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Old 05-28-2020, 01:29 PM   #1
Vadru
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Default Volvo 940 Engine speed signal

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.
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Old 05-28-2020, 02:30 PM   #2
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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.
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Old 05-28-2020, 02:44 PM   #3
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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?
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Old 05-28-2020, 02:51 PM   #4
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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?
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Old 05-28-2020, 03:34 PM   #5
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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.
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Old 05-28-2020, 04:26 PM   #6
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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?
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Old 05-28-2020, 07:29 PM   #7
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Quote:
Originally Posted by Vadru View Post
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知 at a lower RPM than I知 and therefore I get too little fuel around 3200-4000 rpm and then after 4000 it痴 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知 clueless and need advise how to keep testing.
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.
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Old 05-28-2020, 07:53 PM   #8
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Quote:
Originally Posted by cleanflametrap View Post
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.
I read it from this site https://ipdown.net/jetronic.info/tik...s+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痴 pretty good?
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Old 05-28-2020, 08:00 PM   #9
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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...

Last edited by Vadru; 05-28-2020 at 08:17 PM..
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Old 05-28-2020, 09:40 PM   #10
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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:


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/sh...5&postcount=27
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Old 05-29-2020, 05:06 AM   #11
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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..

Last edited by Vadru; 05-29-2020 at 05:13 AM..
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Old 05-29-2020, 05:09 AM   #12
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Here is a snap of the reading:

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Old 05-29-2020, 05:13 AM   #13
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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.
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Old 05-29-2020, 05:15 AM   #14
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I just saw now on the program there is a checkbox called "trigger" maybe I need to check this to get a better reading?
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Old 05-29-2020, 05:16 AM   #15
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Quote:
Originally Posted by ZVOLV View Post
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.
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
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Old 05-29-2020, 06:20 AM   #16
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I think you may need to look somewhere else for your problem. Maybe try a few different MAF's.
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Old 05-29-2020, 06:27 AM   #17
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I think you may need to look somewhere else for your problem. Maybe try a few different MAF's.
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?
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Old 05-29-2020, 06:43 AM   #18
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MAFs are one of the sensors that can give a reading that is "within range" but the vehicle may not run correctly.
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Old 05-29-2020, 06:44 AM   #19
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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.
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Old 05-29-2020, 06:47 AM   #20
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Quote:
Originally Posted by ZVOLV View Post
MAFs are one of the sensors that can give a reading that is "within range" but the vehicle may not run correctly.
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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Old 05-29-2020, 06:56 AM   #21
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I think you are looking in the wrong directly currently. The engine speed circuit obviously is working because the car is running.

The manuals were written for these cars before modern test equipment was widely available. The test procedures may be outdated.
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Old 05-29-2020, 07:38 AM   #22
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I think you are looking in the wrong directly currently. The engine speed circuit obviously is working because the car is running.

The manuals were written for these cars before modern test equipment was widely available. The test procedures may be outdated.
I知 not following any manual, I was just looking at the documentation made by ipdown about the ecu and saw that the volt was lower than he specified, however now I知 not sure about it since my multimeter is a cheap one.
Ofcourse the car runs, however if the voltage really is lower then maybe the fuel ecu is seeing it as a lower rpm than it is? The ezk is still getting the corect signal, so ignition timing will be right, just that the amount of fuel calculated by fuel ecu is MAF table x rpm table, not exactly simple math like this, but you get what I mean and below 3000 rpm it uses the lambda control, so it caps at around 14.7 AFR, but after around 3200 rpm AFR goes up above 15 and then above 4000 it gows down to 11.
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Old 05-29-2020, 08:18 AM   #23
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Basically my thought process is that since fuelpressure and injectors are good it can only be maf or engine speed signal and since the maf seemed to have corect value and the engine speed has too low votage, if my mutimeter is corect. Then I assumed engine speed signal is wrong. But I have ordered a maf now anyway so we will see next week if that solves the problem.
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Old 05-29-2020, 08:24 AM   #24
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MAF's are one of the sensors that you can't really test without just trying another one. They can read "in spec" but still be skewed enough that the mixture will be off. Especially on these old EFI systems.
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Old 05-29-2020, 11:57 AM   #25
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Today when I was driving my sister she asked me what’s making all that noise and I said it’s just my fuelpump and then it occured to me it is pretty loud. Maybe a little too loud and maybe it means I have an air leak in the fuel system. However this is a heavy duty fuelpump after all and it sits directly under the pedals, so it’s quite noisy.

Here is a video of the sound, what do you guys think?
https://streamable.com/qi2av4

Another thing that happened today was that the car didn’t want to start and I had to give it a little gas for it to start, like it didn’t wanna rev up. But then it ran normally.
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