Voltage drop diagnosis

[daven8844x]

Having an issue where when I start my '97 K1500 truck cold, it is great. After the engine warms, and in southern heat, when I make a quick stop at the store and come back out to start, the starter is sluggish and sounds like its barely pulling power.

I ran a voltage meter from battery negative to starter negative, and did a starter test, and it's dropping around 10v. I then ran from battery negative to engine ground, and it was more like 0.2v or so. I am assuming that this means the battery negative cable, and its connection to the chassis, are both fine. So clearly something between the engine block ground and the starter is going bad/melty on the ground side. What can I check in between to track down the issue? And did I miss anything by not checking the positive side?

Thanks in advance!

--Dave

 

[daven8844x]

Nevermind

 

[Iconn]

I just went through this on my 97 1500 express. 5.7 L. I put a disconnect switch on my battery and the switch went bad. But the starter cable is large like a #1 so the battery post connector needs to be rebuilt maybe? I cut the existing cable (good and heavy) and added a cut off end from Napa. The FSR says the engine will not start under 10 Volts. After I put in a new fuel pump assembly it really starts well. Fuel pressure could be an issue. It should be 56 to 62 psi running and hold when key off. Heat. Might need a new fuel pressure regulator.

 

[Erik the Awful]

After the engine warms, and in southern heat, when I make a quick stop at the store and come back out to start, the starter is sluggish and sounds like its barely pulling power.

If your battery cables are good, it could be the bushings inside the starter. I '78 Mazda GLC that the starter would bind up when hot because the bushings were bad. I got good at parking on hills and bumpstarting it.

I just went through this on my 97 1500 express. 5.7 L. I put a disconnect switch on my battery and the switch went bad.

Most battery disconnect switches are cheap garbage. A good one will set you back $60 or more.

 

[daven8844x]

Thanks a lot for the replies. So I went and warmed up the truck and got it to where it would expose the issue. I disconnected the fuel relay to diagnose. Hooked up the voltage meter, etc. When I had my wife hit the starter, the first "chug" sounded awful like it always does. After this chug, the truck usually starts so I never heard anything after that when driving...just sounded bad.

Anyway, in diagnosis mode with the fuel pump relay pulled, after the first awful "chug," the starter sounded normal as it kept trying to start. All subsequent start attempts after that sounded normal too. Voltage drop on ground checked out fine (about 0.25v). Positive cable from battery to starter (and alternator) is brand new and starter is only a couple of years old (direct drive). Alternator and battery are also brand new. So is the fuel pump. Sounds like it may be something else? ICM? Missing heat shield near starter (though I assume the problem would persist through the starter activity)? What would cause just the first turn of the starter to be hard?

I only worry because I had a similar issue a couple of years ago and the truck left me stranded. The mechanic ended up replacing some older wiring that had melted (I want to say something near the transmission, like a safety switch). Any ideas?

Thanks again!!

 

[Schurkey]

Had an extensive reply typed, got a message that I didn't have permission to edit it; and now it's just plain gone.

Verify the battery state-of-charge and actual cold-cranking amps capacity.
Install the missing heat shield
Re-do your voltage drop test. 10 volts of voltage drop is not realistic. HALF a volt is bad news. 10 volts is crazy.

 

[daven8844x]

Had an extensive reply typed, got a message that I didn't have permission to edit it; and now it's just plain gone.

Verify the battery state-of-charge and actual cold-cranking amps capacity.
Install the missing heat shield
Re-do your voltage drop test. 10 volts of voltage drop is not realistic. HALF a volt is bad news. 10 volts is crazy.

Thank you again for the replies. Schurkey, the voltage reading was a mistake. I was clipped to the purple wire instead of the starter case, so was measuring the voltage of the completed circuit during start @ 10v. I do not think there is any voltage drop in the cables.

See my last post right above yours, which outlines the result of the starter test. Basically, just the first turn of the starter sounds terrible (only when the truck is warmed up on a hot southern day), but then it sounds fine. My suspicion is heat on the starter/solenoid. I do have the heat shield in place. Its the original shield and looks old from the bottom. I am going to replace the starter back to AC Delco gear reduction, and I will give the heat shield a closer look at that time.

 

[Schurkey]

First turn of the starter being slow is often a failed battery.

Takes some time for the starter to build RPM; until an electric motor has RPM, it's basically a dead short. Battery with low amperage capacity struggles to supply current.

Once the starter gets some RPM, it draws less current and cranks relatively normally.

 

[Ken K]

A voltage drop of 10Vdc on the negative side from the new battery to block, leaves your starter solenoid with 2.8Vdc (12.8V - 10.0V = 2.8) Use a good jumper cable from battery to starter at bracket or other good ground. If it cranks hot or cold, paint, loose bolts, loose ground wire to block or inside solenoid or starter brushes. Heat can melt to ground but “Zero Drop” as path to ground is complete. Just make sure voltage drop is done while current is flowing with circuit on. The key is low current as, the solenoid is high-current switch. If solenoid completes circuit, then path to ground goes thru brushes, armature to field coil (Unless magnet field). Try jumper cable on ground only. Otherwise rebuild starter... or buy reman. No one rebuilds anything at home today. I would pull my own vehicle in when slow at dealer, go thru starter, alt, or check amps to fuel pump, hook amp probe to scope primary and judge waveform. Do what you need! Best of luck! Retired ASE Master.