New here, have done a lot of searching and cannot find the same issue. 99 Blazer V6 4dr. Problem started like a bad battery, would not crank. Could jump to start. Changed battery, when I hooked it up I noticed the battery was drawn down to less than 9.5 V. Disconnected starter, voltage went back up. Changed starter, battery drawn down to 9.5 V. Disconnected alternator, no change. Put amp meter in line, pulling .98 amps. Started pulling fuses in engine compartment, pulled TBC fuse amp went to .03. Reinstalled fuse started pulling fuses inside cab, pulled courtesy light fuse and things looked normal. Reconnected battery, voltage stays up. Turn key and voltage instantly goes to less then 9.5 V. Does not try to crank. I really feel like I am chasing a ghost as it seems to keep moving around and changing without me doing anything. Now my questions, does this sound like the ignition switch issue? If not what would should be my next step?

Thanx for any help. Not sure where to go next.

Karl

 

Welcome to the forum, let’s get you fixed up. The first thing we need to do is resolve the phantom power problem or at least remove that circuit if the truck will still run. Then we need to fully charge the battery, knock off the surface charge and make sure it holds 12.6v. Then check all the high powered cables for corrosion, damage and solid contact. Are you at the point now where you have no excessive parasitic battery drain?

George

 

All connections are clean and solid. Voltage at battery is 12.6 after charging all night. I am at a point where I do not have an excessive parasitic draw. It is not until I turn the key (even before trying to crank) that the battery falls flat.

Thanx,
Karl

 

Two possibilities:

The battery is defective and cannot handle the load

There is a high improper draw at key on.

Do you have the means to test circuits for high current draw like with a DC amp clamp meter? If so, whats the draw on the main power cable?

George

 

I will try with a clamp and key on tonight when I get home. When I put the amp meter in line I was getting .03A with the key off. Will update when I get it checked.

Thanx,
Karl

 

Can not get clamp to work. Hook meter up inline and get 12A peak and goes down to 8A when key is turned on.

Thanx,
Karl

 

So you have 12 amps of current flowing in the battery cable and that current goes down to 8 amps when you turn on the ignition? Does that 12A continue after the truck starts to completely shut down/go to sleep after about 15 min? Does this current still go to zero when the TBC fuse is pulled?


George

 

Key off all fuses except courtesy lights - .6A
Key off BCM pulled - .5A
Key on BCM fuse installed - 8.65A
Key on BCM fuse pulled - 6.03
Do not really want to leave key turned on as there is a lot of humming coming from fuse box.

Thanx,
Karl

 

So with the key off you get 12A but then pull the courtesy lamp fuse and get 600ma? What is that fuse value?


George

 

I want to make sure that you know how to measure the true parasitic battery drain. You have to close all doors and the hood and be sure that no lights have stayed on. You then wait 15 min for the BCM and all other circuits to time out and go to sleep. If you disconnect the battery cable to put an ammeter inline with the battery cable you might wake something up and have to wait another 15 min hence the amp clamp suggestion.


George