Stock Boost Gauge

[corsair231]

Guys I know the stock dash gauge is notoriously inaccurate but I'm curious about a few things and hoping someone can answer. My truck has been down for a while ( I know, shocker, right?) and one of the things I've done during it's hiatus is rewire some of the dash. I managed to get the stock boost/vac gauge working again. This got me to paying attention to the gauge in other trucks. One of the things I've noticed in photos of different instrument clusters is that when the vehicle is off the gauge does not rest at zero. In fact, they seem to sit all over the place on the vac. side. Is there a set place they are supposed to home to? Plus when I turn my key on it does not go to zero either. Is it supposed to then? Also, where is the signal coming from? I'm sure it is from the map sensor but does it read directly from the sensor circuit or does the computer convert it to a signal for the gauge? And if you change from the stock two bar to a three bar sensor does the chip set up for the three bar make the corrections for the gauge to still read correct or does it just read a percentage of what the sensor sees? I know it is no big deal and I do have a mechanical gauge for boost but I am the kind of person that if it is there, I want it to work. Anyway, I just got curious about the gauge operation and hoping to gain a little insight. Thanks

 

[DaveP's Ghost]

The gauge is a volt meter. The MAP sensor outputs a voltage signal. The gauge reads the MAP voltage directly, there is no "computer intervention". A 2 BAR MAP rests at 2.5 Volts at zero vacuum. The gauge reads 0 at 2.5 Volts. A 3 BAR MAP rests at 1.67 Volts. This roughly equates to 1/3 scale into vacuum.

One way to make the gauge read 0 again after changing to a 3 BAR MAP is to add a 2 BAR just for the gauge. You can tap the engine's MAP grey and black wires for power, and run the output wire directly to the 10-pin connector under the ECM for the gauge. Disconnect the engine harness side and add the new signal there. No need to remove the cluster.

 

[proptop]

Dave,
I am glad that you are still on this site because you have are (in my opinion) a Subject Matter expert and I enjoy reading your post. By chance did you Engineer these trucks LoL!!!!!!
Bob

 

[corsair231]

Thanks Dave, I appreciate the answer. I was sure someone on here knew. I guessed the gauge would not read correct but since I haven't driven the truck with my new combo I've not gotten into any boost to compare my dash gauge and my mechanical gauge. My pillar gauge reads boost only so no vacuum readings to compare either.

That's a good tip about adding a second map sensor. I wouldn't have thought of that but makes sense now that I know.


Now, if only someone could tell me how to fix that useless tach and boost meter in a turbo Regal so it reads correctly.

 

[proptop]

randy,
You can forget that Buck tach as mine has never been right I do use it for/in the staging process. LoL Contact Richard Clarke (Pete) that works down there they can rebuild the dash for you.
Bob

 

[ironbiker]

Factory boost gauge shorts out and turns map sensor off turning fuel off...took me and my mechanics about 3 years to figure out intermittent shutdowns...last mechanic had truck for a solid month before it happened to him...disconnected boost gauge and has not shut off since,(2002).

 

[DaveP's Ghost]

Factory boost gauge shorts out and turns map sensor off turning fuel off..

I have been involved with these trucks for 25 consecutive years. First time I've seen the boost gauge blamed for a driveability problem. I'm not saying it didn't happen, just that I've never seen the topic discussed. There's more to it that could be (or could have been used) for diagnostics.

The ECM responds to the MAP voltage to determine the amount of fuel to inject. A 2-Bar MAP voltage varies from .5V at idle vacuum to 4.5V at 14.7psi. There are two possible conditions for a "short" to affect the MAP signal to the ECM. A short to battery voltage (ECM would interpret this as WOT and go full pig-rich.) or a short to ground (ECM would interpret this as idle vacuum, and cut fuel to idle.) So for a "short" by the boost gauge to affect fueling resulting as a "turning fuel off" would be a short to ground.

Code 34 is MAP Signal Voltage LOW. Specifically lower than .25 Volt (it is never below .5 volt in normal conditions) with TPS less than 20% and RPM less than 1,200. All conditions must be met for .4 of a second. One would think that any short to ground that resulted in the engine cutting out would drop the RPM below 1,200 very quickly and the driver would lift their foot causing TPS to go <20% setting Code 34.

34 is a rare code to see set. There's not much that can set 34, usually it is a bad connection on the gray wire that causes loss of the 5V reference voltage. Did you check for codes? Was 34 present? Did you actually confirm it was the boost gauge? How did you disable the boost gauge?

But I'm mostly curious as to why you joined 5 years ago and never posted until today in a couple of dead threads regarding MAP and boost gauges.?

 

[ironbiker]

I have been involved with these trucks for 25 consecutive years. First time I've seen the boost gauge blamed for a driveability problem. I'm not saying it didn't happen, just that I've never seen the topic discussed. There's more to it that could be (or could have been used) for diagnostics.

The ECM responds to the MAP voltage to determine the amount of fuel to inject. A 2-Bar MAP voltage varies from .5V at idle vacuum to 4.5V at 14.7psi. There are two possible conditions for a "short" to affect the MAP signal to the ECM. A short to battery voltage (ECM would interpret this as WOT and go full pig-rich.) or a short to ground (ECM would interpret this as idle vacuum, and cut fuel to idle.) So for a "short" by the boost gauge to affect fueling resulting as a "turning fuel off" would be a short to ground.

Code 34 is MAP Signal Voltage LOW. Specifically lower than .25 Volt (it is never below .5 volt in normal conditions) with TPS less than 20% and RPM less than 1,200. All conditions must be met for .4 of a second. One would think that any short to ground that resulted in the engine cutting out would drop the RPM below 1,200 very quickly and the driver would lift their foot causing TPS to go <20% setting Code 34.

34 is a rare code to see set. There's not much that can set 34, usually it is a bad connection on the gray wire that causes loss of the 5V reference voltage. Did you check for codes? Was 34 present? Did you actually confirm it was the boost gauge? How did you disable the boost gauge?

But I'm mostly curious as to why you joined 5 years ago and never posted until today in a couple of dead threads regarding MAP and boost gauges.?

The truck shut off intermittently in traffic for several years with wife driving...auto electric mechanic kept truck for several weeks to find problem..my regular mechanic changed map sensor and egr valve with trouble codes that popped up. the gauge was disconnected by later mechanic and truck never quit again.I was told the gauge and map sensor were on the same circuit,when the gauge shorted out the truck quit.