Speed sensor output Voltage
Discussion
Hi everyone,
Just been testing a few of the sensors on the car and the speed sensor output is a little confusing. The bible (which is geat BTW) specifies a 0-12V output swing as the car moves. The output from my sensor is approx 0.25 - 4.5V possibly indicating either a faulty sensor or the fitting of a sensor with a 0-5V output. The ignition is on and the connector plug disconnected from the ECU. The oscilloscope output looks fine apart from the low voltage.
I guess that it may also require the ECU to be connected to 'Pull' the voltage up to the 12V rail.
The car has been de-catted (by the previous owner), could this 0-5V output be possibly part of the mods?
Has anyone else seen a 0-5V output from the speed sensor?
I have also noticed that the fueling resistor is the wrong value for a non-cat car but I assume this is taken care of by the ECU chip change.
Thanks for any help guys,
Gav
Just been testing a few of the sensors on the car and the speed sensor output is a little confusing. The bible (which is geat BTW) specifies a 0-12V output swing as the car moves. The output from my sensor is approx 0.25 - 4.5V possibly indicating either a faulty sensor or the fitting of a sensor with a 0-5V output. The ignition is on and the connector plug disconnected from the ECU. The oscilloscope output looks fine apart from the low voltage.
I guess that it may also require the ECU to be connected to 'Pull' the voltage up to the 12V rail.
The car has been de-catted (by the previous owner), could this 0-5V output be possibly part of the mods?
Has anyone else seen a 0-5V output from the speed sensor?
I have also noticed that the fueling resistor is the wrong value for a non-cat car but I assume this is taken care of by the ECU chip change.
Thanks for any help guys,
Gav
De catting does nothing with the speedo. If the tune resistor is wrong then it is possible that the chip change was not carried out. Quite common for cats to be removed without doing the full job!
Normally the sensor is fed with 12 volts and switches between it and ground. The ground side normally goes to the ECU and I suspect is grounded via a resistor to limit current. It is possible that it needs that connection to switch correctly. I have to say it doesn't sound right as the unit is switching a 12 volt supply!
Having said all that, what is the problem you are trying to address? If the speedo is not working at all I would suspect you have a reason. Also what speed are you turning the wheels as some sensors are very much speed dependent.
Normally the sensor is fed with 12 volts and switches between it and ground. The ground side normally goes to the ECU and I suspect is grounded via a resistor to limit current. It is possible that it needs that connection to switch correctly. I have to say it doesn't sound right as the unit is switching a 12 volt supply!
Having said all that, what is the problem you are trying to address? If the speedo is not working at all I would suspect you have a reason. Also what speed are you turning the wheels as some sensors are very much speed dependent.
Cheers for the info Steve,
With regard to the ECU chip, it does have non-cat written on the EPROM so I guess the de-catting has been reasonably performed.
Is the resistor normally changed during the de-catting process?
I have noticed the speedo jumping occasionally recently, but in general it has been OK during the time I have had the car. Guess I had better get under the car and see what voltage the sensor is being supplied with.
I was only pushing the car backwards and forwards so it could be a speed related problem. I know that some speed sensors will only output 5V. It may be possible that an incorrect sensor has been fitted but has enough output to just operate the speedo. Do you know the input switching levels for the ECU?
Thank you for your help,
Gav
With regard to the ECU chip, it does have non-cat written on the EPROM so I guess the de-catting has been reasonably performed.
Is the resistor normally changed during the de-catting process?
I have noticed the speedo jumping occasionally recently, but in general it has been OK during the time I have had the car. Guess I had better get under the car and see what voltage the sensor is being supplied with.
I was only pushing the car backwards and forwards so it could be a speed related problem. I know that some speed sensors will only output 5V. It may be possible that an incorrect sensor has been fitted but has enough output to just operate the speedo. Do you know the input switching levels for the ECU?
Thank you for your help,
Gav
As you've already said there are apparently two speed sensor outputs 0 - 12 V and 0.5 - 5 V, how you tell whats what I haven't a scooby, but I'd be interested to know as mine still plays up 2% of the time (sstill catted btw).
I'm not sure how you're off for room on the chim but on the S I found it easier to access the the sensor plug from above (didn't want to drop the exhaust either
).
One thing I have noticed though, is that I think my problem is that where the sensor wires comes out of the box (LH side directly below the reversing switch and gear selector for the rover box) they do a 90 deg turn immediately to avoid the chassis, but only by about 5/10mm. So that under full load and or bad road surfaces the slight twisting of the engine and box bangs the wires against the chassis, well thats my theory anyway, I'll have another look when I'm finished with the more important jobs on the car. btw mine was reading 12v (I think
or that could have been the reversing switch that I was chasing at the time ). I'll let you know next time I playing 
.
Good luck.
Harry
I'm not sure how you're off for room on the chim but on the S I found it easier to access the the sensor plug from above (didn't want to drop the exhaust either
). One thing I have noticed though, is that I think my problem is that where the sensor wires comes out of the box (LH side directly below the reversing switch and gear selector for the rover box) they do a 90 deg turn immediately to avoid the chassis, but only by about 5/10mm. So that under full load and or bad road surfaces the slight twisting of the engine and box bangs the wires against the chassis, well thats my theory anyway, I'll have another look when I'm finished with the more important jobs on the car. btw mine was reading 12v (I think
or that could have been the reversing switch that I was chasing at the time ). I'll let you know next time I playing . Good luck.
Harry
My Speedo stops reading from time to time but not too often but would like to sort it if possible. I presume the sensor on the diff must be working or it would not work at all and the connections there are good.
Any ideas what happens between the speed sensor and the back of the Speedo. Has anyone got as far as checking the connection to the back of the Speedo and if so how, looks like a nightmare to get to?
Ivan
Any ideas what happens between the speed sensor and the back of the Speedo. Has anyone got as far as checking the connection to the back of the Speedo and if so how, looks like a nightmare to get to?
Ivan
The sensor is supplied with a 12 volt supply according to the wiring diags I have and I cannot see any sign of a 5 volt supply or converter....
If you are only pushing the car then you are lucky to get any reading at all so that could easily explain why you are only seeing a few volts. The sensor spec not only refers to the output voltage but also the input voltage and if it was a 5 V unit it wouldn't last long when 12 volts is applied and then nothing would work. The 520 uses a special static type for the traction control and instrumentation that trigger when metal is within the gap and they have a built in light to set them up.
If the speedo is jumping it is more likely to be in the head unit or an intermittent connection somewhere which means whipping that dash out and getting to the connections behind and hope that the intermittent fault is co-operating. Not a lot of fun especially if the connection is not co-operating.
>> Edited by shpub on Wednesday 12th February 11:25
If you are only pushing the car then you are lucky to get any reading at all so that could easily explain why you are only seeing a few volts. The sensor spec not only refers to the output voltage but also the input voltage and if it was a 5 V unit it wouldn't last long when 12 volts is applied and then nothing would work. The 520 uses a special static type for the traction control and instrumentation that trigger when metal is within the gap and they have a built in light to set them up.
If the speedo is jumping it is more likely to be in the head unit or an intermittent connection somewhere which means whipping that dash out and getting to the connections behind and hope that the intermittent fault is co-operating. Not a lot of fun especially if the connection is not co-operating.
>> Edited by shpub on Wednesday 12th February 11:25
The sensor is supplied with a 12 volt supply according to the wiring diags I have and I cannot see any sign of a 5 volt supply or converter....
If the speedo is jumping it is more likely to be in the head unit or an intermittent connection somewhere which means whipping that dash out and getting to the connections behind and hope that the intermittent fault is co-operating.
If the speedo is jumping it is more likely to be in the head unit or an intermittent connection somewhere which means whipping that dash out and getting to the connections behind and hope that the intermittent fault is co-operating.
Hi Steve,
I've had the same problem as Ribol. My speedo stops working sometimes and returns back to 0mph reading and stay there for a period before the speedo works again. It's happening a lot more frequently recently, I've tried cleaning the connector at the back of the speedo but without any luck. Could there be a bad connection from either the ECU or sensor or could it be something more serious. Can't plead innocence when pulled over by the police.
Many thanks in advance.
I've had the same problem as Ribol. My speedo stops working sometimes and returns back to 0mph reading and stay there for a period before the speedo works again. It's happening a lot more frequently recently, I've tried cleaning the connector at the back of the speedo but without any luck. Could there be a bad connection from either the ECU or sensor or could it be something more serious. Can't plead innocence when pulled over by the police.
Many thanks in advance.
onny said: Hi Steve,
I've had the same problem as Ribol. My speedo stops working sometimes and returns back to 0mph reading and stay there for a period before the speedo works again. It's happening a lot more frequently recently, I've tried cleaning the connector at the back of the speedo but without any luck. Could there be a bad connection from either the ECU or sensor or could it be something more serious. Can't plead innocence when pulled over by the police.
Many thanks in advance.
If it is intermittent it is a swine to fine. There is no magic fix on this. Could be a bad connection or a broken wire somewhere or a dead sensor or a dead speedo. Could try making sure the earth connections are good but apart from that and assuming it is a problem in the speedo or sensor is to replace/recondition them as the fault may be internal.
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