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Volvo 940 Engine speed signal

V

Vadru

New member
Joined
May 28, 2020
Hello I have some problem with my speed signal going between the ezk and the fuelbox.
From what I read it should be between 7-9v on idle, but I have only 4.8 volt.
Volatage from vr sensor to ezk is ok and I tried with another ezk box with same result.

I also have this weird thing that when I read the hz I can only do it if I connect to signal only, if I connect to ground it displays 0 hz, is this normal or is there bad connection to ground?

The problem that I have with the car is little bit rough idle, idle stays high sometimes and it seems the fuelbox thinks I’m at a lower RPM than I’m and therefore I get too little fuel around 3200-4000 rpm and then after 4000 it’s good again. I have temporarly fixed this by increasing the fuelpressure and it works, so there is not a problem to deliver enough fuel.

Problem with both ezk, or some wiring isssue, or the fuelbox? I’m clueless and need advise how to keep testing.
 
Regina or LH2.4 injection system?

If it is LH2.4 from what I understand. The speed signal goes to the speedometer where it is then sent on to the fuel computer and ezk. I would also try looking at the signal with a scope. It may be fine with your meter only reading an average of the signal.

From your description it seems like it might be intermittent loss of the signal from the axle. I would check the connection on the rear diff and the wiring going to the hole in the body.
 
It’s LH 2.4.
The signal to the ezk looks to be stable and corect voltage, also my tachometer and speedometer is stable and corect. However the signal the the ezk send to fuelbox is too low volt and when connect to ground it don’t show the hz, but when not connect to ground it had good hz reading.

What do you mean by checking cable from rear diff, isn’t that the signal to the speedometer?
 
Unfortunatly I don’t have a scope, I’m using this cheap multimeter Must L mt108t.
Do you know of a good scope that don’t cost too much?
 
From my understanding the VR sensor signal is sent to the speedometer and the analog signal is changed to a digital one. Some model years also use a signal converter to make it a digital speed signal. I also thought the vehicle speed was only used by the fuel ecu. But if it's both and one is working. Then the wiring between them is suspect. If it's a shielded cable and got damaged then you can lose signal strength from lack of shielding.

A cheap scope is a used one on ebay or a close out shop. I bought mine from about 150$ from a college that was selling it's old equipment. It's a Japanese built Leader. A typical 20 year old bench scope. I bought it for working with vacuum tubes.
 
I made a new wire between the two ecus and it didn’t change anything.
However I saw that there is 30 ohm resistance Between ground pin and chassi on the fuel ecu. Could this be enough to lower the voltage from 7-4.8?
 
Vadru said:
Hello I have some problem with my speed signal going between the ezk and the fuelbox.
From what I read it should be between 7-9v on idle, but I have only 4.8 volt.
Volatage from vr sensor to ezk is ok and I tried with another ezk box with same result.

I also have this weird thing that when I read the hz I can only do it if I connect to signal only, if I connect to ground it displays 0 hz, is this normal or is there bad connection to ground?

The problem that I have with the car is little bit rough idle, idle stays high sometimes and it seems the fuelbox thinks I?m at a lower RPM than I?m and therefore I get too little fuel around 3200-4000 rpm and then after 4000 it?s good again. I have temporarly fixed this by increasing the fuelpressure and it works, so there is not a problem to deliver enough fuel.

Problem with both ezk, or some wiring isssue, or the fuelbox? I?m clueless and need advise how to keep testing.
Click to expand...

The problem here is a common one of misunderstanding. What you read was one person's observation with a DC meter of unknown make or response to an AC voltage, not a specification. Unfortunately when in print, it looks like a specification.

Even with a scope, you would have to compare to a known working pair of ECUs to verify the signal, so it is best just to skip those items of measurement that are not properly qualified. Don't get hung up on the difference between 4 and 7 volts when the instruments being used could respond quite differently to the same pulse waveform.
 
cleanflametrap said:
The problem here is a common one of misunderstanding. What you read was one person's observation with a DC meter of unknown make or response to an AC voltage, not a specification. Unfortunately when in print, it looks like a specification.

Even with a scope, you would have to compare to a known working pair of ECUs to verify the signal, so it is best just to skip those items of measurement that are not properly qualified. Don't get hung up on the difference between 4 and 7 volts when the instruments being used could respond quite differently to the same pulse waveform.
Click to expand...

I read it from this site https://ipdown.net/jetronic.info/tiki-index.php?page=LH2.4+Pinouts+and+Diagrams
Which from what I know is reliable and when googling about engine speed volt most posts say it should read 7 or even 8+ volt. And ofcourse my cheap multimeter is not the best, but all the other values from this site match up so I think it?s pretty good?
 
But yeah maybe you are right. The thing is just that I have been trying to figure out why it don’t get enough fuel around those rpms and it would make sense if the fuel ecu thought I was running lower rpm.
The reason why I was so sure was both because of the voltage was not the same and this weird thing that the hz cannot be read when connect to ground.
And I have basicly tested everything else already. My maf reading is good, fuelpump brand new bosch 044, pumps great, no vacuum leaks, good fuel pressure, injectors are flow tested to specs etc...
 
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Do you have any ECU or EZK diagnostic codes? If not, the wiring between ECU and EZK is most likely good.

Some cheap meters have frequency/pulse width measurements ("Hz" or similar) that expect the signal to be AC and centered around 0 volts. The EZK-to-ECU rpm signal is roughly 1 to 11 volts, so the center point is about 6 volts. This may not work with some meters.

Here's what the signals look like on the benchtop with a USB oscilloscope:
EZ116-K-signals-rpm-spark-cps.png


The top trace is RPM signal from EZ116K pin 17 to LH2.4 ECU pin 1 (10V/div scaling)
The middle green trace is the spark signal to the ignition moduel, EZK pin 16 (5V/div scaling)
The bottom trace is the CPS signal from the 60-2 wheel.

For oscilloscopes, you can also get a cheap single channel pocket oscilloscope, but it can't save pictures to your PC. For example, http://www.forums.turbobricks.com/showpost.php?p=6033245&postcount=27
 
Thank you bob!
I just remembered I had one of those cheap scope laying around from some project from school, however it's been laying for quite awhile and I think at 1 time I dropped it so I'm not so sure it is working great...

As soon as I connected it to the signal it started jumping around even with the car off and then when I started the car the signal did not look smooth at all, so either the signal is bad or my scope is crap :p

Any ideas how I can test the scope?

And BTW unfortunetly I don't have any codes..
 
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That screenshot by Bobyx is from a Hantek 1008 scope. I have one I bought years ago for $80. Hopefully the firmware/software is better now. It works if you can get it out of Chinese.

I use a Picoscope at the shop. But I do most my work with a factory scan tool.

Look around. There are tons of cheap standalone and PC based scopes out there for under $200.

Edit- I see you found one.
 
I just saw now on the program there is a checkbox called "trigger" maybe I need to check this to get a better reading?
 
ZVOLV said:
That screenshot by Bobyx is from a Hantek 1008 scope. I have one I bought years ago for $80. Hopefully the firmware/software is better now. It works if you can get it out of Chinese.

I use a Picoscope at the shop. But I do most my work with a factory scan tool.

Look around. There are tons of cheap standalone and PC based scopes out there for under $200.

Edit- I see you found one.
Click to expand...

Well I'm not from USA so the problem is that shipping is so damn slow now with covid that I will probably already fix the problem or destroy the car before it comes here :rofl:
 
ZVOLV said:
I think you may need to look somewhere else for your problem. Maybe try a few different MAF's.
Click to expand...

Well when I meassure the MAF it has the corect reading at idle and goes up smoothly to 5v at around 6500 RPM so I think it should be fine?
 
MAFs are one of the sensors that can give a reading that is "within range" but the vehicle may not run correctly.
 
What I have noticed is that the problem only occures on higher gear, mostly 5 and sometimes on 4th. So maybe it is the MAF after all?
Like I said I can run with higher fuelpressure, but then I can feel the power drop on the lower gears and I get too much fuel above 4-5000 rpm.
 
ZVOLV said:
MAFs are one of the sensors that can give a reading that is "within range" but the vehicle may not run correctly.
Click to expand...

Okay so I will try with another MAF, it was just something I was hoping I didn't need to do because it's quite expensive part in my country.
 
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