System voltages - make sure they're correct!
06-27-2015, 12:45 PM
#1
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System voltages - make sure they're correct!
I just found out something interesting...
For the longest time, my system voltage would start out at 14.5 volts when cold and then slowly drop to ~13.3-13.4 volts when fully warmed up.
Took my alternator into a couple of places and they both said it checked out ok. Also reading around on the net, other people were saying similar things and folks would respond to not worry about it. So I had always thought it was normal... and by and large the car ran good.
But that incident several months ago when I had the bad connection on the sense wire that caused the 16V voltage spike (which resulted in a huge change in how the car ran- for the better), really got me to thinking...
So I whipped out my DMM and (after triple checking all my grounds) I started probing around the various (+) battery and ignition points after the car was fully warmed up...
What I found was the following:
1.) Main battery stud on back of alternator and battery (+) terminal => 14.4V (essentially no voltage drop there)
2.) Horn relay (which was serving as a main junction and where the alternator voltage sense was hooked up => 14.3-14.4V
3.) My main switched ignition junction block => 13.4V
4.) Ignition coil 13.3V
5.) (This was the key finding I think) The (+) battery terminals at the ECM 12.9 - 13.1V!
What was interesting was the the ALDL output was saying 13.4V for battery voltage and the ECM terminal was only showing ~13.0V on the DMM.
But any rate I spent the morning today re-routing various wires and now ALL of my system voltages stay at 14.4-14.5V at all operating temperatures.
The effect of this is that my tune is now way off from what it was. Going to have to start from scratch again since I've essentially been tuning around a system malfunction. I think the ECM was sending out commands based on what it thought was the system voltage.
Overall, I think this may be more important for swap guys who are transplanting a new harness (or re-pinning an old one). My guess is you guys with factory setups had this already figured out at GM.
I'm not sure why my ECM battery terminal was so low (I didn't want to rip apart the dash to figure it out). So I simply routed a new wire from the main battery tap to the ECM and spliced it in.
Any rate, yeah, make sure all your voltages agree with eachtoher.
For the longest time, my system voltage would start out at 14.5 volts when cold and then slowly drop to ~13.3-13.4 volts when fully warmed up.
Took my alternator into a couple of places and they both said it checked out ok. Also reading around on the net, other people were saying similar things and folks would respond to not worry about it. So I had always thought it was normal... and by and large the car ran good.
But that incident several months ago when I had the bad connection on the sense wire that caused the 16V voltage spike (which resulted in a huge change in how the car ran- for the better), really got me to thinking...
So I whipped out my DMM and (after triple checking all my grounds) I started probing around the various (+) battery and ignition points after the car was fully warmed up...
What I found was the following:
1.) Main battery stud on back of alternator and battery (+) terminal => 14.4V (essentially no voltage drop there)
2.) Horn relay (which was serving as a main junction and where the alternator voltage sense was hooked up => 14.3-14.4V
3.) My main switched ignition junction block => 13.4V
4.) Ignition coil 13.3V
5.) (This was the key finding I think) The (+) battery terminals at the ECM 12.9 - 13.1V!
What was interesting was the the ALDL output was saying 13.4V for battery voltage and the ECM terminal was only showing ~13.0V on the DMM.
But any rate I spent the morning today re-routing various wires and now ALL of my system voltages stay at 14.4-14.5V at all operating temperatures.
The effect of this is that my tune is now way off from what it was. Going to have to start from scratch again since I've essentially been tuning around a system malfunction. I think the ECM was sending out commands based on what it thought was the system voltage.
Overall, I think this may be more important for swap guys who are transplanting a new harness (or re-pinning an old one). My guess is you guys with factory setups had this already figured out at GM.
I'm not sure why my ECM battery terminal was so low (I didn't want to rip apart the dash to figure it out). So I simply routed a new wire from the main battery tap to the ECM and spliced it in.
Any rate, yeah, make sure all your voltages agree with eachtoher.
06-28-2015, 09:48 AM
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Re: System voltages - make sure they're correct!
> 3.) My main switched ignition junction block => 13.4V
This is a fairly big drop (1 volt). I've replaced ignition switches due to this.
As for the tune, if it changes a lot then the injector voltage compensations are likely off. In $8D the ECM will use either the IGN+ input or the fuel pump input for the injector compensations (flag selectable).
If the voltage that the injectors had was the same as what the ECM was reading, then a change in voltage shouldn't affect them.
If the ignition input to the ECM drops below 12.4 volts the dwell routine will increase coil dwell time.
RBob.
This is a fairly big drop (1 volt). I've replaced ignition switches due to this.
As for the tune, if it changes a lot then the injector voltage compensations are likely off. In $8D the ECM will use either the IGN+ input or the fuel pump input for the injector compensations (flag selectable).
If the voltage that the injectors had was the same as what the ECM was reading, then a change in voltage shouldn't affect them.
If the ignition input to the ECM drops below 12.4 volts the dwell routine will increase coil dwell time.
RBob.
Last edited by RBob; 06-28-2015 at 11:26 AM. Reason: corrected battery vs. fuel pump voltage
06-28-2015, 10:01 AM
#3
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Thread Starter
Re: System voltages - make sure they're correct!
> 3.) My main switched ignition junction block => 13.4V
This is a fairly big drop (1 volt). I've replaced ignition switches due to this.
As for the tune, if it changes a lot then the injector voltage compensations are likely off. In $8D the ECM will use either the IGN+ input or the battery input for the injector compensations (flag selectable).
If the voltage that the injectors had was the same as what the ECM was reading, then a change in voltage shouldn't affect them.
If the battery input to the ECM drops below 12.4 volts the dwell routine will increase coil dwell time.
RBob.
This is a fairly big drop (1 volt). I've replaced ignition switches due to this.
As for the tune, if it changes a lot then the injector voltage compensations are likely off. In $8D the ECM will use either the IGN+ input or the battery input for the injector compensations (flag selectable).
If the voltage that the injectors had was the same as what the ECM was reading, then a change in voltage shouldn't affect them.
If the battery input to the ECM drops below 12.4 volts the dwell routine will increase coil dwell time.
RBob.
06-28-2015, 11:34 AM
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Re: System voltages - make sure they're correct!
I corrected my original post, keep getting battery voltage and fuel pump voltage mixed up.
The ECM monitors (reads) the IGN+ switched input to the ECM. Plus the fuel pump voltage input to the ECM. The ECM doesn't monitor the battery input.
In $8D the ignition switched input is used for most items. Such as ECM shutdown, idle speed increase on low voltage, extended dwell, ESC system ignore, and over voltage protection (q-driver shutdown & no-step on IAC).
The fuel pump input is used for code 54 (fuel pump relay malfunction). And can be used for injector compensation. Which is flag selectable to be the fuel pump input or the IGN+ input.
RBob.
The ECM monitors (reads) the IGN+ switched input to the ECM. Plus the fuel pump voltage input to the ECM. The ECM doesn't monitor the battery input.
In $8D the ignition switched input is used for most items. Such as ECM shutdown, idle speed increase on low voltage, extended dwell, ESC system ignore, and over voltage protection (q-driver shutdown & no-step on IAC).
The fuel pump input is used for code 54 (fuel pump relay malfunction). And can be used for injector compensation. Which is flag selectable to be the fuel pump input or the IGN+ input.
RBob.
06-28-2015, 05:37 PM
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Re: System voltages - make sure they're correct!
06-29-2015, 08:50 AM
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Re: System voltages - make sure they're correct!
Code:
; 34 ADBAT Ignition VOLTAGE, A/D COUNTS ; VOLTAGE = N/10 ; 35 PPSW FUEL PUMP POWER SWITCHED (SUPPLIED) ; VOLTAGE = N/10
07-12-2015, 09:20 PM
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Re: System voltages - make sure they're correct!
So just to drive this point about voltage consistency home with real world data...
I just got back from the SoCal Supercruise (ended up being ~375 miles worth of driving).
Using Tunerpro RT data and my mpg calculator, I found that I picked up another 2 mpg on the highway due to this fix. Went from ~23 to ~25 mpg highway. Overall, averaged ~20 using the fill-up method (which included quite a bit of lead footing through twisty hills).
I just got back from the SoCal Supercruise (ended up being ~375 miles worth of driving).
Using Tunerpro RT data and my mpg calculator, I found that I picked up another 2 mpg on the highway due to this fix. Went from ~23 to ~25 mpg highway. Overall, averaged ~20 using the fill-up method (which included quite a bit of lead footing through twisty hills).
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