Odd voltage drop? snaking out the actual problem

[redfishsc]

I have two 1999 Suburbans. One is a C1500, other is a K1500, both are 5.7 Vortecs.

This question is mostly for the K1500 but I think something similar happens on the C1500.

Truck just got new head gaskets, new PS pump, new alternator, new battery, and the MFI injector upgrade. I have a good ground connection at the back of the Cylinder 8 head, and a good ground on the base of engine block below the alternator.

Driving around town, the voltage will often drop to 12.5 or lower (reading from an OBD2 scanner app on my android), especially if I have both HVAC blowers going and headlights on. Dashboard gauge confirms this is "roughly about right" . I've seen it drop as low as 11.8

So cleaned up the battery contacts as well as the ground strap that sits next to the battery (admittedly it was a wee bit loose). Also cleaned the positive cable that goes around the engine compartment to the fuse box near the windshield.


I tested the alternator with my multimeter. One contact on the alt's power output screw, other contact on the battery negative. Consistently this reads 0.5 to 1.0 volts higher than the OBD2 reading. Not sure if this is normal or not.


Here's an example of what I saw:

OBD2 voltage: 13.0

Voltage across battery terminals 13.3

Voltage from alt power output to neg battery terminal 13.8


Does this mean the battery could be wonky? Admittedly I did get a cheapo $50 Johnson-Controls from Walmart (600cca). Or should I be looking for other bad grounds?

 

[redfishsc]

Just for reference. I'm showing no voltage drop on the actual alternator output cable.

Vehicle off, I get
12.65v across battery terminals
12.65v across negative battery terminal and alternator output post

For resistance I get anything from 0.0 to 0.4 ohms, and that 0.4 is just "noise" I believe.


I'm going to upgrade that cable anyway just for giggles

 

[df2x4]

I'd be suspicious of your blower motors. I've had one go bad in my Suburban and cause a similar issue before it eventually gave up.

 

[redfishsc]

I'd be suspicious of your blower motors. I've had one go bad in my Suburban and cause a similar issue before it eventually gave up.

That's something I will definitely keep in mind. The voltage still does fluctuate with headlights on (I was driving at night) and neither blower running...... just doesn't drop as much obviously.

I won't need to run the rear one often thankfully.


I'm going to do a "big 3" wire upgrade on it. The only one I'm waffling on is adding a ground from the alternator's mounting bracket to the battery neg cable. I've read comments where that can confuse the voltage regulator, but then again this is done on a daily basis by professional audio shops, and honestly I think this was done by a shop on my 1990 back when I was a stupid teenager that wanted to spend $!!! on subwoofers.

 

[redfishsc]

I haven't yet done the "big 3" but will do it when materials arrive tomorrow. Friday I went through and really cleaned up the ground next to the battery and all the terminal contacts.

However, once I got done with the suspension and U-joint repairs today, I took her out for a drive. Voltage never wobbled. Had the fan blower motors OFF the whole time but headlights on. Voltage was fine, hovered around 13.8 (13.3 to 14.2 was what I saw depending on the situation).

Turn the blowers on and it drops, but not as bad. I think the problem was compound. Lousy grounds/contacts, and these blower motors just suck a lot of ass when you turn then on.

 

[Bob L]

I had a similar problem with my 98 and it turned out to be a bad plug on the alternator.