Voltage drop between power into MS and TS

[Rikard]

I have my MS2 connected to a power supply 13.8V 7A and JimStim to setup a PWM valve.

I noticed that I have a voltage drop when I measure the voltage coming into JimStim (13.7V) and the reading in TunerStudio (12.8-12-9V) is this normal? I have not connected any other hardware to the MS or JimStim and not running anything from JimStim either (rpm=0)-

Is this normal behaviour?

Rikard

[Piledriver]

Could be a calibration issue, the MS "measures" the incoming battery voltage via an analog input across a voltage divider, but component values vary and the tolerances can stack up. (max direct ADC input voltage is 5v)
It is correctable in software, I just don't recall how, might be fixable in TS or in the .ini.

I wouldn't sweat it much if is a small value...
... unless uoy are seeing a significant voltage drop with your meter, that could indicate a cold solder joint or something.

You should measure it at the input of the voltage divider.

The reading does have an effect on coil dwell time and injector PW corrections. (automatically compensates, but it needs to read the right voltage to do that accurately)

[Rikard]

The difference is about 0.9V, I don't know if I should be bothered about it or not. Where can I find the voltage divider?

Edit: I have the 3.57 board if that makes any difference. I tried to find info about the location of the voltage divider but couldn't find any.

Rikard

[Piledriver]

Schematic : http://www.megamanual.com/357/hardware.htm
Layout: http://www.megamanual.com/357/intro.htm

Right side, second page shown as part of the bootloader circuit (dual purpose), R3 and R6 for the divider, measure the voltage going into R3, the adc looks at the other side between R3 and R6.

Reading B&Gs website (and the Megamanual in particular) can cause great confusion if you are using msextra firmware, but it's where the schematics live. The B&G and MSextra firmware use the hardware much differently in places, but same for vbat..

[Rikard]

Thanks Pile.

If I measure at the R3 point as you suggested I get 12.93V to compare to the TS reading of 12.80V. Not a big difference.

So I should not worry to much about it I guess. There is a voltage drop about 0.8V from the input to JimStim to the R3 point, is that normal?

I have another question related to this issue, perhaps I should start another thread.
I have the MS on the test bench to set up a 2-wire PWM idle valve, when I connect that to MS, 13.8V from my power supply on one wire and the other connected to FIDL i get a higher voltage reading in MS average of 13.3V but the voltage varies between 13.6 and 12.8V, this means a 0.5V increase when I connect the PWM valve, this seems strange to me.

I have added a 1N4001 soldered into the wiring loom, but is there anything else I can do to get rid of the voltage variations between 13.6 and 12.8V?

Edit: I can add a capacitor to the boot loader to have a steady voltage but I guess that is just masking the real issue.

Rikard

[jhoefer]

Where is this extra diode? A 1N4001 has about a 0.9V forward voltage drop, so if it's inbetween the power supply and the MS it would explain your difference in readings.

There are also protection diodes in the 12V supply rails of both the MS and Jimstim that have a similar voltage drop that could explain the difference depending on where you are measuring. Although I don't believe they come into effect for the battery voltage reading. So it's likely just the tolerances in the voltage divider resistors.

If you go into Tuner Studio, under Tools, you can choose Calibrate Battery Voltage. You can tweak the settings there. To start, try changing the Max setting from 29.7 to 32.0 and see where your Tuner Studio gauges read. You can adjust from there.

[Rikard]

The diod is soldered close to the PWM IAC and the IAC is connected directly to the power supply so I didn't think it should affect the voltage of the MS but it does somehow through the grounding. I will play around with the setting and see if it gets any better.

Rikard