truck sometimes dies at the stoplight with the a/c on

[6716]

I mean, at least the a/c works, right? I think it is electrical-related, I also notice voltage drops when the a/c is running and I come off the gas. Alternator is nothing special, but it can't have more than 20K miles on it. Battery is within three years.

Also, it is not every stop light every time, but I do notice the volts dancing around the 14 mark up and down at most stop lights.

The first suggestion always seems to be throttle body, which is recently cleaned, so I don't think it is that.

It might be engine-related, I have 5 of 6 new plugs, one was stuck and I left it there.

 

[Expeditor]

I would check grounds and power cables for loose or corroded connections. Weak alternator/battery. I would also chek for excessive amp draw on A/C clutch. Overcharged A/C could also couse engine to bog down.

 

[freddyboy61]

What are the RPMs when stopped with AC off, then with AC on?
When you cleaned throttle body, did you pull PCM fuses or disconnect battery to reset PCM?

 

[6716]

I would check grounds and power cables for loose or corroded connections. Weak alternator/battery. I would also chek for excessive amp draw on A/C clutch. Overcharged A/C could also couse engine to bog down.

I think I want it to be the alternator because that is easy to fix and I could upgrade to a high-performance one and start adding accessories.

I can't find any links for checking the amp draw on the A/C clutch. Got any ideas? What is the normal draw there?

A/C might be overcharged I suppose. I did just have some A/C work done.

What are the RPMs when stopped with AC off, then with AC on?
When you cleaned throttle body, did you pull PCM fuses or disconnect battery to reset PCM?

I don't remember disconnecting the battery, maybe I didn't.

RPMs sit right at 600 as far as I can tell, the action seems to be in the volt meter. I'll keep an eye the tach as well.

 

[aaserv]

" I did just have some A/C work done." Hmmmm nothing seems to be jumping out at me. Could be anything I guess...

 

[gmcman]

I would check grounds and power cables for loose or corroded connections. Weak alternator/battery.

This.

Could be coincidental that the A/C is on. With the brakes on, you are also needing current for two incandescent bulbs. This could put you over the threshold for avail current if your alternator is weak, or you have poor contacts at the battery terminal. Just because the connections are tight, doesn't mean they are making good contact.

Take a multimeter and connect to the battery, then observe the voltage with headlights on, the high beams on, rear defroster, seat heat if avail, and see if it can hold steady at 13.6 or higher. I would also do the same test again with the red lead on the 125A maxi-fuse and see if the readings are the same.

If all that checks out, then I would move to the A/C and the TB to verify the TB is compensating for the A/C cycling.

 

[6716]

First things first, I cleaned up the battery terminals. I took the rubber boot off and inspected everything. There was only a tiny bit of corrosion on the negative terminal where the solid flat terminal piece clamps to the copper strands of the lead. The solid negative terminal piece was not quite as shiny as the positive. I took some steel wool to both of the terminal pieces, as well as the terminal bolts, which looked good where they contact the battery, but were pretty oxidized where you would clamp on jumper cables.

With as much stuff as I can turn on while not in the driver's seat, it holds steady at 13.7. Battery read 12.5 with the vehicle off. I didn't also check at the 125A maxi-fuse, I wasn't sure if I put the negative probe on the black cable there, or at the battery.

And then I guess I wonder how I verify that the TB is compensating for the A/C cycling?

In any case it hasn't died at the light on any test drives so far, so there's that.

Thanks

 

[6716]

Cleaning grounds seems to have been the final thing that needed to be done. Before, when I had just cleaned up the battery terminals the rpms would still stumble at the light and the volts would dip. It didn't stall out, but it still wasn't quite right.

I cleaned the 5 upper engine compartment grounds, and one of grounds on the engine block (G108) but not the other 3 because I couldn't get at them very well from the top. If I go after the other 3 engine block grounds some other time I will remove the wheel and try through there.

Now with those grounds clean the truck idles steady at 600 rpm with the a/c on. The volts move around a little, but not swinging hard like they were doing before. And the volts don't drop hard when I come off the accelerator.

 

[rchalmers3]

I'm a little late to this party, and I'm pleased to hear that @gmcman led you to an easy solution. however....

With as much stuff as I can turn on while not in the driver's seat, it holds steady at 13.7. Battery read 12.5 with the vehicle off.

These readings are low. A fully charged battery should measure at least 12.65 volts, and a recently charged battery with an undissapated surface charge may read as high as 2.2 volts per cell, thus 13.2 volts.

A weak battery usually triggers the alternator to produce a higher charge rate, definitely above 13.7. Therefore I am of the opinion that you are not yet out of the woods, and need to fully charge and test the battery, followed by an output test of the alternator.

Then again, if your junk is working, then....

Cheers,
Rick

 

[gmcman]

These readings are low. A fully charged battery should measure at least 12.65 volts

We also have to factor in device error.

.15 Vdc is not enough, IMHO, to point at the battery. Measurement tools have error and I believe this will fall into most error percentages.

Could have been unused for a number of days.

 

[rchalmers3]

Agreed, and also the measuring places of the battery voltage, be it directly on the posts or on the clamps can cause variances. We do not have this information so I can only go by the numbers mentioned.

In my opinion, the alternator is suspect. That output voltage does not sound, uh, sound.

Rick