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Oil pressure gauge wiring question.

Bmachine

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I just installed a new VDO oil pressure gauge in the spot where the clock used to be. So I’m using the positive wire from the clock to go to the positive of the oil gauge. One problem there, obviously, is that the gauge is now powered all the time. So I will have to use a switched power source instead.

But in the meantime my question is that whenever I turn the battery on (not the motor) the needle goes almost all the way up to 5 bar. Shouldn’t the gauge stay at zero until oil pressure is being sent from the sensor to the gauge?

The sensor is new and has one output for oil pressure gauge and the other for the idiot light. The idiot light portion works just fine, so I’m assuming the sensor itself is fine. I have not turned the engine on yet just in case this is an indication that something is wrong.

Any thoughts?

Bo.
 

Bmachine

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OK, I believe I have found the problem. Going backwards from the gauge I discovered that the sender shows 14 ohms at the gauge port. I believe with the engine off it should be an open line instead of low resistance as it is now. So I think the sender is busted. It was supposedly a new one. Interestingly I couldn’t find any VDO stamping on it. Maybe that’s one hint right there.

PS: Those guys are a real pain to get to. Especially with the Motronic harness right in the way.
 

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Dick Steinkamp

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I had the same problem with a non VDO sender with both idiot light and gauge connections (Porsche). I ended up using 2 senders. The stock one for the idiot light and a VDO one for the gauge.
 

arnie

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Bo,

14 Ohms is quite close to what it should read at 0 bar.

VDO says: 10 Ohms at Zero pressure.

So a new sender won't solve your specific problem imho.

What's abour the gauge itself ? Maybe a ground wire problem ???
 

Bmachine

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So... very little resistance when no oil pressure, meaning almost full voltage at needle and more resistance as pressure builds, meaning less voltage. I’m probably way oversimplifying things. But seen from this angle, it feels a bit counter intuitive.
 

arnie

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Yes, you are a bit too simple with this.

These kind of instruments working as voltage devider. So the measurement is done in the middle of 2 resostors. One of them is inside the gauge (now I'm a bit simple ... ) and the other is the sensor, givi g resistance in regards to the actual pressure.

Take a look at VDO tables for the different kind of sensors.
 

Bmachine

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Thanks Arnie. Very helpful stuff. So according to that link, a standard 5 bar sensor should measure as follows:

Widerstandskennlinie 5 Bar Druckgeber:

0 bar - 10 Ohm
1 bar - 48 Ohm
2 bar - 82 Ohm
3 bar - 116 Ohm
5 bar - 184 Ohm

So maybe my sensor is good after all.

The plot thickens....
 

jmackro

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Bo:

Both your gauge and sender may be good, but the two may not be meant to work together. I know that in the Alfa world, early cars work like the chart you posted above (e.g., low resistance at low pressure, high resistance at high pressure) while late gauges work the opposite way (I may have this backward but you get the idea). VDO has been making electric gauges for a long time and in many variations - is it possible that some VDO gauge/senders work in the low Ω = low pressure / high Ω = high pressure manner while some are the opposite? So perhaps you are intermixing a gauge of one type with a sender of the other type.

If your gauge displays 5 bar at 14Ω, what does it display with the gauge powered, but the wire pulled from the sensor (i.e. at infinite Ω)? If it displays 0 bar, that sort of suggests that it wants a sender that works the opposite way. A better test might be to try it with a fixed resistor of ~ 200Ω (top of the range posted above). A simpler test would be starting the engine and seeing if the gauge goes down as oil pressure goes up.

I'm just conjecturing here - for all I know, all VDO senders work the same way.
 
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Luis A.

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Cool little analog device. What they look like inside:

Gauge/variable resistance side:

IMG_3894.JPG


Warning light side, seen at 0 pressure closing the contact at its start of travel:
IMG_3895.JPG
 

autokunst

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Sorry, this is adjacent to the topic at best. but does anyone remember the mechanical/hydraulic oil pressure sensors/gauges? I had put those in a few of my early cars (70's muscle cars). In hind sight, it seems like a bad idea to run a small pipe/tube with active engine oil under pressure through the firewall and into the interior of the car. But it worked - without impedance translation.
 

Bmachine

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Sorry, this is adjacent to the topic at best. but does anyone remember the mechanical/hydraulic oil pressure sensors/gauges? I had put those in a few of my early cars (70's muscle cars). In hind sight, it seems like a bad idea to run a small pipe/tube with active engine oil under pressure through the firewall and into the interior of the car. But it worked - without impedance translation.
Isn't evolution fascinating?
Mechanical oil pressure gauge -> oil pressure gauge by wire -> transmission by wire -> drive by wire -> Don'tDrive by wire.
;)
 

Bmachine

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Bo:

Both your gauge and sender may be good, but the two may not be meant to work together. I know that in the Alfa world, early cars work like the chart you posted above (e.g., low resistance at low pressure, high resistance at high pressure) while late gauges work the opposite way (I may have this backward but you get the idea). VDO has been making electric gauges for a long time and in many variations - is it possible that some VDO gauge/senders work in the low Ω = low pressure / high Ω = high pressure manner while some are the opposite? So perhaps you are intermixing a gauge of one type with a sender of the other type.

If your gauge displays 5 bar at 14Ω, what does it display with the gauge powered, but the wire pulled from the sensor (i.e. at infinite Ω)? If it displays 0 bar, that sort of suggests that it wants a sender that works the opposite way. A better test might be to try it with a fixed resistor of ~ 200Ω (top of the range posted above). A simpler test would be starting the engine and seeing if the gauge goes down as oil pressure goes up.

I'm just conjecturing here - for all I know, all VDO senders work the same way.
Excellent thoughts as usual, Jay. I'm going to check on which guts the gauge uses.
 

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autokunst

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Drive by wire? No thank you. We won't even allow an automatic transmission in our stable. All/only 3-pedal cars! Sometimes a bit of devolution is in order. :cool:
Isn't evolution fascinating?
Mechanical oil pressure gauge -> oil pressure gauge by wire -> transmission by wire -> drive by wire -> Don'tDrive by wire.
;)
 

Bmachine

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Bo:

Both your gauge and sender may be good, but the two may not be meant to work together. I know that in the Alfa world, early cars work like the chart you posted above (e.g., low resistance at low pressure, high resistance at high pressure) while late gauges work the opposite way (I may have this backward but you get the idea). VDO has been making electric gauges for a long time and in many variations - is it possible that some VDO gauge/senders work in the low Ω = low pressure / high Ω = high pressure manner while some are the opposite? So perhaps you are intermixing a gauge of one type with a sender of the other type..
I just checked my records and the oil pressure gauge came from a Porsche 928. Does anyone know if those function “backwards”?
 

Luis A.

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I just checked my records and the oil pressure gauge came from a Porsche 928. Does anyone know if those function “backwards”?
They don’t. The 928/944 sender has the resistances as outlined in a post above. Low resistance(10-15 Ohms)=low pressure(0) and high pressure (5bar)=~180-200 Ohms.
 
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