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Factory oil temp sensor in a RS4V without gauges?

Chi-Town
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Chi-Town Contributor

Chi-Town

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So like the title says my 97 rs4v has a oil temp sensor but no triple gauges as it was originally factory navigation.

My current setup is not using the factory oil pressure sensor as I'm running a link ecu and a real sensor. 

I did leave the oil temp sensor in. So now as I was working on first start up and bleeding coolant I get the oil light on the dash faintly as the oil temp gets to about 60-70c. Oil pressure is 45psi at idle at the same time. 

I'm wondering if the temp sender doing this?

 

Duncan Grand Master

Duncan

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The pressure light likely has a different sender to a pressure gauge, its been too long for me to remember for sure. Just have a look for another sender down near the oil filter.

If you are confident in your rewired pressure sender reading correctly, you can be pretty sure it is just the dash light's sender failing, it is a pretty common thing.

Chi-Town Contributor

Chi-Town

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The factory oil pressure sender is no longer in the car that's what is confusing me.

In the Taarks adapter I have an aftermarket Bosch style pressure/temp sender and the factory temp sender only. Oil pressure is perfect.

Where does the factory oil temp sender go to if there were never any gauges? Why was it there from the factory?

GTSBoy Grand Master

GTSBoy

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2 hours ago, Chi-Town said:

Why was it there from the factory?

To plug the hole. The engine plant may not have known whether the car it was going into had a gauge or not. It was a long time ago and the integrations might not have been fully modern. Or they might not have cared because the extra inventory and processes to save a few cents on the sender might have cost more anyway.

But please tell me you are not still confusing the idea of a pressure gauge sender, and an oil pressure light switch. The switch will be out there. In a separate hole. Probably with only one wire running to it. Running the light.

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Chi-Town Contributor

Chi-Town

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The original pressure sender (switch) had the standard connector and the temp sender had the single terminal connector also.

The pressure switch is gone and those wires eliminated from the harness. 

I left the temp wire as I too used the temp sender to plug the extra hole.

I was hoping for an easy answer but I guess I'm going to need to trace some wires to see where my ghost oil pressure light voltage is coming from or maybe I'll just pull the bulb out as there's no pressure switch any longer.

Duncan Grand Master

Duncan

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I see what you posted, but how could a warning light be going on and off if there is no sender for it?  The only other explanation is that the wire that used to go to the sender from the warning light is earthing somehow in the loom or at the connector, or that you have connected the warning light wire to something (maybe the temp sender?).

 

jdniss Contributor

jdniss

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On 11/06/2025 at 5:20 AM, Chi-Town said:

My current setup is not using the factory oil pressure sensor as I'm running a link ecu and a real sensor. 

 

On 14/06/2025 at 9:22 AM, Chi-Town said:

..or maybe I'll just pull the bulb out as there's no pressure switch any longer.

Slightly off topic - I've just wired oil pressure sensor into my LinkECU as well - I tapped into the existing dash cluster oil pressure light wiring, and fed that back into an Auxiliary output in the ECU.

That now lets me control the original dash oil pressure light based on a range of conditions triggered by the LinkECU - means the factory light isn't entirely redundant now that the factory oil pressure switch is no longer installed.

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GTSBoy Grand Master

GTSBoy

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And yet....OEMs put both guages and lights into cars, fed by separate primary elements, for a reason. Oil pressure sensors fail at a significantly higher rate than oil pressure switches. When the HMI has 2 outputs, each with its own separate input, the redundancy makes it far more likely that the operator will notice a problem - particularly if one of those systems has a fault.

I'll keep my oil pressure switch feeding the dash light, thanks.

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  • 2 weeks later...
Chi-Town Contributor

Chi-Town

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On 15/06/2025 at 4:14 AM, MBS206 said:

Sounds like the oil pressure light is wired into the oil temp sensor...

As it warms up, resistance on temp sensor reduces, hence more current can flow and the dash light gets brighter.

This is kind of what I was thinking but the temp sender wire and the two pressure switch wires run through the starter subharnes and I eliminated the two pressure switch wires completely. 

@GTSBoy I have a can gauge with unusually bright warnings should the oil pressure fall so the factory light isn't needed.

I need to dig out my wiring diagram and see if I can sort this out.

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