Engine/Transmission (1998.5 - 2002) Map Sensor Problem...

[JR2]

I got my new OBD2 software and cable and this let me watch the operation of my MAP sensor. Well watching it has led me to believe that it is bad. I monitored it at idle and got a reading of 58-59 in HG (which is about 29 psi). Then I drove around and watched it... . same reading. Then I unhooked the boost fooler on the Comp box and plugged the map right in and got the same reading... . hmmmm I says. So then I plugged into my Dad's truck, his reads 29 in Hg (about normal for AK). So I guess I have a problem. My truck has set no codes other than the couple I caused unhooking the map to set a code I could try to clear. . So I am off to cummins NW to try and get a sensor. .



The funny thing is that the truck runs very well and has been getting decent mileage. I thought it woud throw an overboost code if the press was past 21 or at least defuel..... maybe the boost fooler is hiding the bad sensor. I don't know... but I will report back when I find out.



JR2

 

[Jetson]

Do any of the readings change while you are watching them? I wonder if it is a software problem. The MAP signal should be one of the more variable signals.

 

[Bigramguy]

I was thinking the same thing. It could be the refresh rate on the scanner software is too slow. If you are observing many parameters at once,the refresh rate slows WAYYYY down.



Did you take a reading with the map disconnected ? If the MAP reading changes,it may be a bad MAP. If not,maybe a software problem with the scanning software ?

 

[JR2]

The readings change but not by much... I just went and got a very small power inverter so I can power my lap top in the truck (batt is toast) so I can monitor it while driving. It still seems strange that it reads 28 psi at idle and my dads reads 15 psi at idle (I know the 15 is atmospheric press as the MAP measures absloute press). I will go for a short test drive while hooked up and see what happens. Be back in 20 minutes.





John





By the way, Cummins NW has to order the Map for me..... they just called and said the factory does not have any... but it may be here tomorrow...

 

[JR2]

Well I just got back for a drive across town. The map sensor read from 60 to 75 in Hg. That correlates to 29 - 36. 8 psi, with the 29 being at idle. The difference is about 7 psi which is about the average boost I run when driving around town, but I did a couple of almost WOT runs that had 25+ psi durning the run and those do not even show up. The graphing feature on the software I have will not graph the MAP output because it says the parameter is out of range.

I unplugged the Comp box (this un plugs the map sensor and I got a reading of 29 in hg which is correct, but there is no sensor there. I also just turned off the comp box and that made not diff.



I have a new sensor coming overnight from Cummins NW and assuming it gets here tomorrow I will report any change I see... .



JR

 

[icarus33]

If you're using the OBD-II software from Alex Peper, you may not have a real problem with your MAP. I have seen some very unbelievable readings on the pressure that when converted to psi make no sense. They do not correspond even close to my boost gauge and I would believe the gauge first. Don't know what the issue is, but realize you may be chasing a ghost. Update with what you find out.



ic

 

[StakeMan]

Mine says the same thing. There is not 58 in hg in Denver.

 

[JR2]

What I don't understand is that it reads correctly on my Dad's 98. 5 but will not read right on mine. I am still getting a new MAP sensor and we will see what happens. I may have a chance to try the software on a truck that also has Road Relay 4 so then we can compare with the Alex Pepper software sees and what Road Relay sees... . I will let you know. .





John

 

[Little Bill]

John,



MAP sensor output voltage changed in '01 when they switched to absolute pressure. . Most ODB software has not been updated to account for this and they read boost as imdicated boost + 14. 7 psi. At idle, the old software will show 29-30 inches Hg. The new sensors put out 1 volt per atmosphere. At idle they output 1 volt. At 14. 7 psig they output 2 volts.

 

[JR2]

Little Bill - at idle I show 59 in Hg, which is 30 psi... that is 15 over atmospheric??

 

[StakeMan]

TTT



Did you change the map sensor yet, JR2?

I rechecked mine today it said 54 inhg(26 psi) with key on and motor not running. I will change it today and see what happens.

 

[JR2]

No I did not get it changed... . Cummins is having a hard time getting me the part as they did not have one in AK and the Cummins NW wearhouse in Wash did not have one either. I have them shipping it next day air so maybe it will get here today.



John

 

[StakeMan]

Got a new map sensor today. No change in the readings.



Anybody need a new map sensor?:-laf

 

[JR2]

I got a new one too... $86 at Cummins... . I did not get it installed but I am guessing that I will see the same as Stakeman... I wonder what the real story is... Maybe Marco could shed some light on how the ecm looks at boost... as far as I can tell the engine thinks its getting 15 psi boost a idle.



I am off moose hunting for the weekend so I will install the new sensor on Monday and see what happens.

 

[Little Bill]

Check the output voltage from the MAP. If it is ~ 1 volt at idle and 2 volts at 14. 7 psig then it's OK and the ODB software is wrong.

 

[StakeMan]

TTT



Change it yet, JR2?

 

[Huff N Puff]

WATCH OUT FOR OBDII DATA

All you guys out there checking data from the OBDII connecter. The program has a task manager built in to the system. When data changes from what it last read that dosn't coincide with what it is presently reading,it will substitude in the last known good value,and estimate from known good sensors, and ignore the sensor it thinks, is faulty. Until it has evaluated the sensor for a few drive cycles. If it continues to test bad, by the PCM,it will then turn on the check engine light. If the data from the sensor tests OK within PCM paramatters it wont turn on the check light but will set a pending code if in 40 warm ups it dosn't detect another code it will erase the code. It will run the system by estimating from other known good sensors. To test the sensors you need to look at actuall voltages in the wiring with a lab scope or 1million ohm impedance volt meter. The systems are pretty intelligent so be careful with the data that dosn't make sense. Also be careful after you have made repairs,and you are evaluating your fixes,cause the computer will evaluate the repair for awhile before putting the sensor backon line. You will need to erase codes or disconnect the battery for a while. Hope this helps Merv

 

[JR2]

Stakeman - Nope, I went moose hunting and then did not get a chance due to weather and my lack of a garage to work in. Now I am at work for a couple of weeks and then I am off to Mexico for a week so sometime towards the end of next month I will get to it. After you found what you did I don't expect any change.



Mervs post above is interesting. I can see the values changing for the sensor but they are all too high. Anyway I suspect that the ECM knows what to do or I would have set all kinds of codes and such. I will post here once I get it fixxed.

 

[JR2]

Well I have not changed the MAP sensor yet but I did check two more trucks of various years and they seem to have the problem too. . One of the trucks has a road relay 4 on it and the road relay reads correct, just the OBD2 stuff does not. Still not sure about the problem. I emailed Edge but never got a reply. . I am going to post my problem in there forums and see what happens.