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We have a 2011 Chrysler 6.1L Hemi SRT. We can not get any diagnostic reading at the DLC or OBDII port. We have checked all the wiring, fuses etc.

Our readings at the DTC connector are as follows: CAN bus Hi pin 6 2.09 V, CAN bus Low pin 14 2.75 V. Shouldn't this Hi pin read the high voltage and both together equal 5 V?

Measured the resistance at the CAN bus, with battery disconnected; we are getting 45.6 Ω between pin 6 and 14. This should measure 60 Ω. Can not work out why it is ready that low, has anyone seen this before?

Seen a similar question on here, but it was never answered.

winny
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Antony
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    "*This should measure 60 ohms.*" Why? Because of the termination? It doesn't have to be super precise. There could also be three 120 Ohm terminations. Then again, you aren't getting a signal. – DKNguyen Feb 15 '22 at 04:28

4 Answers4

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Our readings at the DTC connector are as follows: CAN Bus Hi pin 6 2.09 V, CAN Bus Low pin 14 2.75 V?? Shouldn't this Hi pin read the high voltage and both together equal 5 V?

You probably measured the line voltages with a DMM in DC voltage measuring mode. The common-mode voltage of the bus is 2.5V and the bits are represented by 1V above or below this level, depending on the dominant or recessive state of the bus lines. And since the data rate is way faster than the sampling rate of your DMM, it's not surprising to see that your DMM is showing some random values around 2V.

Measured the resistance at the CAN bus, with battery disconnected; we are getting 45.6 Ω between pin 6 and 14. This should measure 60 Ω.

You are assuming that there should be one 120R resistor at both ends of the line. This is mostly the case in automotive but is not that a critical requirement because the distances between the nodes and the central computer are relatively low.

Two things came to my mind:

  • There might be a third 120R resistor connected somewhere across the bus, making the effective DC resistance 40 Ohms. Adding the wiring and the connection resistances to this can give around 45 Ohms.
  • One of the transceivers might be dead so this can make the effective DC resistance lower than 60 Ohms.

Again, this is a car and the distances are relatively low. So the accuracy of the termination resistance is not that critical. If we were talking about hundreds of meters at 250 kBaud then the termination accuracy or tolerance would be a thing to consider. Don't misunderstand. This doesn't mean that the termination is not important here.

Rohat Kılıç
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  • I'm just trying to get some sort of communication from the dtc/ obd2 as I cant tune nor adjust anything, it throws lights but cant diagnose anything very frustrating. I only done these measurements from watching other people do it. It is just a means of troubleshooting but still stuck against a wall. Has anyone got anything other than what I should be reading either multimeter or oscilloscope? This vehicle obviously has an electrical issue even auto electrical technicians have given up on it. I want to solve this problem can anyone suggest anything other than readings etc. – Antony Feb 15 '22 at 22:20
  • @Antony Instead of an oscilloscope, you could connect a USB-to-CAN Analyzer. – Velvel Feb 15 '22 at 22:35
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Our readings at the DTC connector are as follows: CAN bus Hi pin 6 2.09 V, CAN bus Low pin 14 2.75 V. Shouldn't this Hi pin read the high voltage and both together equal 5 V?

You can't measure this accurately with a plain multimeter, you must use an oscilloscope. It's a high speed digital pulse signal. The "high" in CAN High means that it goes high=3.5V in case of a dominant bit. Whereas CAN Low goes low=1.5V in case of a dominant bit.

Either signal idles at 2.5V. In case the bus load is low, both signals would measure close to 2.5V if measuring incorrectly using a multimeter. You won't get 2.09V at CANH ever and you won't get 2.75V at CANL ever. You are measuring this the wrong way or you are using the wrong tool (again, use an oscilloscope).

Measured the resistance at the CAN bus, with battery disconnected; we are getting 45.6 Ω between pin 6 and 14. This should measure 60 Ω. Can not work out why it is ready that low, has anyone seen this before?

The most common reason for that would be that there's voltage present when you are measuring. But it's also not that important to have exactly 60 ohm on most vehicle applications since they have relatively short distances. I often plug in my CAN listener lab stuff onto an existing terminated CAN bus and then I could be adding 1 or even 2 additional 120R terminators. Out of experience, I (non-scientifically) know that going below 60 ohm is rarely a problem (assuming some <20 meters of bus) but going above it could be. One would have to dissect a CAN transceiver in-depth to tell the reasons why, but that's another story.

Lundin
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  • I'm just trying to get some sort of communication from the dtc/ obd2 as I cant tune nor adjust anything, it throws lights but cant diagnose anything very frustrating. I only done these measurements from watching other people do it. It is just a means of troubleshooting but still stuck against a wall. Has anyone got anything other than what I should be reading either multimeter or oscilloscope? This vehicle obviously has an electrical issue even auto electrical technicians have given up on it. I want to solve this problem can anyone suggest anything other than readings etc – Antony Feb 15 '22 at 22:21
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I've experienced this problem, the reason you are seeing 45ohms instead of 60 is because the probe device you are using has its own 60 terminator resistor enabled, 3 120 ohm resistors in parallel are 40 ohms, more than likely it means there's an extra termination somewhere in the system, the 120 terminations should always be at the ends of the important components such as ECU and CPU, I've seen a couple of Ford engines come with an extra 120ohm termination in the wire harness. Check your ODB device to make sure the termination resistor is disabled. Generally the Can bus pins are complete opposite of each other in terms of signals, 2.5V is considered central ground, or working ground. so In low can line you will see negative signals and in the Hi can line you will see positive can signals.

Davide Andrea
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TimWolf
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  • I'm just trying to get some sort of communication from the dtc/ obd2 as I cant tune nor adjust anything, it throws lights but cant diagnose anything very frustrating. I only done these measurements from watching other people do it. It is just a means of troubleshooting but still stuck against a wall. Has anyone got anything other than what I should be reading either multimeter or oscilloscope? This vehicle obviously has an electrical issue even auto electrical technicians have given up on it. I want to solve this problem can anyone suggest anything other than readings etc. – – Antony Feb 15 '22 at 22:20
  • now I've never tried this before, so disclaimer on trying it, but from the ford engines I've worked with a couple of times, one 120ohm termination is in the ECU, if you remove the ECU based on the datasheet you should theoretically be able to see the can lines and measure the termination to be at 120ohms, and then also use the same connector to measure the terminations on the rest of the can line from the ECU line, I'm not sure what happens if you disconnect the ECU from the wire harness, assuming that you know to remove the battery when working on it, but it should give you access – TimWolf Feb 16 '22 at 08:44
  • You do really need an Oscilloscope to see the signal integrity, I would however try to find out where the termination fault is occurring or how it even occurred in the first place. if this is a off the shelf build I would recommend looking up the software that can turn off a termination, that should give you an idea if there's 3 terminations in the lines. – TimWolf Feb 16 '22 at 08:46
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I get exactly the same reading on a fiat obd and the reason is that the obd is wired differently. Pin 6 is the slow speed hi Pin 14 is the slow speed low Pin 2 is the high speed hi Pin 10 is the high speed low. Disconnected battery pin 6 to 14 is 48 Ohms Pin 2 to 10 is 61 Ohms. Check with oscilloscope. Pin 6= 2v to 5v Pin 14= 4v to 0v Pin 2= 2.5v to 3.5v Pin 10 =2.5v to 1.5v

You are seeing the slow speed Canbus not canbus low! Very little on the Internet re the slow network. Hope that helps.

Keith Ian Spencer
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