Voltage Drop

[mobilevet]

Hi,

I'm experiencing a (to me) weird voltage drop.

When I crank up the voltage is up around 14. With some driving time, it will gradually drop down one dash/mark on the voltmeter - no problem. But if you do any of these three things: turn the front A/C on, the back A/C on, or just turn up the fan all the way (no A/C on) you will get a voltage drop of a dash/mark or two. If you turn them all on together (or any combination) you get a cumulative drop to the point that the needle is only half way between the 9 and 14. Also, if you do this for too long will get a code 35 or 36. The SES will go off if you turn all the blowers/A/C's off again (I assume the SES codes are due to the low voltage).

I checked my voltage at the batteries (idling no A/C, idling with A/C) and both batteries showed 14-13.5 volts.

Any ideas as to what's going on here??? and what to do about it???

Thanks

[DennisG01]

Just a shot here, I'm sure some others will give you a more definitive answer.

I wonder if a tired/bad battery(s) could be the culprit? One thing I can say for sure is that good batteries (at least my experience) will read about 12.6v (without the alternator running, of course). You could check what your batteries read after you shut down and then again the next morning. I'm just thinking that maybe the alternator can't keep up with the total demand if one or both batteries are going south. Of course, another basic thing to check is your battery cables and grounds.

[Beedee]

Check your battery connections, where the cables from one battery join to the other battery is a problem, Pull the plastic away and get in right behind, don't be suprised to find lots of corrosion. The stock GM system is pretty hokey. I think there is a cure for that in the members section, simple fix.
Also, check your grounds. You might even have to take them apart and clean and reassemble.
Do a search on grounds, should be plenty of information, it is a common problem.
Hope I helped.
Brian

[rhsub]

I had a 94 Sub 5.7 with a 100 amp alternator and it did the same thing at idle when you turned on frt and rear fans but it would come back up slowly to near normal once you picked up the rpms. If yours doesn't come back up i would check the alternator output under load. if you have a bad diode in the alternator it will only charge approx 2/3 of its rated output which is not enough to operate everthing

good luck
ron

[mobilevet]

Thanks. The batteries were new less than 2 years ago, but I will check them cold and see what they are showing. Also, I think you're right, a thorough going over of all the connections is probably warrented (I did read several old posts on that - thanx). Will let you all know - thanks for the thoughts.

[mobilevet]

Thanks. The batteries were new less than 2 years ago, but I will check them cold and see what they are showing. Also, I think you're right, a thorough going over of all the connections is probably warrented (I did read several old posts on that - thanx). Will let you all know - thanks for the thoughts.

[Bnave95]

http://www.thedieselpage.com/members/batterm.htm
One of the best upgrades to date :cool:

[mobilevet]

Checked the batteries cold - 12v+ each. Started checking the grounds, realized that the two grounds from the battery were R&R'd when I upgraded the water pump (passenger side bolted to a stud off the thermo crossover, drive side bolted to a stud off the intake manifold). Found each pig-tail ground off the batteries, both tight, passenger side was R&R'd when I removed the air-snorkel. I also R&R'd the positive terminals when I did the waterpump and sprayed all with battery terminal protection. Found the ground on the back of the block (passenger side) was tight but R&R'd it along with a double-strand braided ground coming up from what seemed to be the transmission bell area (?). When I took the bad PMD off (see "missing"/Code 16 posts) and re-hooked up the one on the firewall, I re-connected the ground from the PMD back to the ground on the origninal wiring harness from the pump. Is there another ground on the IP that I need to find and R&R? Did I miss any grounds???

What about checking the load amps? I did recheck the batteries at an idle and under a load (front and rear AC + fan on high) and they were at 14 volts. My multimeter said it has a max test of 10 amps.... someone said something about alternators putting out 100 amps???? This right? Sounds like it could mess up my meter. Anyway, lets say the meter is okay and I'm wrong about the 100 amp deal, to test them amps do you unhook the cable from the alternator and put the meter in-line (series) then crank up and take the reading?

One last note, belts. My belt has a chirp. I wonder if this could be part of the voltage problem? The chirp is regular but not constant. It almost sounds like it is when a certain spot on the belt hits during a revolution. Any ideas??

thanks

[joe bleaux]

Do not hook up your ten amp meter. As you said, you will fry it or most probably just blow a fuse inside the meter.

Joe

[moondoggie]

Good Day!

You have no need to check the amperage output of your alternator. You can with great ease check the result of your alternator's output amperage. Start the truck. At idle, with all electrical accessories off, you'll generally see 14.0 - 14.5V at both batteries (measured with a DVM, NOT you dash meter). Turn both A/C fans on full speed. The battery voltage might drop some at idle, but ought to be back to 14.0 - 14.5V by 1500 rpm or so. If it drops considerably & won't come back up with higher rpms, the most likely culprit is your alternator.

Most of my GM vehicles continue to show 14.0 - 14.5V with the fan(s) on high, even at idle rpm. The ability to put the batteries at 14.0 - 14.5V reflects the fact that the alternator has adequate output amperage. If it didn't, it would be reflected by low battery voltage.

Measuring battery voltage with the vehicle off is of limited value for your situation, IHMO. Freshly-charged lead-acid batteries will show ~ 13.2V until the surface charge is removed, then they'll show ~ 12.6V, but there are other variables. If your system passes the above test, your alternator is putting out at least 2/3 of its rated output amperage. If it cranks OK, your batteries are probably fine. You don't have your location in your info, so I don't know how cold it gets where you are; winter temps add another whole variable to this.

If you really need to know if your alternator is putting out rated amps, most good parts stores will test this for you at no charge. The alternator on your truck is about a 5 minute job to remove, so if you just gotta know, it's no biggie to find out for sure. Personally, I've never had a need to do so.

Grounds are a HUGE potential problem with these trucks, especially as they age. Do some searching, there should be TONS of info on this.

Good Luck & Blessings!

[Robyn]

The 94 is notorious for this sort of crap. The issue is most likely with the cables.
Power from the Alt on this year truck comes to LH battery and then crosses to the RH via the large cable over the header support.
Power then feeds from the RH battery via a #4 cable down along the frame with the supply line to the starter and then behind the engine and up into the junction box just above the AC unit.

Remove the cover from the junction box and check the power at the large stud in there.
You need to see 14.2-14.7 with the engine idleing and all accessories turned on (lights too)

If the power level drops off then its a cable issue or an alternator issue.

I have dealt with this very thing on two 94's and a 95 The 95 has a slightly different wiring setup but the results are the same.

Once the power level drops off just so far the ECM gets unhappy and things get HINKY.

If your voltage is not up to snuff at the power stud in the junction box start by making sure the alternator is putting out properly.
If this is good then the cables are to blame.

A fresh #4 can be run from the RH + of the battery across the inner fender around under the air filter assembly and into the junction block.
You can simply cut the old one off at the battery and trim it back outside the junction box and leave it alone until such time as the engine is out then clean out the remaining wire where it traverses in back of the engine and up to the junction.

These things are very common and easy to fix.

Now if you do find that you have 14.5 at the power stud then there can be an issue with the ignition switch.

Im betting you have cable issues.

best

Robyn

[mobilevet]

Thanks for all the advice. I did check the batteries with accessories (not lights though) on and they were still getting 14 - 14+.

Does anyone know if I found all the grounds? (see previous post)

I will check the other cables ASAP.

Thanks again...

[Robyn]

Be sure your getting the 14.2- 14.7 at the main chassis feed terminal in the junction box.

Robyn

[mobilevet]

Okay.... I checked the hot post at the junction box and with a load (lights, fans, A/C) it did drop down to about 12v. So, I removed both batteries, all the cables (pos and neg) and where they attached (except the end of the hot attached to the starter) cleaned the connections, re-did the posts according to the article suggested earlier (thanks), and put everything back together. I found a wad of corrosion inside the plastic housing/cover on the driver side positive cable - cleaned all that up. I re-wired the cable from the passenger side positive to the junction box. Now, my hardware store didn't have 4guage wire, so I got 8 gauge wire, which didn't look too much different size wise from the cable that was there to start with - hope that was okay. So, after all that now, at idle with all the load, the junction box post shows 13v. (I didn't try reving it up - couldn't, didn't have anyone to rev while I was watching the meter)...

Any idea if this is adequate? Or, do I need to do more, something different??? Would having other accessories wired to the batteries make any difference (e.g. trailer brake and controller)??

Don't see how it would matter, but I did not cut the wire that originally went to the hot junction box post. I just un-hooked it, wrapped it up good in electrical tape, taped it back on itself, and stuck it down out of the way... that ok?

Thanks for ideas..

[Robyn]

Now we are getting somewhere :D

At least we know its not a bloody ghost someplace in the wiring.

If the main battery cables had that much corrosion under the red jackets then the core of the cable itself is no doubt badly corroded.
These are flash soldered and once the core corrodes then the power transfer through the cable drops right down the toilet.

Be sure to have the alternator tested to make sure it can produce 14- 14.5 Volts at its maximum rated amperage.

If the alt is ok then the main battery cables need to be replaced.
You do need a nice number 4 cable on the power feed to the chassis supply stud.

You must see at least 13.5 V preferably 14 at the + stud in the junction box with the engine running and the accessories on.

Now if you can get 13.5 to 13.8 at the power stud it will be ok.
With only 12 showing you were seeing much less on some of the control circuits that have very small wires.

I cant stress enough that the cables must be replaced. A set of NAPA cables (parts store) will get you up and flying.

Be sure the ground cables are new too if you have not replaced them recently. Many times the ground on the LH battery will fail and these can fail at the stud on the engine as well as the terminal end. When this happens the alternator can't charge the system properly and things go down hill real fast.

Good grounds, good plus cables and good alternator and you should see things fall right into line and work sweet.


Best

Robyn

[JCM5]

I had the exact same problem on mine.
What I found was very simple.

One the grounds going to the dash was 2volts diff. from the engine ground. Ran a ground wire between the two and my gauge came up 1-2 volts.

Second, you might want to hear this but you most likely have a dead cell in one of your batteries. Mine checked out OK as you descrbed, sit idle with all the ACC units on and the gauge would drop down to 10volts after a minute. Turn off the ACC and the gauge would go right back up. 2 year old batteries are still new but it happens. Change them out.

Alternator at load, you would think it would maintain 14volts but when you have a dead cell in the battery it will suck it down.

[Robyn]

The best thing you can give your little diesel creature is a nice set of High performance (CCA) jell cell batteries.
They perform very well and dont leak all over and eat up the truck.

best

Robyn

[moondoggie]

Good Day!

Please forgive my previous, misleading post. Sometimes, in the face of overwhelming evidence to the contrary, I think I have something to offer. :o Robyn has forgotten more about these trucks than I'll ever know; mostly her advice is solid gold; & others here too. :D

Blessings!
(signature in previous post)

[mobilevet]

Thanks for the thoughts. I have the supplies to replace the corroded line and upgrade the hot to the junction with a #4.

But, I'm thinking that I'm still missing some of these grounds??? I found 3 (see previous posts) plus a small short braided line from the rear bumper to up around the fuel tank inlet.... am I missing some more???

[mobilevet]

New #4 cable from hot on passenger-side batt to junction box. New #4 cable crossover from hot to hot on batteries (this was the one corroded).

Result: Improved. Measures 13.0 at the junction box with full load (front and rear A/C, Blower fan on high, and lights on)

What next??

[joe bleaux]

I know that you have had your troubles with this and just have to be tired of addressing it but, really, #4 cable is not big enough.

If this is the biggest cable from the passenger or any side battery, it should be much, much larger. I would start at at least 1 ought cable.

The circular mils rating is what counts and it is not a linear but logarithmic function of current capacity or resistance per foot versus gauge rating.

Joe



New #4 cable from hot on passenger-side batt to junction box. New #4 cable crossover from hot to hot on batteries (this was the one corroded).

Result: Improved. Measures 13.0 at the junction box with full load (front and rear A/C, Blower fan on high, and lights on)

What next??

[JohnC]

Bigger is better, but a #4 ought to be fine. The resistance is on the order of .0003 Ohms/ft. The voltage drop on a 4 foot length at 500 amps is only .6 volts. What's your starter draw?

[joe bleaux]

The problem is not when the starter is running and spinning the engine. The problem is to have enough instantaneous power to overcome the inertia of a static engine.

Me? I would want much larger cable.

Joe



Bigger is better, but a #4 ought to be fine. The resistance is on the order of .0003 Ohms/ft. The voltage drop on a 4 foot length at 500 amps is only .6 volts. What's your starter draw?

[Robyn]

I spoke of the #4 cable. (about the size of a large pencil)
This is only the power feed from the battery to the chassis Junction box by the AC

The crossover and the feed to the starter need to be 1-0 (One Ought) the grounds from the batteries to the engine can also be 1-0 too.

The #4 is fine to supply the power to the junction box. You wont need more than this to supply the truck.
The 1-0 is needed to supply the huge draw of the starter.

Hope this helps

Robyn

[joe bleaux]

Robyn,

Actually, someone told this to me before. Perhaps, it was you. Anyway, I forgot that the junction box is not the feed to the starter.

I was thinking that the box fed the starter and that is why I said that #4 was too light.

I may remember now, then again.........

Joe





I spoke of the #4 cable. (about the size of a large pencil)
This is only the power feed from the battery to the chassis Junction box by the AC

The crossover and the feed to the starter need to be 1-0 (One Ought) the grounds from the batteries to the engine can also be 1-0 too.

The #4 is fine to supply the power to the junction box. You wont need more than this to supply the truck.
The 1-0 is needed to supply the huge draw of the starter.

Hope this helps

Robyn