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Boost Gauge Question


RobertISaar
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having never owned a TGP and likely never will(well, LG5 anyway) due to all of the build threads gone awry here, but i'm curious about the boost gauge in the cluster. what does it use for a signal? is it directly run off of the 2BAR MAP's signal wire? alldata doesn't show pretty much any TGP specific information.

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much appreciated.

 

i had assumed it worked that way, since being 2BAR, 2.5 volts (atmospheric pressure) would drive the gauge to a half position, which is where the stock gauge switches between pressure and vacuum.

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If it's like the 2-bar map on the L67 it goes like this:

 

Range 14.3psi to -14.3psi

 

14psi = 4.84V

 

0psi = 2.45V

 

-14psi = .07V

 

each change 1 psi = .17 volts

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Off topic, but the Saab had a rubber hose connected through the firewall to the back of the instrument cluster for the boost gauge.

 

I wonder why GM didn't do that :lol:

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Off topic, but the Saab had a rubber hose connected through the firewall to the back of the instrument cluster for the boost gauge.

 

I wonder why GM didn't do that :lol:

 

Because GM did not use a mechanical gauge. :lol:

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If it's like the 2-bar map on the L67 it goes like this:

 

Range 14.3psi to -14.3psi

 

14psi = 4.84V

 

0psi = 2.45V

 

-14psi = .07V

 

each change 1 psi = .17 volts

GM 2bar sensor output.

14.3psi-4.9v

14psi-4.84v

13psi-4.67v

12psi-4.5v

11psi-4.3v

10psi-4.16v

9psi-3.99v

8psi-3.82v

6psi-3.47v

4psi-3.13v

2psi-2.79v

0psi-2.45v

.50 in/hg-2.03v

10 in/hg-1.19v

20 in/hg-.35v

29 in/hg-.02v

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interesting, the factory manual for the TGP lies.... :lol:

 

gauges that are ACTUALLY powered by D10 are boost gauge, speedo and tach.

gauges that are ACTUALLY powered by D5 are the oil pressure, fuel level and coolant temp.

 

in reality, there is a SMALL amount of leakage from one to the other, but that's where each gauge gets it's primary source of power from. also, the boost gauge's factory scaling must be horribly off in at least this cluster, i fed 3.35(5PSI or so) volts to it and it almost hit the 15PSI mark. is this common?

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A w-body manual lying about something!!! I am shocked!!! :willynilly:

:lol::lol:

 

TGP cluster boost gauges are not really trustworthy. I have not datalogged my TGP enough to know if it's normal for 5psi to read as 15, but I understand they are VERY optimistic. I would easily believe your results.

 

However... during my TGP datalogging yesterday I did notice that the Vacuum reading exactly matched my mechanical boost gauge as well as the data Tunerpro was telling me. So perhaps it's not a linear discrepency.

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maybe... i'm tracing the boost gauge on the PCB now, seems simple, but Jarek mentioned he would like this project done sooner rather than later, so i don't think i'll mess with it too much, maybe a few pics so i can reconstruct it on the laptop and see how easily it can be adjusted.

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here is the entire boost gauge circuit.

 

XXiRh.png

 

excuse the general shittiness of it, but that is the entire portion of the cluster's PCB that deals with the gauge.

 

i already have a few theories as to how to "tune" the gauge.... but it may have to wait until i come across another cluster....

 

then again, perhaps someone wants to donate a cluster and pay for testing parts and receive a cluster that can either be scaled to their liking or corrected to actually be accurate?

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Well... I do have a spare cluster... as far as I know nothing about it is accurate anymore. It's got all the symptoms of old tgp cluster syndrome.

 

I may be willing to provide it for your testing needs.

What sort of costs are we talking about for testing parts and stuff?

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not much... i mean, pots are cheap(~.10 to .50 each), resistors are cheap(penny a piece for pretty much any value), caps are cheap....

 

the only "expensive" parts are the ICs and maybe the couple of transistors in there, and i highly doubt any of those would top the $1 mark.... assuming they can be found these days anyways. but those seem to RARELY go bad.

 

i ALMOST have all of these parts just scattered around.... but new parts would be a significant plus.

 

and then the possibility of other custom scaled gauges come up.... like a coolant temp gauge that goes from 120-200, sets off the check gauges light at 190.... all kinds of wierd stuff. but that would require a custom overlay to make sense.

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I like the idea, maybe start a thread about it, it would probably get a lot of interest. I would be willing to support the cause if it means one day my gauge cluster will work right!

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getting it to work to factory specs isn't difficult at all...

 

getting it to work BETTER than factory isn't that bad either.

 

getting it to work exactly the way you want it to? that's where some thought is required...

 

think of the fuel gauge in these clusters.... now think of the fuel gauge in a 1.5W... ever notice how they jump all over the damn place with even the slightest movement of the car when it's not either completely full or completely empty? if you dive into the circuit for the gas gauge on those you'll see that there is VERY little to no dampening of the needle movement. you can add a resistor and capacitor and you now have movement that is as fast or slow as you want.

 

for this specific cluster, think of the fuel and boost gauges... the boost gauge seems to respond damn near instant to changes from when i was playing with this cluster. fuel gauge? takes ~15 seconds to go from 100% empty to 100% full. think that's too fast or too slow? change a resistor or capacitor and now you just changed needle movement speed.

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Makes sense.

 

Yeah, these fuel gauges are just barely more useful than a full/empty dummy light. :lol:

 

I need to learn more about reading circuits. I'm not a complete newb, but I get lost rather quickly reading most schematics.

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honestly, the 95-99 MC and 95-01 lumina fuel gauges are 3 position: full, empty, somewhere in between. i think when i pull the dash to remove the ABS lamp in the monte, i'm going to add in that RC mod and have it be somewhat more useful in situations other than parked with no wind.

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Yeah, my 94 GP is like that too. I go by the DIC miles remaining data, as sketchy as it may be... it's miles better than the fuel gauge.

 

Gauge in my GF's 05 Cavalier has taken to sometimes reading so far beyond the full, even when only 1/4 full, that it blocks the coolant temp needle from sweeping properly. Dunno yet what is causing that.

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  • 1 year later...

an update for this thread: so, there is only so much that can be done with the boost guage... the way GM made the circuit, if the needle itself acts non-linear(most seem to), then there is nothing that can be done to make it correct at all positions. you can somewhat easily make it 100% at one position and live with the inaccuracy in other places.

 

or you could add some type of intelligent control and simulate and skew the 2BAR MAP sensor signal that the cluster uses. the easiest way i can see to do that would be via the ECM. there is exactly one PWM output left open in a TGP calibration, and that is the one that would have been used for EGR3 with other masks. could patch the code and insert a 2D table that determines the duty cycle of the circuit based on MAP voltage.

 

though to be 100% accurate with what the PCM is commanding, a little more math would be required. as far as the ECM is concerned, any MAP value above baro value is considered boost. denver, for example, is roughly 5,280 feet above sea level, barometric pressure is around 84kPa. if you have your calibration setup to push a full 14.7PSI(101.4kPa) of boost, you're up to a total of about 12PSI(185kPa), not the nearly 15PSI(203kPa) you might be expecting to see on the boost guage.

 

in any case, it's a thought i've been having. that circuit can be switched at 64Hz(or 32 or 16), so some filtering via a resistor and cap would be necessary, otherwise the guage might dance a bit since it is certainly of the fast moving type.

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How much to do mine :)

honestly, the 95-99 MC and 95-01 lumina fuel gauges are 3 position: full, empty, somewhere in between. i think when i pull the dash to remove the ABS lamp in the monte, i'm going to add in that RC mod and have it be somewhat more useful in situations other than parked with no wind.
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