How to check Speed sensor is working through the BCM on automatic VR 1993

[commvr93]

Hi,

I have an automatic holden VR 1993 model. I had a problem for a while with the speedo on the instrument cluster. I replaced the instrument cluster but still have the problem.

On my previous two threads in this matter I checked everything to make sure that there are now cables or connections cut or severed.

I am now in a situation where I am doubting the Body Control Unit is not sending a signal to the speedo on the instrument cluster. How can I check that?

The speedo is connected through two cables 1) Violet/White coming from the ECU, 2) Grey coming from the BCM. Look at diagram page 261 of the Gregory service manual.

All other functions connected to and from the BCM are working fine.

[88GreenVN]

Have you replaced the speed sensor? - I'm guessing yes...

[Cheap6]

If you have a multimeter with a frequency function or even a tach. function, backprobe the terminals for each of the two signal wires in turn, referencing them to earth with the other multimeter lead. With the car rolling you should get a varying frequency signal from each of them. Backprobe with a dressmaker's pin slid, carefully, between the terminal and terminal connector.

[commvr93]

Have you replaced the speed sensor? - I'm guessing yes...

Hi,

I did not replace the speed sensor for the following reasons:-

1) As suggested by other experienced members I did the code testing from the ECU. I got 12 all the time. I kept the coding test going for 10 minutes and still got code 12. Which means that the computer is not reading any wrong signal from the speed sensor.

2) The car is an automatic transmission system. It drives beautifully even without the speedo working. If there is something wrong with the speed sensor it will not change gear smoothly. There is somehting wrong beyond the ECU. It is between the ECU and the instrument panel. This is why I am suspecting now the BCM ( Body control module).

Please correct me if I am wrong.

[commvr93]

If you have a multimeter with a frequency function or even a tach. function, backprobe the terminals for each of the two signal wires in turn, referencing them to earth with the other multimeter lead. With the car rolling you should get a varying frequency signal from each of them. Backprobe with a dressmaker's pin slid, carefully, between the terminal and terminal connector.

Hi,

I do not have a multimeter with a frequency or tacho measuring. I have two multimeters one is an old analogue and the other is a basic digital one.

I think I did the test you have suggested on one of the wires which is the V/W coming from the ECU. This is what I did " I backprobed the V/W wire to the positive terminal on my analogue mutimeter and the earth to the negative terminal on the multimeter. With the car running I got a voltage over 12 maybe around 14.5 volts. It sayed for few seconds and dropped back to zero. I switched off the car, took the ignition key out and retried the same test with me driving the car around the block. It gave me the same reading as before and dropped back to zero."

I did not try it on the other wire, the Grey one coming from the BCM. I rather wait to hear from you again.

[seraphic06]

The ECU is not perfect, it doesn't always log a code. Change the sensor to at least eliminate it as a problem.

[delcowizzid]

the gearbox would be in limp home mode and change gears erratic if the sensor was buggered

[Cheap6]

Hi,

I do not have a multimeter with a frequency or tacho measuring. I have two multimeters one is an old analogue and the other is a basic digital one.

I think I did the test you have suggested on one of the wires which is the V/W coming from the ECU. This is what I did " I backprobed the V/W wire to the positive terminal on my analogue mutimeter and the earth to the negative terminal on the multimeter. With the car running I got a voltage over 12 maybe around 14.5 volts. It sayed for few seconds and dropped back to zero. I switched off the car, took the ignition key out and retried the same test with me driving the car around the block. It gave me the same reading as before and dropped back to zero."

I did not try it on the other wire, the Grey one coming from the BCM. I rather wait to hear from you again.

It's really not a great idea to be using an analogue meter on electronic stuff as they can draw enough current to damage things. At best you will probably get an erroneous reading and it's likely that when connected to a circuit they will alter what it is you are trying to measure anyway. The GM-H specs. are that any meter should have at least 10M Ohm internal impedance for accurate, safe readings.

What you did is about it for the test, just not with a suitable meter. You will be able to get something suitable for < $50, with frequency function. Whether or not you think that is worth it...

You might try using the digital meter (>= 10M Ohm, yes? ) backprobing voltage - it might give you a random sample of the voltage pulses to give you something that changes with speed - maybe try it at a few constant speeds and average the readings for each of those speeds - but I haven't tried that on a speed sensor ECU output. If it's a square wave, and I think that it is, that's not going to work unless you average it over time.

There will also be a speed sensor input to the PCM somewhere too (I don't have a VR wiring diagram ATM) but as delcowizard has said and you have concluded, there would probably be other symptoms with that absent.

[88GreenVN]

Hi,

I did not replace the speed sensor for the following reasons:-

1) As suggested by other experienced members I did the code testing from the ECU. I got 12 all the time. I kept the coding test going for 10 minutes and still got code 12. Which means that the computer is not reading any wrong signal from the speed sensor.

2) The car is an automatic transmission system. It drives beautifully even without the speedo working. If there is something wrong with the speed sensor it will not change gear smoothly. There is somehting wrong beyond the ECU. It is between the ECU and the instrument panel. This is why I am suspecting now the BCM ( Body control module).

Please correct me if I am wrong.

Fair enough - dont sound like the speed sensor is the problem hey.

[commvr93]

Hi,

Thank you for your information. I will try with the digital meter first. If it does not work I will go to J-Carr and look for a frequency guager/meter. One thing I did not test as yet, is the circuit continuity of the grey wire coming from the BCM to the connector at the back of the instrument cluster.

I will let you know how it goes. Thanks again.

[commvr93]

Hi,

It makes sense to me too. The car is driving smoothly and the gears change normally. There is no erratic movements in the gear.

[Dangerous Dave]

find someone with a good scan tool / tech-2 and go for a drive with it plugged in. will show you all the sensors working as you drive.

will even show you each wheel speed individually if you have ABS...