Scope Pics (Battery)

[MrAl]

Hello there,

Here are some scope pictures of the battery voltage as various things are turned on in the car for those interested. My main purpose for this is to keep an eye on the health of the battery over time.

Note that the vertical scale is not zero on the line it usually would be so i quote the more important voltages here. The zero line is more like 6 volts below the normal zero line, which is about 1 volt below the yellow arrowhead on the left.

First, there is the starter (pic labeled "STARTER"). As shown the voltage comes down just a little as the key is turned on, then when the key is turned to start the car (cold start sitting for 2 days beforehand) the voltage dips sharply down to about 7 volts. Then it quickly climbs up to about 9.5 to 10 volts. I've never been able to see the sharp dip before in my other cars because i did not have a portable scope back then so i dont know what that should look like, but when it climbs back up a little to around 10 volts i've seen that on my Hyundai as well so i consider that normal. I also suspect the sharp dip is normal too though as the starter motor would draw a very high current as it starts to wind up to cranking speed. As the starter turns we see the voltage go up and down slightly as the load from the non linear force of the engine turning makes it harder for part of the rotation than for other parts of the rotation. The main point is that once the motor starts, the battery voltage climbs up to about 11.5 volts, then about 4 seconds later the regulator/alternator kicks in and brings the voltage up to about 13.8 volts or so. That is the end of the starting interval of the engine.

Next comes turning on the headlights (pic labeled "LIGHTS"). The voltage drops slightly and then the regulator regulates it back up to normal voltage.

Finally the heater (pic labeled "HEATER"). The voltage falls more than with the headlight than when first turned on, then the voltage goes back up just before the heater turns on. After about 2 seconds we see the second dip which is when the motor actually starts to run, then the regulator regulates it back up again.

Scope pics like this can be used to monitor the electrical system components. A future test should show similar results or else something could be going wrong. This is especially true of the starting period where if the battery voltage goes down too far (after the quick spike) it would mean the battery is getting old.

Not shown is the operation of the all five door locks at once, which did not make much difference in the voltage.

If you have any other ideas for tests like this please let me know.

What i forgot to do though (was in a hurry to go somewhere) was try to capture the alternator waveform which would tell me if it is working properly. I'll have to try that next time.

[jimmy57]

Volvo and most others phased in time delay voltage regulators in the 90's to fix the issue where cooling fans would come on at idle and the extra load would dip rpm. Earlier before uncontrolled acceleration was a part of our lives the IAC would have a faster response but this caused some RPM oscillations at big load changes. When they introduced time attenuated signal change to IAC then many cars developed "wet dog syndrome" whee the fan would come on an the RPM drop (4 cylinder cars mainly) would be to a speed low enough that whole car would do a shake like when a dog gets out of a bath, pond, pool, etc and shakes. The Voltage regulator timed response fixed it (and then people would complain about lights dimming momentarily....you can't win sometimes). The newest cars have the ECM control the VR and the cooling fans and the cooling fans are staged to ramp up speed with the VR being controlled and the electric throttles controlled to orchestrate all this.
The latest thing is to quit charging on acceleration, with a time limit of 10-15 seconds, and to go to high rate (>15V) during coast when injectors are shut off (FREE charging!!). At steady speeds after an initial battery charge after starting, the charge rate is 13.2 +/- V so the engine load (and fuel use) is less but that is enough to supply everything with no battery discharge. If you were to watch a voltmeter or scope on the newest cars you'd think the voltage regulator was screwed.

Your car has the time delay VR.

[MrAl]

Hi,

Very interesting, thanks for that information. I am finding out new things about this car all the time.

Yeah i thought that time delay was rather strange. That means if you have a battery that is not very good in there when the car starts the battery voltage may drop too low to keep the car running properly until the VR starts to pump up the voltage i guess. I think i may have experienced this once when maybe the battery was low one day. It was raining and the car started to shake and then shut down. Lucky it started again and i had to go park on the side of the road with all electric stuff turned off and wait for the battery to charge up a little more. It was cold and rainy and i had made several short runs and stops so that was probably worst case.

Yet another strange thing about the newer cars that shows up.

[abscate]

Jesse...driving in NJ....Route 1 , I80, 287, 87

75 mph is low 10 percentile on speed. I get crappy mileage at 75...drops to 31 mpg

[beigg]

Problem with the old IACV method for some vehicles is carbon buildup and sudden erratic driveability. The ECM would not be able to catch up to monitor and control fast enough resulting in emissions exceeding its values.