. You can test a temperature gauge with pans of hot and cold water and a thermometer and a multimeter. (And a pressure gauge similarly but most of us wouldn't have the kit.)
  6. Does the Aim MXS manual say that it has distinct preset ranges (see 4) or is everything variable? Have you got a link to it?
  7. You can probably leave the old instrument connected and match the old and new readings under controlled conditions: cold and then hot for temperature, engine off and then at, say, 4,000 rpm for pressure.

Jonathan

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revilla

revilla

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I did measure the resistance of the standard Caterham sender across a range of temperatures. I plotted the curve in Excel. It was a while ago and I'm sure I posted it on here, but I can't for the life of me find it now using the search and I can't find the original files on my laptop. So I'm afraid that's not a lot of help. What I can confirm is that it was a simple negative temperature coefficient resistance and that after many hours of research I want able to match it with any of the standard senders available on the market. It didn't match anything for example from the Smiths or VDO ranges. I even got as far as trawling through vintage tractor parts catalogues but it does appear to be a really archaic or proprietary calibration. There were plenty of senders which would match it over a small range of the curve but nothing that was reasonable close over the full scale. My guess is that it was something in use around the time of the inception of the Seven and then as instruments and senders have evolved they've kept the same calibration to maintain compatibility.
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Grant Dryburgh

Grant Dryburgh

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Excellent thanks for the help folks *thumbup*

Just have a final question regarding the tacho signal. I'm using the white wire with black tracer which goes from the original tacho through pin 2 of the grey plug and to the ignition coil. What kind of signal am I going to see from the coil ? 

The RPM parameters I have are Max rpm : adjustable in increments from 6000, 7000, 8000, then 10000 and beyond, I'd assume I would set this to 8000 rpm given the limter is around 7200-7500 I believe? 

I then have an RPM factor, options are *2 /1 /2 /3 /4 /5 & /6 guess this depends on the signal from the coil ? 

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revilla

revilla

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On a non-VVC EU2 engine the tacho is driven directly from the switched negative side of the coil. So the waveform is actually pretty nasty! The good thing is most tachos will have an input stage that can deal with it.

The waveform is basically:

  • Battery voltage between firing events.
  • Pulled down to ground for a few ms to "charge" the coil with current.
  • A nasty positive spike as the driver turns off. Will typically be clamped internally by the ECU at somewhere between 30-90V. Can't remember exactly what voltage Zener is used inside MEMS1.9. Without the ECU clamping it can be hundreds of volts.
  • A series of high frequency oscillations decaying back to steady battery voltage for maybe 1ms as the actual spark burns.

Repeated again for each firing event. You will get one pulse per cylinder per engine cycle, or half that per revolution. 

Just for information on VVC engines (EU2 and EU3) and all EU3 engines the tacho is driven directly by the ECU. The ECU output is a an open collector switch to ground, so there's a pull-up resistor inside the tacho and the voltage waveform is a clean square wave between close to ground and close to battery voltage.

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Grant Dryburgh

Grant Dryburgh

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Hi Andrew thanks for that info

This is the section in the manual regarding RPM input. 

6.2 RPM via a 5-50 V Square Wave Signal or coil (150-400 V) In case your engine is not managed by any ECU, MXS Strada can read the signal from the low voltage of the coil (whose peak can be from 150 to 400 V) or from a possible square wave (the peak can be from 5 to 50V).

ScreenShot2019-12-29at14_47_20.png.7e4d53c9d5f81cb1347b111f40fc08cb.png

ScreenShot2019-12-29at14_47_35.png.78db61efd45e575adac1e1f8d441917b.png

The diagram I assume is showing the rpm being picked up from Point 1: the switched negative side so is this the same as the EU2 non vvc tacho supply wire (White w/Black Tracer) ?

If so should I be seeing 1 pulse from the coil per Revolution ? therefore I need to use /1 for Rpm Factor ? 

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revilla

revilla

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Yes that's basically the same circuit so the tacho should work fine with the coil signal, but the difference is you have a distributor and the one single coil is therefore servicing four cylinders, each of which fires every second revolution of the engine. So overall you will get 2 pulses per revolution.
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revilla

revilla

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The only possible complication factor I can think of (without being able to see the manual for your dash) is whether there's an implicit factor of 2 already built into the calibration to allow for the fact that a single cylinder only fires every second revolution. Worst case setting a factor of /2 will give you a tacho that reads twice or half what it should which will be fairly obvious and you can just adjust the factor if required. It's not going to do any harm.
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