1995 OBDI MAP sensor limits?

[WarWagon]

What is the limit of the boost that the GM 6.5 MAP sensors can read? I appear to be stuck at around 17 PSI +- 1 PSI. Is this the MAP sensor limit, computer or both?

 

[THEFERMANATOR]

It's only supposed to be able to read 2 bars, so 29.4 absolute(atmospheric + boost). If your at sea level they can only reliably read to 14.7 since atmospheric is 14.7.

 

[buddy]

That makes sense. But I have seen mine hit over 20psi calculated boost before, I would use it because my Banks gauge only goes to 15. Although that could be from a high MAP reading or low Baro reading. I wish I had taken snapshots on the scanner. So I asked a GM computer designer and he also said it was set up to read 2 bar of boost over atmospheric. Of course that didnt make sense why it would call for a 2 bar sensor. I would go test that now to verify, but the IP is failing and GM4 wastegate wont close all the way anymore, and I dont drive it normally since last Fall with diesel prices so high in CA. Got to fix it sometime though.

 

[Slim Shady]

They make three bar map sensors, but will the program (tuning) be able to use the information from the higher boost reference numbers?

 

[THEFERMANATOR]

A 1 bar map reads vacuum and atmospheric, a 2 bar will read reliably 29.4 absolute, and a 3 bar will read 44.1 absolute. They can read higher, but not and be accurate. There was some mention that GM used 2.5 bar map sensors in the 6.5, but I'm pretty sure when I crossed the number it came out as a 2 bar.

 

[Slim Shady]

A 1 bar map reads vacuum and atmospheric, a 2 bar will read reliably 29.4 absolute, and a 3 bar will read 44.1 absolute. They can read higher, but not and be accurate. There was some mention that GM used 2.5 bar map sensors in the 6.5, but I'm pretty sure when I crossed the number it came out as a 2 bar.

http://www.robietherobot.com/storm/mapsensor.htm

how to identify the gm map sensors and this for the discriminating buyer

http://www.auberins.com/index.php?main_page=product_info&products_id=150


http://www.autozone.com/autozone/ac...-268u?itemIdentifier=891284&_requestid=119491

Can our programming use the added information ?

 

[buddy]

It may be possible to adjust the PCM's voltage to pressure table.

 

[Slim Shady]

It may be possible to adjust the PCM's voltage to pressure table.

Would it make a difference, to have the extra data now the way the pcm voltage to pressure table is?

 

[buddy]

Unless using the PCM to control the boost, it wouldnt make any difference except for data logging or using the sensor as a check to your gauge.

 

[Slim Shady]

Then it is a waste of money to get a three bar map sensor.

 

[THEFERMANATOR]

Then it is a waste of money to get a three bar map sensor.

If running stock tuning, going to a 3 bar map allows you to run higher boost without codes or defueling. If you wanted the ECM to control boost still, but go higher than 2 bars then a 3 bar map would work great as well. I know I'm in the minority as I liked the vacuum system.

 

[Slim Shady]

If running stock tuning, going to a 3 bar map allows you to run higher boost without codes or defueling. If you wanted the ECM to control boost still, but go higher than 2 bars then a 3 bar map would work great as well. I know I'm in the minority as I liked the vacuum system.

Yep finding out that the OBD 1 program has no altitude compensation, boost does not affect fuel, and only rpm and throttle pedal affect fueling. Pretty rudimentary, thinking of going to a manual pump, if my ds4 goes upside down.

 

[buddy]

The OBD1 system does have altitude compensation, it just wasnt utilized in F programs. It can be adjusted, so that boost or rather lack of boost reduces fueling.

I had discussed this with a GM programmer before and confirmed again, that the program is written that the OEM sensor should have been or is/was a 3-bar MAP, but the boost control system was limited to control up to 2-bar, even though its scaling allows you to request up to 22psi of boost. GMTDScan should be using the raw voltage from the sensor and conversion, so it should be able to show accurately up to 3-bar if the sensor is a 3-bar. If you look at a manual it says that the voltage on the boost line at idle or just ignition on should be about 1.4V. That corresponds to atmospheric pressure on a 3-bar MAP.

So using a 2-bar MAP may actually be holding back a bunch of people using the stock system. Will look into it further, but does anyone have the original OEM part number for the boost sensor?

 

[Slim Shady]

The OBD1 system does have altitude compensation, it just wasnt utilized in F programs. It can be adjusted, so that boost or rather lack of boost reduces fueling.

I had discussed this with a GM programmer before and confirmed again, that the program is written that the OEM sensor should have been or is/was a 3-bar MAP, but the boost control system was limited to control up to 2-bar, even though its scaling allows you to request up to 22psi of boost. GMTDScan should be using the raw voltage from the sensor and conversion, so it should be able to show accurately up to 3-bar if the sensor is a 3-bar. If you look at a manual it says that the voltage on the boost line at idle or just ignition on should be about 1.4V. That corresponds to atmospheric pressure on a 3-bar MAP.

So using a 2-bar MAP may actually be holding back a bunch of people using the stock system. Will look into it further, but does anyone have the original OEM part number for the boost sensor?

I can get the part number off of my map but I am not sure if the number will cross. So is there any benefit to me installing a 3 bar map sensor?

 

[Slim Shady]

The OBD1 system does have altitude compensation, it just wasnt utilized in F programs. It can be adjusted, so that boost or rather lack of boost reduces fueling.

I had discussed this with a GM programmer before and confirmed again, that the program is written that the OEM sensor should have been or is/was a 3-bar MAP, but the boost control system was limited to control up to 2-bar, even though its scaling allows you to request up to 22psi of boost. GMTDScan should be using the raw voltage from the sensor and conversion, so it should be able to show accurately up to 3-bar if the sensor is a 3-bar. If you look at a manual it says that the voltage on the boost line at idle or just ignition on should be about 1.4V. That corresponds to atmospheric pressure on a 3-bar MAP.

So using a 2-bar MAP may actually be holding back a bunch of people using the stock system. Will look into it further, but does anyone have the original OEM part number for the boost sensor?

If you look back at my post with the links, the first link has the gm part numbers listed about half way down the page.

 

[buddy]

I need to just go check if the voltage feedback on that wire with just ignition on is really about 1.5V, then what is said to be 2-bar would really be a 3-bar sensor. Or if the manual is wrong. I also sent a message to Stefan to find out what conversion he used for calculated boost, if assumed a 2-bar or 3-bar sensor in GMTDScan.

And I was more looking for a service manual reference for the original GM part number specific for the 6.5TD.

 

[millsroger]

All MAP sensors automatically include "altitude" correction. Their 'at rest' reading at sea level is different than at altitude. They are a barometric pressure monitor and read up or down from the existing "atmospheric" pressure. Also, one pound of "pressure" eqauls about 2 inches of mercury on a MAP. That's the difference in "gauge" reading referenced to zero and a MAP reading referenced to 30" barometric at sea level.

 

[buddy]

The sensors dont have altitude correction, they just read lower voltage at lower pressure, just like vacuum. The correction would come in the calibration of fuel and boost in the PCM program. To adjust those parameters based on MAP input. Technically the BARO sensor is another MAP on 94 and 95 trucks, and they can be different at idle, but there is a comparison that cant be too far off. Im at sea level pretty much, and I did measure the intake MAP as 2.4V with just ignition on, so its a 2-bar sensor. I ordered a 3-bar sensor though and with some help have a program that will use the correct voltage conversion for it instead. So will see how that works out with GMTDScan.

 

[millsroger]

Here is the problem with the PCM chart and the MAP. The chart has an expected voltage with key on and engine off. That voltage is different for non turbo and turbo charts. A one bar MAP for a non turbo sits at about the top of the range at 30" and measures down from there to an idle of 12-15". The chart is built with that in mind. A two bar MAP for low boost turbo's sits in the middle of the range at 30", reads down to the same 12-15" at idle, (no turbo spool up) and up to an absolute max of 60" or two 'atmospheres'. It also gets inaccurate near the limit so the realistic limit is about 50" and that corresponds to 10 psi of boost. (50"-30"=20"/2=10psi) The PCM chart needs to be built with that voltage swing in mind. With turbo pressures above the 10-12psi range you need a three bar MAP. With these the resting point is 1/3 up from the bottom and needs a chart built with that in mind and enough data points below and above that to track it. The sensor reference is only 5 volts and there is a big difference between a one bar MAP starting out at 4.5 volts going down from there to a two bar MAP starting out in the middle at 2.5 volts or a 3 bar MAP that starts out reading 1.5 volts and going both ways from there.

Bear in mind that the numbers given are for illustration purposes, not absolutes. Just that I have done alot of supercharging and turbocharging and have found that changing MAP is only half the job, the other half is the right chart in the PCM to go with it.

 

[millsroger]

Am new to the Chevy diesels and mine does not have the xtra BARO sensor, I believe that it reports the difference in atmospheric to manifold pressure to control the EGR and not as an "altitude compensator" for fuel management.