System voltage drop during start

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

rmoehle

New Member
Joined
Aug 13, 2015
Messages
2
Location
Kirkland, WA / USA
Hello elec-xperts,
My engine computer began dropping offline while cranking. I ran some tests, and the system voltage is >12V at power-on, but the bus voltage drops to ~8V while the starter cranks. I feel like this is below the operating voltage of the ECU.
I'm far from an electrical expert - what could be wrong and what could I do to fix it?
Thanks!
Robert
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
12,851
Location
Memphis, TN
How old is the battery? Sounds like it has lost some capacity. I think a split buss/ emergency buss would be great. Starter on main batt and ECU on E buss batt and then tie together or something like that.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,291
Before you blame the battery, take some voltage drop readings across the master contactor. Those things have huge copper contacts in them that oxidize with age and burn with use, both factors introducing resistance. And it only takes a tiny fraction of an ohm to cause a huge voltage drop at the bus. Ohm's Law, see? Volts = Ohms times Amps. A six-volt drop only requires .024 ohms at a 250-amp draw. Starters use that sort of current, bigger engines draw more.

Put the voltmeter on each side of the contactor, on the big terminals. Positive toward the battery side, negative toward the bus. It should read system voltage with the master off. Crank the engine; the voltage should drop to zero or very close to it.

Weak starting is often due to this or to the same sort of thing in the starter contactor, or both. Lots of batteries get replaced because of bad contactors.
 

Aerowerx

Well-Known Member
Joined
Dec 1, 2011
Messages
5,544
Location
Marion, Ohio
+1 on all this.

I was also going to suggest corrosion on the battery terminals. I had a 2003 Honda Accord that wouldn't start, even though it was an almost new battery. Turns out it was the plated copper lugs on the lead battery terminals, with a layer of corrosion in between.
 

Derswede

Well-Known Member
Joined
Jan 6, 2016
Messages
924
Location
Central North Carolina
Might try one of those large capacitors, with a blocking diode to keep it from being dragged down voltage wise. I do think that borrowing a known good battery and retrying it would be worthwhile as TFF is right, a good battery should not drop that much. (oh, and at Harbour Freight, they have a "jumpstart" battery/kit with a lithion iron battery, snagged one for $48 on sale. Will get another and use the battery in my motorcycle. Need to see how much just the batteries are going for. Dan's info is spot on as well, many times a major problem can be fixed with a simple cleaning.

Derswede
 

TerryM76

Well-Known Member
HBA Supporter
Joined
Sep 8, 2012
Messages
606
Location
Tempe, AZ
Be sure and check your engine to airframe ground. I have seen many problems associated with that from having loose or corroded connections. Might be a good idea to take apart and clean those connections if you are living in a humid climate.
 

Winginit

Well-Known Member
Joined
Jun 12, 2016
Messages
811
Location
x
You didn't say if there was actually a problem occurring when cranking the engine. Did you just take a reading to inspect the system, or is there an actual problem with slow cranking? As always when working on an electrical problem the very first steps are to verify battery condition, and then disassemble all relevant connectors and clean them. They can appear to be fine even though they aren't. We had a company pickup that refused to start one day even though it never had a previous problem. We paid a tow bill and the high costs of dealership labor. The problem was identified as a bad ground connection on the engine. We were charged for the replacement of the stud that secured the engine ground. A week later the truck did the same thing again. This time the company maintenance man took a look at the problem. Turns out that the dealer charged us for a new stud but had only cleaned the external part of the existing stud...which appeared to be in perfect shape. When the maintainance man removed the stud, he found lots of corrosion surrounding the studs threads in the block. Perfect on the outside, but badly corroded inside. Cleaning of the affected area and a new stud and everything was fine again. One of the most common causes of electrical issues is the simple lack of good grounding. Its one reason you see so many grounds in automotive systems today.
That being said, most ECUs will operate erratically, or not at all with a discharged battery, often indicating other faulty parts when the problem is actually incorrect voltage to the ECU. The sensors for the ECUs signals though operate off a 5 volt reference.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,005
Location
Rocky Mountains
DT has the right answer - do a volt drop test (under load) on each connection, including the battery terminal/battery clamp. Measuring the resistance with an Ohm meter doesn't always tell the whole story. Voltage drop under load will show problems more definitively. Once you have eliminated any high resistance double check with a clamp on amp meter.

If this all checks out then a load test on the battery is in order. It's been my experience that checking in this order will find the problem in the first few steps most of the time.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,291

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,005
Location
Rocky Mountains
Hadn't really thought about it until this thread, but my favorite ammeter for this kind of work is an inductive meter and it just slides over the wire from the side. After some more looking the "clip on" analog ammeter that I bought from Snap-on years ago could not be found with a several minute internet search. I could also not find any mention of how it actually works. No way am I going to tear it apart just to figure it out - It's way too handy. ;)
 

rmoehle

New Member
Joined
Aug 13, 2015
Messages
2
Location
Kirkland, WA / USA
Thanks all. For some reason, I didn't get notified of updates to this thread..
Cranking is strong, but downstream systems are suffering during crank. I think you're on to something with resistance. I trickle charge the battery every week.
I recently tested voltage at the coil packs during start, and these drop to nearly 0.
So, 12v switch box (&ECU) down to 8v, coil packs down to 0.5v. I'm attaching a simplified schematic of the 12v system - any advice pertaining to this issue is greatly appreciated.
Thanks,
Rob
 

Attachments

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,005
Location
Rocky Mountains
Double check the grounds - especially in the starter circuit. You also need to know the amp draw during cranking.
 

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,211
Location
CT, USA
Use a DC voltmeter and check for voltage across each connection while cranking, starting from the battery. If there's any significant voltage across any junction, there's resistance, likely corrosion. Correct it and move on to the next point. Don't forget the connections to ground.

Dana
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
13,119
Location
Port Townsend WA
I think a voltage drop to about 9 volts while cranking is average.
I think some car engine ignitions are designed to work at 9 volts. The ignition (key switch) provides two positions for crank and run. When the cranking stops the ignition system then has a resistor to keep the coil at 9 volts so it doesn't overheat at 12 volts.
A new battery is 12.6 volts.
Something to consider if an auto engine ignition is installed in an airplane that normally have mags.
 
2
Top