I can't believe my luck.
For the past ten years, I have been through 9 Alternators at various amps due to a nasty voltage drop when the AC or Defrost was on and the car was under load. I had tried everything to alleviate the problem to no avail - I mean everything. I knew that the problem went away if I played 2 feet - one on the brake and one on the throttle - to get the rpm's up to 8 grand or above[edit: sorry, I meant 800 or above].

A couple of days ago, I was playing with a hunting idle problem since my engine rebuild; and, thanks to tech tips for resetting idle speed and TPS voltage, I inadvertently resolved this 10 year voltage mystery.

I had to tweak the idle speed and TPS voltage a bit away from factory specs to get my idle to at least 800 rpm under load and the TPS voltage is set at .533 - just a touch less than the factory .54 volts.

Car now idles great, voltage holds under load - even with this temporary 105 amp alternator - and I have my AC back problem free.

Unbelievable how important the right idle speed and TPS Voltage can be.

Hopefully, this stumbling, bumbling fluke of mine will help someone out there.

 

YOU cannot, legitimately alter your idle speed with a "tune up". The idle speed is burned into your chip and the ECM regulates the idle via the IAC, based on engine temperature, A/C operation, and other inputs. If you have set your idle speed with the throttle stop (minimum air adjustment) screw, you have circumnavigated some of the advantages of the ECM controlled idle. The correct "minimum air adjustment" establishes an idle speed, without the IAC, which is way too slow to make you happy. I can think of a couple of conditions that may have caused your previous problems, but if you are happy with your home grown, unconventional tune up, I guess that is all that counts.

RACE ON!!!

 

Don't know exactly what you did, but as CFI-EFI points out, idle is controlled by the ECM. Minimum idle (IAC fully extended) should spec out between 425 and 450 rpms. TPS .54 +/- .08 volts. Actual idle speed should be +/- 50 rpms (in Drive; +/- 100 rpms in Park/Neutral) from whatever the ECM targets for idle as shown on your scanner (My '89 targets 575 and I have an actual or 600 to 610 and with all of my accessories running I've always been on battery voltage with an engine temp of 205 to 212). A basic check of IAC control is to note idle speed; turn it off and then restart it - idle speed should be higher than what you initially noted and then return to that same, initial number after a few seconds. Maybe you just got everything back to specs?

Did you ever try an alternator rated with a higher low amperage output (I tried a 120 amp years ago and it didn't do anything)? I know that few manufacturers even publish that lower spec, but some of the aftermarket guys promise better #'s. The other thing to consider would be a custom chip that increases targeted RPM by 100 - don't know if that will keep it emissions friendly though. I would make sure that all of your Vette cylinders are making equal power as a weak one can wreck havoc on good idle control. Also, if you're pre '90 and running an Aux cooling fan, that zaps the little bit of remaining idle amperage that's powering everything else. It isn't needed (nor was it designed for anything other than extra profit for GM), so if the cooling system isn't maintaining temps within specs with only the 1 fan, I'd look into that too.

 

When I put it to factory specs, though, the problem persisted. The ECM always reset the idle and it continued to hunt. Then, I'd measure the voltage and it'd be above the .54 to which I set it. So, rather than setting the screw at 400 in drive (with paper clip removed and the IAC disconnected as per the tech tip), I bumped it to 500. But the idle still hunted and the TPS voltage again moved above the .54 until I set the TPS a bit lower to the .533 volts. Now the ECM seems to have "learned" the new settings. The TPS is steady at .533 volts and I have smooth idle at 800 even with my cam. When I put it under load with the AC Clutch engaged, the idle now adjusts itself accordingly which had never happened before.

Didn't know that was unconventional; I had read that adjusting the TPS voltage was a legit way to change idling characteristics - in fact, I believe there's an adjustable TPS for sale out there. I guess I was fluky for sure; but, 10 years of conventional attempts to get that voltage drop under control failed.

If you find some time, I'd really appreciate it if you'd share
those "conditions" that may have caused my previous voltage drop problems under load.

Thanks for the feedback.

 

I'm pretty sure that 800 rpm and .533 volts aren't factory specs but my engine (stock block but 383 with 1.6 roller rockers and 580 cam lift, ported and polished, headers, yada yada) isn't factory anymore either - even though the problem occured with my stock engine and I had a gut feeling that I needed more RPM to make the problem go away. Tried all types of alternators to no avail; cylinders are great; and, without that aux fan, the car overheats with the AC clutch engaged (another issue in another thread for some even with the aux fan on).

I'm just so relieved that I've got my AC back in order.

Nevertheless, thanks man for the feedback!

 

I've been sucked in. I've been had. Duped. I should know better. A perfect example of what happens when one ASSUMES. My fault. Usually, when no year is given, I demand one. I took the clue of, "I had to tweak the idle speed and TPS voltage" of it not being the latest of the L98s that have non-adjustable TPS's. I donno if it's an L98, an LT1, or an LT4. It might not even be a C4, for all we've been told. NOW, a few posts down the line, it's a 383, and some of the efforts were to tame a heretofore unmentioned non-stock cam. My mistake. My stupidity. I should have known better. Gone!

RACE ON!!!

PS. Also, you need to learn how to read a tachometer. I don't think your engine is REALLY idling at 8 grand (8000 rpms).

 

Maybe you have a pulley to lower the parasitic loads on the engine in order to gain hp at the clutch. These make your serpentine belt run slower which will make your alternator run slower than stock at idle. All alternators cannot deliver their rated current at idle rpm, but unless you have a high electrical load, your alternator at idle should be able to maintain 14.3 v (dash reading) cold, and 13.3 v hot. My stock 87 with a/c, headlights, radio, compartment fan, radiator main fan going, the alternator at stock idle will maintain 14.3 v cold and 13.6 v hot. High current alternators do not work any better than stock alternators at idle speeds.

 

If it overheats with the a/c engaged it may have a restricted condensor/radiator or the pressure is too high (which could also be a restriction). A fully charged and functional a/c system has liquid refrigerant leaving the condensor (on average) at anywhere from 170 to about 240 psi (it depends on outside air temp). Temps at those pressures range from about 115 to 145 degrees, well below the thermostat. It has to have unrestricted airflow to maintain this pressure, and if your Vette is pre '90 (or if it's not a Vette - all a/c/ cooling systems work pretty much the same way), the main fan needs to be on from idle to about 35 mph (and if it's not coming on that's something else to look into) and the fan shroud needs to be in decent shape. You can check for high pressure by aiming a garden hose at the condensor. If it stops overheating, you may just need to improve air flow (unbolt the shroud and remove whatever debris is between the radiator and the condensor) or you might have to investigate a little further and get whatever is plugging up the a/c lines unplugged. If you didn't replace the radiator when you swapped in the new engine, you might want to consider doing that now (particularly if it's 10 to 15 years old) and new condensors are fairly inexpensive too. Once the a/c charge is right and there's good air flow, you won't need the secondary fan (on any car that was designed to cool with just the one fan which is about all of the pre '90's OEMS). The aux fan is taking another 5 to 10 amps from your idle output of about 30 to 35 amps - the main fan is taking another 5 to 10; and a good blower motor another 10 to 15; so there is little amperage, at idle, to spare. Also, your engine temps (assuming you've kept the ECM parameters stock) should/will be lower with the a/c on then with it off. That's why a/c operation is exempt from emissions regs - it has to run at lower temps in order to function correctly - and its another reason cars need a decent thermostat.

 

It's an L98, 1988 block. Internally, it's a 383 bottom end the rest you know.

Buddy, I certainly don't appreciate your tone - self-depricating as it is; I was just sharing something that worked for me that might have helped others with similar problems. That kind of tone does wonders to encourage others to share stuff, huh?

I can, indeed, read a tach among other things; and it idles where it idles.
My mistake about 8000; I meant 800. I apologize for that. I'm an idiot and should be flogged for the mistake, I suppose, huh?

Speaking of reading, give it a try and look again: I had the idling/voltage drop problems even when the engine was stock, so I don't know where you're going with that.

Moreover, the "uncoventional tune", as you put it, was right from the tech tips section here; I don't claim to be a vette guru nor a mechanic; I'm just a guy who got lucky with a fix for my set up.

You said you could think of some other things that caused the voltage drop.
I'd still be interested in hearing your thoughts about that; unless, of course, you just feel like flaming some more. Whatever. I'll just keep to reading and enjoy forum that way rather than be subjected to such venom -subtle as it is.

Man o Man

 

Thanks man! I'll be sure to check all of that out.

 

Thanks for the advice! I already had undersized pulleys; I thought that would make the belt spin faster? No worries, though, problem is gone now that it idles properly. Take care.