OBDLink

[jag]

Apparently there are different ways to read the PID'S, or I'm just not getting it.

I thought WIKI was gospel.

 

[Me78569]

The PIDS just are not there.  take Boost for example, I read it from the canbus string in a PNG.  It's where it should be and makes sense, BUT say baro, which is in the same PNG is not reported.

 

The slot is there but no info is reported to it.

[Mopar1973Man]

9 hours ago, jag said:

This is a very interesting subject, & the knowledge is outstanding,

But what I don't understand is that the PID's are all relating to something. For instance, say the boost reading is in a PID group under pressure's, so why can't we read the rest of the headers under the pressure PID group like trans gov pressure, or oil pressure?

Same goes for temps, if we can read IAT & ECT, why can't we read the rest  of the PID'S under say the temp PID group. I myself would love to see the Injection pump fuel temp. 

I see on Mads they use PID groups, PID short name & PID long name, but don't use numbers.

OK, now I'm really lost!

 

 

When you manually call the PID's from the OBDII port the ECM and PCM report exactly what information is available. Remember this is only for smog reasons. Also remember the classification of our truck is "NON-OBD Compliant" (05 Hex) So the rest of the data does not have to exist for public use. 

[woodtrucker]

Spacehiker, I play guitar (20 yrs) and I'm a recording engineer (by hobby) and understand your logic. That's called making the room flat or tuning the room by sweeping frequencies 20-20k hz and notching with an eq. It's usually corrected bc the room generates unwanted noise at certain frequencies. 

These guys were using some formula's to try and convert from abs pressure to gage pressure with no luck.

For me to understand the problem (so I can ask one of these smart people I work with at Virginia Tech to help us, is the issue in that the map reading the scan tool is giving us in correct for a conversion to gage pressure? I would like to think that some formula should be able to convert this to psi for us. I have no idea what it should be but I could ask some smart math people to help us.

 

 

 

[Mopar1973Man]

Problem is its actually a 2 byte value. The MAP sensor is capable of seeing up to 40 PSI. So you have to work with A and B byte value and figure out to do the math on it. The PID standard only sees 1 byte value so this why it terminates premature. 

[woodtrucker]

Could you combine a 2 byte value to one with a formula you think? Like an if, then, type thing? 

If pressure reaches x value then use B byte value? 

[Mopar1973Man]

Got to use a outside program like OBDLink or similar that can work with two bytes formulas. @Me78569 managed to figure out the formula for the early series MAP sensors but never quite got the later series quite right. Offset numbers are different and range of bytes is different too.

[Me78569]

@Mopar1973Man 

 

My truck reports map values as a 1 byte string.  So long as the max value is under 255 in whatever format it is you shouldn't need more than 1 byte.

 

 

To read a 2 byte string you would just do

 

A + (B * 256)

Edited January 14, 2017 by Me78569

[Mopar1973Man]

maybe I goofed... Now I've got to look at the tablet and see what I played with last...

[Me78569]

Yea, if the ecm reprots boost in KPA you would be allowed 255 in a 1 byte value, which translated to psi would be 72.xx 

 

(kpa * 2) / 7 = psi.

 

It's actually 6.93492728 or something like that but you get the point

[woodtrucker]

On 1/14/2017 at 10:04 AM, Me78569 said:

@Mopar1973Man 

 

My truck reports map values as a 1 byte string.  So long as the max value is under 255 in whatever format it is you shouldn't need more than 1 byte.

 

 

To read a 2 byte string you would just do

 

A + (B * 256)

 

 

I tried this formula and MAP just read 0 the whole time. I'm wondering if that was because I had something else set up wrong though. 

 It should have output something...hmm

[Me78569]

I very much doubt the truck uses 2 bytes. It is prob ((a * 2) /7) - 11 for map

[woodtrucker]

ok. thanks. I'll give that a try!

[SpaceHiker]

Sorry for the disappearance, but what I'm finding with the map sensor, at least on the early ones, is that it doesn't register anything below standard atmospheric pressure. Apparently Cummins doesn't care if we're at 14,000 feet, the sensor will read sea level anyway and consider boost to be 0. If any of you have had your trucks at 14,000 feet, let the engine cool off and try to restart it, you know how badly these trucks run up in the rare air anyway, at least until they can build boost. You literally have to drive one foot on the throttle and one on the brake to get some boost going before they will run right.

 

So readings start at 100kpa which is approximately the average sea level pressure, then go up from there as boost builds. This makes sense if all Cummins is trying to do is measure relative boost above standard pressure. It's actually the same thing our gauges do, as I described previously.

[Me78569]

10 minutes ago, SpaceHiker said:

Sorry for the disappearance, but what I'm finding with the map sensor, at least on the early ones, is that it doesn't register anything below standard atmospheric pressure. 

That's not what I am finding.  

 

When I read boost from the ECM I see it come off "bottom" right away.  It might send boost over obd differenly, but canbus at least seems to read boost at 0+ at idle regardless of altitude,  same is true when I take my truck to sea level.  My truck is a 00 tho so maybe there is a ambient pressure sensor being used where as 98-99 dont?

 

late trucks however seem to read boost differently and I cannot explain it yet. I will know more in the summer when temps come up again.

[SpaceHiker]

9 minutes ago, Me78569 said:

That's not what I am finding.  

 

When I read boost from the ECM I see it come off "bottom" right away.  It might send boost over obd differenly, but canbus at least seems to read boost at 0+ at idle regardless of altitude,  same is true when I take my truck to sea level.  My truck is a 00 tho so maybe there is a ambient pressure sensor being used where as 98-99 dont?

 

late trucks however seem to read boost differently and I cannot explain it yet. I will know more in the summer when temps come up again.

 

That's very interesting because I see the opposite. My truck will read 100-102 for a long time before it starts picking up. That might be the difference between the early sensors and the late sensors. It seems I might have bought the wrong truck for high altitude operation lol. But after 9 years I guess I'm stuck with it.

[Me78569]

My truck uses a early sensor though....  

 

All I can say is our trucks electronics are very much a testing groudn for dodge /cummins to figure out the Electic injection control system on a diesel.  

[SpaceHiker]

Well, since I've taken it upon myself today to solve the mystery of my truck's poor performance on 2-stroke oil (see cumminsforum), as a side effect I solved the mystery of the boost formula.

 

The correct boost formula that works 100% accurately at any elevation is: (A/6.8947)-14.6

 

This means that in any OBD software, you need to add a custom PID for boost. For the mode and pid, enter 010b or 01 0b (depending on your software's format). For the minimum value enter 0, for maximum value enter 100 or 60 or whatever you want (22 suffices for my stock truck lol). For equation, enter the above formula: (A/6.8947)-14.6

Then for decimal point precision, just use 1 decimal point.

 

This formula does not depend on atmospheric pressure at all. And in fact, the whole discussion about atmospheric pressure is completely overblown because it really only concerns our trucks when they are at idle. You have to keep in mind the function of a turbocharger. It is by it's nature a perfect altitude compensator and will always try to achieve a certain equilibrium within it's mechanical limits. This means that as soon as we begin applying fuel with a press of the throttle, intake pressure comes up above standard atmospheric pressure anyway. It just takes slightly longer at 10,000 feet than it does at 2,000 feet.

 

For the above reason, and since I'm at 9,240 feet, I see my boost sit at 0.0 for a little while before it starts coming up. That time where it's at 0.0 is when the turbo is reaching 14.6psi. It's a very short period of time, but it is noticeable at my elevation (lasts about 2 seconds after applying throttle). My mechanical boost gauge shows the same thing.

 

Hopefully that clears up some of the issues that have been bubbling in this thread for a while. Just keep in mind that my truck is a 98.5, I don't know if the newer trucks report a different data format on 010b, but I can't see why they would.

Edited February 3, 2017 by SpaceHiker

[Me78569]

Intersting to say the least.  The ecm reports boost as 2 bytes per kpa over canbus not 1 byte per kpa.  Good work.  

[SpaceHiker]

Oh yes, then there's that conversation. I forgot to mention that one. The people who were discussing that were making some important mistakes at various points in that conversation:

 

1. Some were thinking the ECM handles boost as psi rather than kpa.

2. Pretty much everyone assumed the ECM handles map readings as an integer, when it is almost certainly floating point.

3. People were confusing the canbus with obd.

 

So on the canbus, the ecm is taking raw voltage readings from the output of the map sensor and converting them to a floating point (decimal) number in kpa. What we see on obd is the rounded output, because decimal precision is not necessary and the obd spec doesn't have the space for it.

 

Logically then you would need 2 bytes on the canbus for the floating point value and 1 byte on the obd for the rounded integer.

Edited February 4, 2017 by SpaceHiker