Hello, New Member here and banging my head up a wall. 2002 Blazer 4.3l, 96K, The story, about 10k ago the previous owner began to have starting problems, mostly when cold, After cranking over serveral time it would fire up. Then after that for the rest of the day no problem starting. She had the shop try to diagonse. They replaced the enitre Distriptor, Fuel Pump, with no change. When I took over, I changed the plugs, fuel filter, cap, battery. Still the same issue. Now here is what I am finding.

The car starts almost immeditaly when jumped. I tested the battery terminals while turning over without a jump and the draw from the starter brings my voltage down to 8 volts while trying to turn over and the battery starts at 12.8V. I am now thinking the starter is pulling too much power and takes way from creating spark to start.

My question is how many volts should it be pulling and how do I tell if the starter is really the problem. Thanks and Im sure I forget something to say, but Ill start here.

 

Sounds like a bad battery. Check the connections first. Then, you can put an inductive ammeter (amp clamp) on the battery cable to check cranking amp draw. It probably shouldn't be over about 200, but that's just a ballpark. Or, you could do it the easy (but expensive) way and just try replacing the battery.

 

A couple of things:

1) Where are you measuring the system voltage during cranking, is it directly on the 2 battery terminals?

2) What is the voltage during cranking when the truck is being jumped?

3) What is the CCA rating and the age of the battery?

4) How does the starter sound during cranking? Is the sound and speed normal?

5) Are all power and ground connections clean and tight with no frayed or damaged wires?


Has the battery been properly load tested? This is where a calibrated load (like a large carbon pile resistor or large wire coil) is used to simulate the starter current draw and then observing the battery voltage during the simulated cranking load? This is the only proper way to determine if the battery is up to the task. You can have a bad cell such that resting battery voltage is normal but the voltage drop under a high current load through that cell causes the overall battery voltage to drop below spec.

If you are getting 8V at the battery terminals during cranking then something is indeed wrong and that was a good test to start with. If the battery is fully charged and the starter load is normal then you should get at least 9.6V at the battery terminals during cranking and a system in good condition will probably show more like 10.8 volts on a warm day. That's at the battery terminals. All wires, connectors, relays, coils, etc act like resistors and resultantly have a voltage drop across them in accordance with ohms law > V=IR. In the starter circuit the current is hundreds of amps so even with thick starter power cables in good condition with no corrosion there is a significant voltage drop on both the power and ground side and the starter will see up to another 1 volt less than the battery terminals. Any broken wires, corrosion, inadequate replacement cables, etc then resistance and voltage drop goes up and the starter sees even less.

Can the starter cause excessive amp draw? Yes it can if its starting to fail. That's why I asked about the speed and sound. Do you have an amp clamp meter? If so then watch the DC amp draw during starting on the large positive cable, ignoring the first few seconds. I don't know the spec for our starters offhand (I'll look in the shop manual later when I am home) but typically you should see 100 - 200 amps with a max of 250 amps. A battery and cable system in good condition is capable of much more than that. A bench test of the starter is some information and will show less current under a no load condition but the real test for all current draw and voltage drops is under the circuits full load.


George

 

OK George thanks for that info. I had the previous battery replaced which was about a year and a half old. load tested at Napa and they said it was bad so it was still covered under a prorated warranty and I went and had a brand new battery swapped in. So the battery is definitely not gonna be my issue. When I get home I will top the battery off again and then try the amp test around the positive terminal cable and see how many amps the starter is drawing.

Another thing that has me somewhat worried that I forgot to mention is after the vehicle starts and then check the alternator sending power to the battery in it is an upwards of 15 V. After the vehicle has run for a bit or driven the volts are hovering then about 14.5v

 

Don't rule out the battery or anything for that matter, new stuff dies all the time, especially if you are overcharging as may be the case. We can talk about that later. For now crank current and battery terminal voltage at crank is a good next step.

Unfortunately, when people start the parts cannon on unconfirmed failures they may be reducing the overall reliability of the truck because many after market parts are of poor quality vs the AC Delco parts already in the truck. I have 205,000 miles on my truck and literally the only part I ever replaced that was better than OEM was a Moog Idler arm.

George

 

Starter current goes up with mechanical load (more cylinders or gas to diesel). I confirmed 150 - 200 amps for 6 cyl gas.


George

 

Sorry George just getting back to all this again so my voltmeter does not read amperage in DC volts how old can I test what the draw is while trying to start?

 

This is the type of clamp meter that can measure high DC current:


In the absence of a meter capable of 250 dc amps then you wont be able to test the starter amperage draw under a full load. There are such things as shunts which are a low valued high powered resistor that you pu inline and measure the voltage drop across the resistor to calculate amperage draw. Your probably better off just getting a proper meter. The only other option would be to remove the starter and have it bench tested.

How confident are you that you have a good battery and good connections all the way to the starter? You can consider having the battery load tested.

What is the battery voltage during starting while you are also jumping the vehicle?

What is the voltage at the starter main power lug while cranking? IOW, how much of a drop is there from the battery positive terminal to the starter lug?


George

 

I measured my starting and charging behavior today. 215 peak amps, 98 steady state amps and 10.5VDC, Battery was fully charged with a 12.65V resting voltage. My charging voltage at start up was 14.6V.


George