Oil pressure gauge faulty?
12-18-2021, 03:48 AM
#16
Drifting
Fred, when I rewired the whole front of an 86 32V, I was surprised to see the oil pressure sender worked backwards to most others. That is to say, open circuit from the sender gave full scale deflection, closed circuit [grounded] gave '0' deflection.
Your GTS will have a 3 wire sender though and I don't know if they work the same?
Your GTS will have a 3 wire sender though and I don't know if they work the same?
12-18-2021, 05:40 AM
#17
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Thread Starter
Fred, when I rewired the whole front of an 86 32V, I was surprised to see the oil pressure sender worked backwards to most others. That is to say, open circuit from the sender gave full scale deflection, closed circuit [grounded] gave '0' deflection.
Your GTS will have a 3 wire sender though and I don't know if they work the same?
Your GTS will have a 3 wire sender though and I don't know if they work the same?
The 3 wire sender was introduced with the digi dash models in MY1989- why they felt a need to do this I do not have a clue. All I can say is that the 2 wire sender used in MY87 and MY88 works perfectly well and the sender variables are the same. I did a pressure calibration test some years ago and I struggle to remember the details but as I recall the resistance increases with pressure as Adrian suggests in his post but I am far from certain about that. That is not to say your MY should be identical given it may well be different spec item strange as though it would seem. Unfortunately I do not have a copy of PET for your model year to check against.
In the process industries it is quite the norm to have reverse acting transmitters for safety reasons depending on what is being monitored. I also understand there is some feature about the VDO gauges in the dash panel that can cause them to drive full scale if there is an earth fault condition and that can happen with the 3 pin gauge. The extra pin is an "insulated earth" and is what might be expected to be used if the starter motor had a separate earth to the battery which is not the case with the 928 as all models are earthed through the engine and the strap that sits behind cylinder No4.
Someone like Alan may well understand the insulated earth concept but for sure I do not- i just know of its existence presumably brought about by a perceived need to avoid false earths corrupting the signal value. With a new 3 pin transmitter and new wiring my dash panel gauge was under reading oil pressure by about 1 bar- when I changed out the sender to a 2 pin type the reading immediately went from what was previously reading 4 barg to 5 barg and then the gauge display reading is pegged [maxxed out] at that value- in reality the actual pressure may well be higher especially when the engine is cold and then the actual value will be somewhere between 5 barg and the pressure release valve set pressure of 8 barg.
12-18-2021, 11:32 PM
#18
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Mike,
The 3 wire sender was introduced with the digi dash models in MY1989- why they felt a need to do this I do not have a clue. All I can say is that the 2 wire sender used in MY87 and MY88 works perfectly well and the sender variables are the same. I did a pressure calibration test some years ago and I struggle to remember the details but as I recall the resistance increases with pressure as Adrian suggests in his post but I am far from certain about that. That is not to say your MY should be identical given it may well be different spec item strange as though it would seem. Unfortunately I do not have a copy of PET for your model year to check against.
In the process industries it is quite the norm to have reverse acting transmitters for safety reasons depending on what is being monitored. I also understand there is some feature about the VDO gauges in the dash panel that can cause them to drive full scale if there is an earth fault condition and that can happen with the 3 pin gauge. The extra pin is an "insulated earth" and is what might be expected to be used if the starter motor had a separate earth to the battery which is not the case with the 928 as all models are earthed through the engine and the strap that sits behind cylinder No4.
Someone like Alan may well understand the insulated earth concept but for sure I do not- i just know of its existence presumably brought about by a perceived need to avoid false earths corrupting the signal value. With a new 3 pin transmitter and new wiring my dash panel gauge was under reading oil pressure by about 1 bar- when I changed out the sender to a 2 pin type the reading immediately went from what was previously reading 4 barg to 5 barg and then the gauge display reading is pegged [maxxed out] at that value- in reality the actual pressure may well be higher especially when the engine is cold and then the actual value will be somewhere between 5 barg and the pressure release valve set pressure of 8 barg.
The 3 wire sender was introduced with the digi dash models in MY1989- why they felt a need to do this I do not have a clue. All I can say is that the 2 wire sender used in MY87 and MY88 works perfectly well and the sender variables are the same. I did a pressure calibration test some years ago and I struggle to remember the details but as I recall the resistance increases with pressure as Adrian suggests in his post but I am far from certain about that. That is not to say your MY should be identical given it may well be different spec item strange as though it would seem. Unfortunately I do not have a copy of PET for your model year to check against.
In the process industries it is quite the norm to have reverse acting transmitters for safety reasons depending on what is being monitored. I also understand there is some feature about the VDO gauges in the dash panel that can cause them to drive full scale if there is an earth fault condition and that can happen with the 3 pin gauge. The extra pin is an "insulated earth" and is what might be expected to be used if the starter motor had a separate earth to the battery which is not the case with the 928 as all models are earthed through the engine and the strap that sits behind cylinder No4.
Someone like Alan may well understand the insulated earth concept but for sure I do not- i just know of its existence presumably brought about by a perceived need to avoid false earths corrupting the signal value. With a new 3 pin transmitter and new wiring my dash panel gauge was under reading oil pressure by about 1 bar- when I changed out the sender to a 2 pin type the reading immediately went from what was previously reading 4 barg to 5 barg and then the gauge display reading is pegged [maxxed out] at that value- in reality the actual pressure may well be higher especially when the engine is cold and then the actual value will be somewhere between 5 barg and the pressure release valve set pressure of 8 barg.
I'm a big fan of earth pin senders after needing Loctite thread sealant on one years ago and destroying the earth integrity. Sealing washer type do much better than tapered thread type.
12-19-2021, 03:07 AM
#19
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One presumes Porsche went the way they did for a good reason. I reverted to the earlier approach out of frustration because I could not make the 3 pin sender work and some of our proessional friends advised the older system works which in my case turned out to be spot on. The 3 pin system uses a "rubber bullet" of a connector that was not available at the time- whether or not that was a factor I do not know. I used a red size crimp connector to hold the wire- the other end slipped on tightly over the pins and has worked well for other listers.
Both Porsche sender types used on S4 and GTS models deploy a taper thread as I am aware so the metal to metal earth interface "should" be sound even if it is not needed on the insulated earth models..
Last edited by FredR; 12-19-2021 at 03:08 AM.
12-19-2021, 08:09 PM
#20
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Mike,
One presumes Porsche went the way they did for a good reason. I reverted to the earlier approach out of frustration because I could not make the 3 pin sender work and some of our professional friends advised the older system works which in my case turned out to be spot on. The 3 pin system uses a "rubber bullet" of a connector that was not available at the time- whether or not that was a factor I do not know. I used a red size crimp connector to hold the wire- the other end slipped on tightly over the pins and has worked well for other listers.
Both Porsche sender types used on S4 and GTS models deploy a taper thread as I am aware so the metal to metal earth interface "should" be sound even if it is not needed on the insulated earth models..
One presumes Porsche went the way they did for a good reason. I reverted to the earlier approach out of frustration because I could not make the 3 pin sender work and some of our professional friends advised the older system works which in my case turned out to be spot on. The 3 pin system uses a "rubber bullet" of a connector that was not available at the time- whether or not that was a factor I do not know. I used a red size crimp connector to hold the wire- the other end slipped on tightly over the pins and has worked well for other listers.
Both Porsche sender types used on S4 and GTS models deploy a taper thread as I am aware so the metal to metal earth interface "should" be sound even if it is not needed on the insulated earth models..
So, off came the 3 wire S4 unit.
