oil pressure sensor question

[madmaninshed]

Hi, We have fitted an oil pressure sensor advertised as compatible with VDO gauges to an S3 with 2.0 duratec engine. The car has a VDO oil pressure gauge. We checked the wiring on the car and it is correct. We wired the sensor as per instructions supplied. When the ignition is turned on, the gauge reads high (over 10)and when the engine is running it moves from over 10 to 2 bar approx. Is the resistance the wrong way for our gauge please? We went for this sensor as the advert states "VDO". Thanks for any suggestions. Cheers, Paul.

 

[Paul Deslandes]

when you say you wired the sensor as per the instructions, what did they say? How many terminals does it have? The standard VDO sender just has a single terminal and, as far as I can remember, the resistance of the sender, measured between the terminal and the case, increases with rising pressure. If your sender has more than one terminal, maybe there are options for connecting it. Can you post a link to the unit you bought.

[Stationary M25 Traveller]

Usually with a single wire sender, if the gauge goes to full scale with the ignition on, there is a fault or mis-connection to earth ?

[madmaninshed]

Thanks for the replies. Sorry, I can't remember how to do a link....here is the item:

http://www.ebay.co.uk/itm/271093748262?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

[Paul Deslandes]

looking at the website here it looks as though you may have bought the US spec unit which goes from 240 to 30 Ohms with increasing pressure, i.e. in the wrong direction. If you have a multimeter, check the unit's resistance with the engine not running. If its 240 Ohms then that's your problem. If its about 30 Ohms then it should work although the reading may be in error. I'd put my money on 240 Ohms. I think the correct sensor spec is 10 to 180 Ohms.

[madmaninshed]

Thanks Paul, I measured between the 2 terminals and once the dial settles down it reads 0. I tried 3 different meters. The unit was completely disconnected.

[Paul Deslandes]

A reading of zero on the Ohms range means its a short rather than open circuit so the unit is either faulty, an oil pressure warning light switch (which would be closed with zero or low pressure) or the instructions are wrong and you should be reading between one of the terminals and the case.

 

The ebay details say that you should connect it up using the two terminals, rather than using the case as the earth but it might be worth measuring between each of the terminals and the case.

 

I assume you're setting the multimeters to measure low resistance, e.g. 200 Ohms range.

 

If all else fails I'd be inclined to contact the seller so he can check that he's sold you the right unit. As said in earlier post, VDO make gauges to work with senders that vary their resistance in either direction. If its the Caterham VDO gauge then I'm pretty certain that they require a sender that increases its resistance with oil pressure. I've got one in the garage and try and test it a bit later to be sure that I'm correct on that point, and let you know.

[madmaninshed]

It turns out that it is a faulty sender, and the very nice people who supplied it will refund. We have decided to go mechanical. Thank you very much for all the help, which is much appreciated. Almost ready for the rolling road at last!!!!!

[Paul Deslandes]

glad you've resolved it. Going mechanical is a good idea and far more reliable as these senders appear to fail with monotonous regularity. I used a Racetech combined pressure and oil temperature gauge, which doesn't look out of place on the 7 dash, with the temperature sender plumbed into the dry sump oil tank.