Oil Pressure Gauge not working but it's not the sender - Alfa Romeo Bulletin Board & Forums
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post #1 of 19 (permalink) Old 04-09-2019, 02:58 PM Thread Starter
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Oil Pressure Gauge not working but it's not the sender

oil pressure gauge stopped working. The warning light still works as it should so I have oil pressure. Also, grounding the wire on the gauge sender does nothing and voltage readings from the sender on a multi meter show it is sending a signal.
So what's the usual cause of this? Wiring or actual gauge? Or is this rare? Everything else in the dash cluster works as it should. I've searched and nearly every search leads to a faulty sender.
Anyone have a similar problem in the past?

thanks

bob

77 Alfetta Sedan, 87 Milano, 86 Porsche 944 Turbo, 87 VW Scirocco 16 Valve
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post #2 of 19 (permalink) Old 04-09-2019, 09:11 PM
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How hard is it to get to the gauge for some testing?

78 Spider, Webers, header, Shankle cams, bumperless, lowered, Konis
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post #3 of 19 (permalink) Old 04-17-2019, 12:18 PM Thread Starter
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Originally Posted by slowcreek View Post
How hard is it to get to the gauge for some testing?

Not that hard but just wondering if there's a known failure point for the gauge other than the sender.

bob

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post #4 of 19 (permalink) Old 04-17-2019, 01:02 PM
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Not that hard but just wondering if there's a known failure point for the gauge other than the sender.
Crappy ground at the gauge.

78 Spider, Webers, header, Shankle cams, bumperless, lowered, Konis
78 Sport Sedan, header, Spica
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post #5 of 19 (permalink) Old 04-17-2019, 04:09 PM
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The gauges are 3 wire: 12V, GND and sender. Loss of any one of the three will result in no indication.

Ed Prytherch
79 Spider
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76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #6 of 19 (permalink) Old 04-17-2019, 04:15 PM
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The gauges are 3 wire: 12V, GND and sender. Loss of any one of the three will result in no indication.
True!! Time to gain access to the back of the gauge and put your multi-meter to work. We can guide you through the process.

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post #7 of 19 (permalink) Old 04-19-2019, 10:38 PM Thread Starter
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Yeah, did all that and couldn't figure out what was what or what was supposed to be what so I put it back together still not working.

I have an extra dash cluster and that wouldn't work either. So I hooked a battery to 2 of the terminals and both gauges bounced. Then I measured the signal going into the gauge and it basically reads 5 volts with the ignition on. Then when i started the engine and gained oil pressure, the signal would change from between 5 volts to 5.75 volts in response to the pressure, I think. It was hard to tell as these crappy digital MM's react so slowly to changing voltages.

Not sure what this tells me....

What voltage should be going into the gauge? Which pins? All the testing I did was only with 2 pins at a time, both in getting the gauge to bounce (with one AA battery) and when measuring the signal input from the sender at the gauge.

How do I test the gauge properly?
How do I measure the sender signal?

All the other gauges work fine and I believe they all share a common ground so that doesn't appear to be the cause.

bob

77 Alfetta Sedan, 87 Milano, 86 Porsche 944 Turbo, 87 VW Scirocco 16 Valve
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post #8 of 19 (permalink) Old 04-20-2019, 08:01 AM
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The sender is a variable resistor. The terminal at the gauge to the sender has a voltage that is driven by the gauge and the gauge responds to the current that flows between this terminal and ground, so measuring voltage does not tell you everything.
To test a guage on the bench, connect 12 volts between the gnd and power terminals and then connect a 100 ohm resistor between signal and ground terminals and the gauge should read about 50%. Shorting signal to GND should read full scale. O psi is about 250 to 300 ohms.
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Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #9 of 19 (permalink) Old 04-20-2019, 12:31 PM Thread Starter
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Quote:
Originally Posted by alfaparticle View Post
The sender is a variable resistor. The terminal at the gauge to the sender has a voltage that is driven by the gauge and the gauge responds to the current that flows between this terminal and ground, so measuring voltage does not tell you everything.
To test a guage on the bench, connect 12 volts between the gnd and power terminals and then connect a 100 ohm resistor between signal and ground terminals and the gauge should read about 50%. Shorting signal to GND should read full scale. O psi is about 250 to 300 ohms.
So does the testing I did tell me anything? And I guess just measuring a signal from the sender will not verify that it's working correctly? How do I test the sender? I thought senders either worked or didn't.

And 100 ohm resistor?

bob

77 Alfetta Sedan, 87 Milano, 86 Porsche 944 Turbo, 87 VW Scirocco 16 Valve
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post #10 of 19 (permalink) Old 04-20-2019, 12:51 PM
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How soon do you need the resistor. I can mail one.
To test the sender you have to be able to pressurize it and measure the resistance. The resistance should fall with increasing pressure. The range is about 0 - 270 ohms.

Ed Prytherch
79 Spider
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76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #11 of 19 (permalink) Old 04-22-2019, 12:32 PM Thread Starter
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How soon do you need the resistor. I can mail one.
To test the sender you have to be able to pressurize it and measure the resistance. The resistance should fall with increasing pressure. The range is about 0 - 270 ohms.
Hmmm, so you're telling me that despite the name of this thread, my problem is probably that the sender is not working correctly? I have another in storage so let me try that first before I commit to further testing of the gauge. Thank you for the resistor offer; I may take you up on it.


I also have a strange horn problem which maybe you can help with: the horn isn't working but I can hear the relay click when pressing the horn button. Furthermore, there is a strange sound as if the horn is trying to sound but just can't, almost like a motor without enough current to spin. Strangely enough, I have an 87 Scirocco with the same exact problem which I haven't been able to solve in many years of trying. Both cars have the same exact sound when trying to sound the horn. Both cars have dual horns and both cars' horns used to work. I have checked the grounds and wiring but haven't been able to measure the current due to me being a sole car freak trying to solve this with a harbor freight MM.

bob

77 Alfetta Sedan, 87 Milano, 86 Porsche 944 Turbo, 87 VW Scirocco 16 Valve
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post #12 of 19 (permalink) Old 04-22-2019, 01:53 PM
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If you grounded the sender wire and the gauge didn’t move then you have a gauge or wiring issue, not a sender issue.

Tom

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post #13 of 19 (permalink) Old 04-22-2019, 02:19 PM
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If you grounded the sender wire and the gauge didn’t move then you have a gauge or wiring issue, not a sender issue.
Agree.

Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #14 of 19 (permalink) Old 04-22-2019, 02:22 PM Thread Starter
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Originally Posted by Gubi View Post
If you grounded the sender wire and the gauge didn’t move then you have a gauge or wiring issue, not a sender issue.
Thanks, I'll go give that a try right now.

I measured for a signal coming from the sender which I did find but after reading the posts above, I suspect that simply measuring for any signal might not accurately identify a failed sending unit. But not sure of this either....

bob

77 Alfetta Sedan, 87 Milano, 86 Porsche 944 Turbo, 87 VW Scirocco 16 Valve
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post #15 of 19 (permalink) Old 04-22-2019, 03:49 PM
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May not be your problem, but the stock Alfa supplied senders are pretty unreliable, usually cr*pping out eventually. It's a given. Did in our Milano, and several times with the 164s.

Del

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previously owned since 1964:

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