Ford Mustang Forums banner

1 - 11 of 11 Posts

·
Registered
Joined
·
103 Posts
Discussion Starter #1
The problem I cannot figure out is how the ground on my oil pressure sensor can go from 0.3 ohms when the car is off and jump to 6.5 ohms, when I turn the key on? I’m measuring the ground from the back of the connector on the gauge to a grounding bus bar at the driver’s side kick panel area.

This came up because my autometer, electric full sweep, oil pressure gauge was reading 0 with the engine running. I assumed it was a bad sensor so I pulled it out and decided to measure the voltages and resistance, with the sensor sitting in my hand. The resistance to ground was a high 38 ohms on the ground wire and the supply was a correct 5 volts, but surprisingly the output voltage was 0.70 or so, which is correct. I held an air pressure nozzle to the end and the output voltage went up to around 3 volts, so I guess the sensor is fine – until I touched the sensor to ground – then the sensor’s voltage dropped to 0.40 volts. I’m really confused.

Now it’s possible the sensor, or the gauge really is bad, but why would the ground resistance change with the key on?
Thanks for any help,
Howard
 

·
Registered
Joined
·
23,245 Posts
First, ignore any ohm readings, they are useless, unless your are looking at a PTC, NTC or some thermistor or RTD.
 

·
Registered
Joined
·
23,245 Posts
Is this a 3 or 4 wire transducer?
 

·
Registered
Joined
·
23,245 Posts
what part # do you have?
 

·
Registered
Joined
·
4,970 Posts
It's an autometer 4352 Oil Pressure which uses the 2246 sensor.
It sounds like you might have a bad ground connection, but you cannot read resistance in a circuit with voltage applied. The only way any resistance reading is meaningful is with all power OFF.

The Autometer ground wire grounds via the gauge black wire. That wire should go to a GOOD solidly connected body shell ground connection.

If you want to check the gauge ground with the key on and power applied, you have to measure VOLTAGE from the ground pin to the vehicle chassis. Any voltage over a small fraction of a volt indicates a ground problem.

You could have a bad sensor. Measure the ground pin resistance to the sensor shell without any power and nothing else plugged in. Just the sensor itself.
 

·
Registered
Joined
·
23,245 Posts
Does the needle auto calibrate on every power up?

That is like a bulb test for you, if you do NOT see the full sweeping upon power up, something is up.
 

·
Registered
Joined
·
4,669 Posts
Is there a way to test the gauge like the short sweeps by grounding the signal wire and seeing if the gauge maxes out?

Also what did Autometer say on how to test? What are you using for grounding points for the gauges themselves? I've had lots of electrical issues over the years and a lot of them were faulty connections (both ground and power) more often than a failed component.
 

·
Registered
Joined
·
103 Posts
Discussion Starter #10
Thanks alot guys for the help, I really appreciate it!

The Autometer ground wire grounds via the gauge black wire. That wire should go to a GOOD solidly connected body shell ground connection.
Yes, I have several of the gauges grounded together to a 12 gauge wire and routed to a grounding bus bar at the driver's side kick panel area. I know you said resistance without power is meaningless, but I measured the resistance of only the bare connector to the bus bar and it's 0.3 ohms.

If you want to check the gauge ground with the key on and power applied, you have to measure VOLTAGE from the ground pin to the vehicle chassis. Any voltage over a small fraction of a volt indicates a ground problem.
When I measured the back of the powered gauge (I had to use paperclips to access the connectors rear terminals), there was no voltage to ground for neither the gauge ground nor the sensor ground. Oddly, I noticed that if I touched the paperclip, there was a slight voltage - around 0.17V.

You could have a bad sensor. Measure the ground pin resistance to the sensor shell without any power and nothing else plugged in. Just the sensor itself.
There is no resistance/continuity between either of the 3 pins to the shell, when the sensor is sitting my hand with nothing connected.

Does the needle auto calibrate on every power up?
Yes, the needle makes a full sweep and then goes directly to 0.

Is there a way to test the gauge like the short sweeps by grounding the signal wire and seeing if the gauge maxes out?

Also what did Autometer say on how to test?
When I hold the sensor in my hand (connected), I can pressurize the gauge with my air compressor and the gauge responds fine. Holding pressure steady and just touching the shell to an engine bracket (ground), the gauge immediately goes to 0. - strange.

Autometer just wants me to send everything in.
 

·
Registered
Joined
·
23,245 Posts
sounds like the transducer is short to the casing at the 5 volt supply

you know you can get a 0-10v voltage generator for a couple hundred dollars.....

you can then see if the gauge works.....as well as anything on the 5 volt ref circuit of the car.
 
1 - 11 of 11 Posts
Top