Hi to all.
My digital speedometer shares an input signal with the ECU. With both connected the speedo reads properly, however it trips a fault code on the ECU. With the speedo out of circuit the ECU does not read any faults.

I measured the voltage from the speed sensor to the ECU and it measures 0.117 to 4.73v pulse (4 times per wheel rotation). With the speedo in circuit the voltage drops to 0.19 to 1.73 volts, which i assume is why the ECU finds a fault as it needs at least 4.7v.

I really dont know how to resolve this, i have looked at using an op amp to somehow compensate for the drop in voltage but cant figure it out. I need some pointers as to how I can fix this problem. Any help is much appreciated.

 

Is your digital speedo an add-on?

 

Yes, its a nordskog digital speedometer. It puts out a voltage of 0.68 on the same line to the ecu.

 

Was there some failure in the original speedo? The topic is a bit iffy here, as we are pretty serious about messing with automotive electronics. See - http://forum.allaboutcircuits.com/showthread.php?t=40361

The best solution, assuming a failure in the original, is to go to Nordskog and get a refund/unit that will not load the necessary signal. It does not seem reasonable for them to sell a device that does not function properly - loading the speed signal is classed as improper functioning.

How confident are you that you made the connections properly? Is there a spec sheet with the add-on unit? A figure stating input impedance would be quite interesting. Does the speedo hook in parallel with the speed signal (and with the ECU), or does the signal have to pass through it?

 

Was there some failure in the original speedo? The topic is a bit iffy here, as we are pretty serious about messing with automotive electronics. See - http://forum.allaboutcircuits.com/sh...ad.php?t=40361

I appreciate your concerns regarding safety and am also like minded. The original speedo was cable driven. The new gearbox which is installed comes with an electronic sender not a cable drive and there is no way to convert it back to cable drive.

The best solution, assuming a failure in the original, is to go to Nordskog and get a refund/unit that will not load the necessary signal. It does not seem reasonable for them to sell a device that does not function properly - loading the speed signal is classed as improper functioning.

Unfortunately Nordskog are no longer in business, they folded this year. The installation instruction sheet states that the speedo will work with any OEM vss equipment, however it did not say it would interfere with the ECU!


How confident are you that you made the connections properly? Is there a spec sheet with the add-on unit? A figure stating input impedance would be quite interesting. Does the speedo hook in parallel with the speed signal (and with the ECU), or does the signal have to pass through it?

The connections are correct, I do not have a detailed spec sheet for the unit only instln instructions. How can I measure the impedance?

The speedo is in series with the speed signal. There is only one output wire from the sensor and this is split to feed both the ecu and speedometer at the same time.
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There is only one output wire from the sensor and this is split to feed both the ecu and speedometer at the same time.

That puts the unit in parallel. If it were in series, the wire would be cut and led to the speedo, and there would be an output wire from the speedo that would attach to the line running into the ECU.

You might get an idea of the input impedance (resistance, really) by measuring the resistance between the input wire and ground. You probably have three wires involved - power, ground and signal.

The resistive figure is somewhat important, as it's going to take a buffer amp to isolate the speedo input from the speed signal line to prevent the loading you are experiencing. No wonder the outfit went under.

 

Ok please bear with me, I hope I am answering your questions properly.

I disconnected the speedo and measured 1.143K ohms between its input wire and ground.

From the sensor wire to ground it measures an open line ! ...however between the sensor wire and the speedo it reads 5.44 k ohms.

Does this make sense to you?

There are only two wires to the speedo. one is input and the other a common ground.

There are three wires on the sensor: chassis ground/12v in/output pulse of 0.117 to 4.73 which feeds the ecu and speedo.

 

I'd suggest attaching a multimeter to measure the current flowing into the speedometer. If it's any more than a few mA I would be concerned. A solution would be a simple buffer op-amp. You'd also need a voltage regulator and a few assorted components.

 

Thanks for all the input,
I have just measured the current.
Igntion on/wheel rotated through 1 revolution and a meter between the input and speedo: reads 0.01 ma.

Where do I go from here?

 

Hmm that's odd. I can't see any way the unit is sinking current. What mode did you have it on? mA, µA, 10A?

Well, since this idea will either work or it won't, you could wire up a simple buffer amplifier, which duplicates the signal on the output. If you want, I could draw the schematic up. As I said, you would need some components, you will also need a breadboard or a stripboard to build it on and a soldering iron...

 

Its really confusing, I just re-measured the current on the 10a setting

it reads 0.002amp on DC.

I re-measured the voltages again!

Speedo out of circuit: +0.117 to +4.73.
Speedo in circuit: minus(-) 0.573 and + 4.08.

As you can imagine, I really am elated with the attention being given this. Your offer is much appreciated.

 

Its OK for there to be a discrepancy in the different ranges with a meter, even expensive meters suffer from this. For example if you measure 10.5 volts on the 20 volt range you may get 10.57 volts but on the 200 volt range you may get 10.6 volts and on the 1kV range you could even get 11 volts.

You live in Canada so parts shouldn't be too tricky. Any major electronics stores nearby? I'll have a schematic up in a few minutes so you know what to look for.

Also, are you trying to say the fault is intermittent? This could be indicative of a fault in the unit, such as a bad solder joint, which is repairable and is easier than making a circuit up.

 

I do not believe the fault is intermittant. I get very consistant results. i.e. speedo functions normally with the ecu either in or out of circuit, and the ecu functions normally with the speedo out of circuit. Its only when they are coupled that the ecu throws the fault code.

And yes, there is a major electronics store nearby,and my soldering gun is warming up!

 

Ok, see attached circuit. Post here if you need some help.

Components list:

• U1: Any 7808 +8V voltage regulator, 78L08 ideal (small size)
• U2: Any LM358 type or compatible dual op-amp (if choosing compatible op-amp make sure output can go to ground)
• Z1: BZXC18 or equivalent 18V zener diode, 500mW rating.
• Z2: BZXC6V8 or equivalent 6.8V zener diode, 500mW rating.
• C1: any 470 microfarad 25 volt (or 35 - 50 volt) capacitor.
• C2: same as C1.
• R1: 10 ohm 1/4W resistor 5% tolerance carbon or metal film
• R2: same as R1 but 47 ohms
• MISC: Stripboard or breadboard to build it on, solder

The auto environment is a very dangerous place for electronics, SgtWookie says you can get +60V spikes on the +12V line and I'd be inclined to believe him. For that reason, I have included a lot of protection on the inputs: R1, R2, Z1 and Z2 all play a part in protection as does U1 to some extent.

 

What can I say, thanks so much for your help. This has been a real problem for me for the past few months, and obviously I came to the right place for help.

I will put this circuit together and post my results once its completed.

Thanks once again.

 

Great... keep us posted.

I and I'm sure everyone else here is happy to help...

 

To Tom66,
I guess your'e wondering why i have not posted for such a long time! I have just moved into a new home in another part of Canada and am still in the throes of unpacking. I did get around t building the op amp before i moved but have yet to test it out on the car, which by the way is currently buried in a pile of boxes. As soon as i get sorted I will let you know the results.

Again my apologies for not posting sooner.

 

To Tom66,
I guess your'e wondering why i have not posted for such a long time! I have just moved into a new home in another part of Canada and am still in the throes of unpacking. I did get around t building the op amp before i moved but have yet to test it out on the car, which by the way is currently buried in a pile of boxes. As soon as i get sorted I will let you know the results.

Again my apologies for not posting sooner.

Thanks for the update. Keep us posted.

 

Tom66
Hi again, I tried the circuit today. Unfortunately it did not work as expected. With the board in place the VSS did not trip a fault light while driving, however I have no speedo reading, it shows 0 kph.

Is there anything else I can do?

 

Remove Z2.
Replace R2 with a 10k Ohm resistor.

The LM358 is a dual opamp. You need to connect the output of the side you're using to the non-inverting (+) side of the unused side, and the output of the unused side to the inverting (-) input of the unused side. Otherwise, it may oscillate and cause problems.