Voltmeter issues on ‘83

[BobQ]

Just finished up doing the LMC tach conversion in my ‘83 K2500 6.2L. Took it for a ride this evening to double check everything before putting the instrument bezel back on. Tonight, the factory voltmeter starting reading high (15-16 volt range) just before the red zone on the upper end. It’s never read that high and I’ve never had any issues with it up until tonight. I checked the batteries with a multimeter and they were about 12.7 volts with the engine off and about 14.5 volts with it running and the gauge was still reading high. Do you think the voltmeter is going wacky or is there something else that could make it read higher all of a sudden even though the charging system itself seems fine?

Thanks.

[2INSANE]

Just finished up doing the LMC tach conversion in my ‘83 K2500 6.2L. Took it for a ride this evening to double check everything before putting the instrument bezel back on. Tonight, the factory voltmeter starting reading high (15-16 volt range) just before the red zone on the upper end. It’s never read that high and I’ve never had any issues with it up until tonight. I checked the batteries with a multimeter and they were about 12.7 volts with the engine off and about 14.5 volts with it running and the gauge was still reading high. Do you think the voltmeter is going wacky or is there something else that could make it read higher all of a sudden even though the charging system itself seems fine?

Thanks.

6.2 J code? Do you have dual alternators and dual batteries? If you have a 24v starting system on your J code, these volt numbers are normal on the 12v side of the electronics. Check the voltage on the output side of the alternators and plz report back.

[BobQ]

Yeah, it’s a civilian J-code truck with a regular Delco 12SI alternator (78 amp).

[arveetek]

Check your grounds, especially the small grounds that go from the negative battery cable to the fender; that is your main body ground. If that body ground is not good, your gauges will go all wonky, especially the voltmeter. I had that happen on my '81 one time.
I ran a new ground from the battery to the fender, and it fixed my issues.

Casey

[2INSANE]

Yeah, it’s a civilian J-code truck with a regular Delco 12SI alternator (78 amp).

The civilian 6.2 is a C code with a single alternator and 12v starting system. So i am assuming if you have a j code, it has been swapped at some point in time.

Anyhow, with a 12v starting system and a single alternator at 12.7 volts with the motor off and 14.5 when running, I think your electrical system and alternator is doing what it is meant to do and charging your system and you should be fine.

If you are getting in the 16v range on your tach but 14.5 on your multimeter I would suspect the tach calibration is off and not reading correctly. Did you wire the tach directly up to the battery that shows 14.5v with the multimeter? Also what is the volts coming out of the alternator when running?

[DmaxMaverick]

The civilian 6.2 is a C code with a single alternator and 12v starting system. So i am assuming if you have a j code, it has been swapped at some point in time....

The C/J codes merely specify the emissions class, and the 8th digit of the VIN. The K2500 is likely a J code, and almost certainly a J code if it isn't a 7200 GVWR CA emissions model.

[2INSANE]

The C/J codes merely specify the emissions class, and the 8th digit of the VIN. The K2500 is likely a J code, and almost certainly a J code if it isn't a 7200 GVWR CA emissions model.

My first truck was a 1983 K2500 GMC 6.2 C-Code. I owned it for 20 years and eventually dropped in a 6.2 J code military motor I bought from Boyce Equipment. Sold it 2 years ago. I really regret selling that street King as the 6” all spring lift kit was too stiff to drive off-road… One of the banks sidewinder kits I had on hand would have been an amazing addition to it. But awe well…The Oem C-code did not have an egr on it like a lot of C codes do. It was odd for that year of truck. I always thought only military or commercial 6.2 diesels had the J Code. Never heard of civilian models having a OEM J code. Learn something new every day!

[arveetek]

Never heard of civilian models having a OEM J code. Learn something new every day!

Quick reference: C code = EGR; J code = no EGR. Pretty much all 1/2 ton diesel vehicles were C code, and most 3/4 ton and all 1 ton vehicles were J code, give or take a few models and GVWR ratings as Greg stated.

Nice K2500 you had there!

Casey

[DmaxMaverick]

If a C code has no EGR, someone swapped the intake. A common upgrade.

[2INSANE]

Quick reference: C code = EGR; J code = no EGR. Pretty much all 1/2 ton diesel vehicles were C code, and most 3/4 ton and all 1 ton vehicles were J code, give or take a few models and GVWR ratings as Greg stated.

Nice K2500 you had there!

Casey

Thanks!

If a C code has no EGR, someone swapped the intake. A common upgrade.

So I just got off the phone with the guy that bought my old truck. It is a C code. I specifically remember taking apart the motor. The smaller C code intake that was on it for sure had no egr or a place to put one. I remember comparing the C model to the J and noticed the difference with the larger air ports. There are a couple non egr C code 6.2 diesel intakes on eBay right now. Interesting debate.

Anyhow, Bob sorry for derailing your thread. Any new updates on alternator output voltage? Did you try using the multimeter on the positive wire connecting to the voltmeter gauge to check that reading?

[BobQ]

Thanks a lot for the tips. I will check the grounds.

[BobQ]

I checked/cleaned grounds and cleaned alternator connections, no change to the gauge reading. Took voltage at the output of the alternator and it’s around 14.95-15 volts. Might pull the gauge next.

[DmaxMaverick]

It appears your gage is reading within about 1V of actual. That's as accurate as you can expect of GM instrumentation. Also consider margins of error. If your voltage is consistently too high, such as 15V, I'd suspect the voltage regulator. They're cheap, but if it's bad and you don't replace it, rotten eggs and destroyed batteries are not cheap. Don't wait for the voltage to hit 18V, which will kill both batteries in about 10 minutes. Stock up on baking soda if that happens.

I've had this happen a few times through the years with the internal regulators, and all but one ended with no casualties. The time it didn't was with an 83 S-10. It went to 15V, then to 18V after about 15 minutes. I was on a highway trip, and in between towns. I turned on all the lights, blower to high, and slowed down to keep the revs lower. Held it to just below 15V for the 20 miles to the closest parts store. Super-fast wipers were helpful, which was what queued me to the problem. I made the repair in the parking lot (was in WA Cascades, and it was pouring down rain, of course), then went on my way. No problems, I thought. The battery flattened in a week. It was less than 6 months old.

[BobQ]

Thanks for the help. I’ll check that out.

[2INSANE]

I checked/cleaned grounds and cleaned alternator connections, no change to the gauge reading. Took voltage at the output of the alternator and it’s around 14.95-15 volts. Might pull the gauge next.

Sounds good! Yeah if the back of the gauge is reading 14-15 but saying it is 16, then it is calibration being off for sure. Dmax has a point. It is also possible the alternator is starting to go bad. Keep us posted! A pic of your cluster would be cool to see

[DmaxMaverick]

Sounds good! Yeah if the back of the gauge is reading 14-15 but saying it is 16, then it is calibration being off for sure. Dmax has a point. It is also possible the alternator is starting to go bad. Keep us posted! A pic of your cluster would be cool to see

No, just the regulator. If the alternator is cranking out 15V and isn't squealing, it's fine. The gage being off by only 1V is about normal. I've seen worse. The regulator is internal (the part the connector plugs into, usually white when new, burned white when bad), and very easy to replace. All that's needed is about 15 minutes (alternator off the engine), common tools and 2 toothpicks or paperclips (to hold the brushes during reassembly). If there's visible corrosion, put a couple drops of PB Blaster on the ENDS of the loooong bolts where the threaded holes are, about 30 minutes before turning them. I usually put a dab of grease on the end bearing at reassembly, but that's about it. Very simple. A lot of alternators get replaced when that's all that's wrong. They will usually last through 3 or 4 regulators. You can replace the whole thing if you want, but that's $150+ vs. $15-20. I can walk you through it, if necessary.

For example:

https://www.autozone.com/batteries-s...723/130047_0_0

[BobQ]

Sorry for the delay guys. Took the alternator to a shop and it was putting out too much voltage. They said it was actually putting out about 15.5 volts, but should be around 14.7, which it is now after they replaced the regulator. So, I figured this was my whole issue. Nope. I put the alternator in and the voltmeter gauge still read high, just like before. I got a new OEM-style voltmeter and it now reads lower and closer to the actual voltage. Seems like all is good now. Thanks again for all your help.

[BobQ]

Here’s a picture of the cluster with the LMC tach. So, far it’s been working well.

[2INSANE]

Called it! Hehe! Glad we could help! Thank you for the update!

[DmaxMaverick]

Yes, a great outcome, and the cluster looks very nice.

Called it! Hehe! Glad we could help! Thank you for the update!

Called what? Let's keep it real. The regulator failed, and the original gage was reading typical GM accuracy. More accurate gages are always more desirable, but that's what we get with OEM gages. They are most effective for tracking trends, rather than actual values. The voltmeter did its job, in this case. Many/most other brands only have a +/- or H/L range, or just an idiot light, with no actual voltage indication, which gives them more room for inaccuracy.

The internal regulators are very easy to replace, although many folks don't have the confidence or interest in changing the simple, cheap part themselves. Alternators are very simple. The internal regulator is just a small block of plastic fastened in place with a couple screws. The most difficult part, which isn't difficult at all, is reassembly. It has spring-loaded brushes like a common electric motor, which have to be held with a pin until the housing halves are bolted together. There are holes in the housing, just push the brushes into their slots and insert the pin through the hole to hold them, hands-free. Remove the pins once assembled and the brushes spring into place. It takes longer to gather tools than to complete the replacement. That's it.

The analog gages (not late-model steppers) are calibrated with a resistor on the back of them. If you want more accurate gages, you can replace the resistor with a new or different value to make them more accurate. The OEM and +/- resistor values are published. That takes about 5 minutes with a solder iron, and costs about a dollar or two for the common parts. Or, you can spend a bunch on a new/other gage, or pay someone $$ to do it.