LT Texan

I converted to the Delco alternator with no real problems.

Steady 13.5 - 14volts. Even at idle.

But I started noticing something strange.

When the engine heated up, the volts dropped to about 12.5 (all per the dash gauge)

Before i start digging around, anyone car to venture a guess? Heat soaked alternator?

tmpusfugit

I installed a new Bosch 130 amp alternator today and noticed the same behavior (digital voltmeter plugged into cigarette lighter) On my '84 the voltage started off at about 13.85 vdc but dropped down to 12.8-13.0 vdc after 10 miles on the road.

Iwanna928

I did a delco 130 conversion. The alt was bumped up to 140 amps. Sometimes the gauge reads almost 15 other times it reads 12. I know that when the system is read that the gauge is off by at least 1. It always puts out 14.6 or so w/ a electric fan and a 520 wat amp. I don't truat any of my gauges. I am going to look into some accurate A piller gauges. It goes on the to do list!

dr bob

Alternators are less efficient when hot. Almost all voltage regulators have temperature compensation to protect the alternator when it's hot. Just put the plastic alternator hood and the cooling hose back on.

Maleficio

Mine drops to 12 or 11 after the engine gets fully warmed up. I wonder if my alt ducting is full of leaves?

Landseer

Am running a pair of unducted Delcos and a pair of ducted Bosch. The dash voltage readings all seem to vary with temp and with accessory selection. I think you are ok.

The only thing that has me concerned about the Delco is potentially flooding it. With no ducting and no belly pans it runs very close to the road. Was in some heavy rain during a 450 mile ride yesterday and had no problems but it worried me. Will put belly pan on this week and that should provide reasonable protection.

LT Texan

Thanks for all the replies.

I think a bit more "observing" with my multimeter is in order.

Also, pulling the pod and cleaning lots of contracts and grounds everywhere (again and again and again...)

And another project on the list - Delco air ducting!

Maybe looking at some heatshielding too.

tmpusfugit

I have cleaned all the grounds, tried replacing the main battery ground strap, and have pulled the pod and cleaned and tightened the screws on the VM as well as the rest of the connections (while replacing the ODO drive gear) all to no avail. Using a DVM the readings remain a bit low, well below the 14.5 vdc I see on my other cars...

I have not yet pulled the relay panel and refurbished the fuse connections and the like.

The new 130 amp Bosch alternator does "put out" a volt more than the refurbished 95 amp Bosch that was there previously. By the way, I have recent experience with the test machines at places like AutoZone, O'Reily, and Pep Boys as being less than 100% accurate, try a record of 0 for 2 so far in the last 30 days...

I am wondering about the value of the resistor connected to the D+ terminal? Given I am using an entirely different than original alternator I am thinking I may need to do some investigation into that...I know Sharkskin did a fair amount of work to that point (see http://members.rennlist.com/sharkski...Alternator.htm) so it might seem a different value resistor could be needed to raise the upper limit on the regulator voltage from the alternator?

dr bob

The resistor in question is the one in the exciter circuit from the ignition switch. Located in the dash pod and wired in parallel with the indicaor light, the resistor-and-bulb circuit provides initial excitation current to the alternator. As soon as the alternator begins generating power, excitation current is drawn internally from the alternator B+, and no current flows through the bulb and resistor anymore. This self-excitation allows the bulb to go out when it's working, since there's no current flow. Once the alternator starts making power, the value of the resistor makes no difference in what the regulator decides is the right amount of field current needed to make the desired B+ terminal voltage. What it does affect is when the alternator first starts generating. If the resistor doesn't flow enough current (too much resistance) the alternator will need to spin a lot faster before it starts making power initially. There's usually enough residual magnetism in armature to kick off generation by about 4000 engine RPM's.

Bottom line is that if you are seeing alternator output voltage greater than normal battery resting voltage, the light and resistor are already done with their job, and it's on to alternator capability and wiring/connections for reasons why voltage may not be higher.

Remember that the regulator is working to keep its own terminal voltage at whatever its rating is. If you want to measure alternator voltage, you'll need to attach your meter to the alternator terminals, not to the jump post or the battery. Resistance in the connections and wiring, combined with electrical loads, will cause jump post and battery terminal readings to be different. How different? Depends on resistance in the wires and connections, including grounds, and the amount of current flowing in that particular circuit. Looking from the alternator towards the battery, the alternator is pushing current to charge the battery, plus the fuel pump and injection. On the S4+ cars, the cooling fans also connect to the rear wiring. Looking towards the jump post, there's a stub connection to the ABS pumps, and the connection through the jump post flows current to everything else the car that's running. So lights/wipers/HVAC/horn/radio/windows/seat motors/etc all work to reduce voltage as their current demands go up, and all through that jump post connection.

tmpusfugit

Dr Bob,

Thank you for the education regarding the resistor/bulb/exciter circuit. I was about to run down the wrong path (not for the first time either)


The largest issue I have at the moment is the voltage guage in the dash is off by over 1 volt, which is just irritating.....and the alternator regulates at abput 12.85 vdc, which seems low compared to other cars I have measured recently. So, the gauge shows under 12 vdc.......

Perhaps I am tilting at windmills. I think I need pull the POD again and go through all the connections carefully, and calibrate / adjust the voltmeter to 12 vdc while I have it out....I am thinking there is a pot for adjustment on the board....off to do some more reading, and again thank you for pointing me in the right direction..

Jim

dr bob

Make sure your 'new' alternator is doing its job. My original had all the signs of working OK, including decent dash gauge readings under most conditions. But it just wasn't keeping the battery charged, even displaying 13+ volts at the jump post. The Genuine Bosch exact replacement has the gauge sitting just shy of the 14V line at idle with lights/HVAC/etc running. The original tests OK on the POLAPS bench tester too. So I agree that the testers at those places are hit-or-miss. Bottom line is that I could easily remarket my cleaned and cosmetically-refreshed original as "good" based on the POLAPS testing and the dash gauge, yet I know it wouldn't solve many problems for someone else. Get a real alternator from one of the known-good vendors if you have doubts about what yours is doing for you.

LT Texan

dr bob,

Surely your battery doesn't know if the electrons flowing across it are generated from a Delco or Bosch alternator.

If either keeps the battery reading >13V across the + and - terminals, hard to believe one will charge it and the other will not.

Maybe it's like wingnuts?

dr bob

Dan,

I have a shingle around here somplece that says "EE" on it, and went through a lot of testing and analyzing of the problem. Cleaned connections, ohm'd wiring, tested for voltage drop between components. Evrything said that the battery is charging and the alternator is working. Tom M in the PacNW sent me a used alternator, definitely worn out, so it went as a core. The new one sat on the bench here for a couple months while I scratched my head, measured, etc. Finally I had to put the belly pans back on after almost trashing the spoiler on a parking bumper, so WTH, I'll put the new on in 'just because'. Now the battery is charging fine.

I haven't disassembled the original one yet. Maybe I'll find that one of the stator windings is open, or a diode or two has failed. Either one of those failures would alllow 13+ at low load but fail to deliver adequate current when load went up. It really doesn't matter though; the Genuine Bosch replacement is doing the job fine now. If a bargain-basement rebuilder worked on the OP's unit and didn't do all the testing of the diodes and windings, one could have an apparently-fine unit that doesn't deliver when it's needed. Users can test with full load (fans/headlights/HVAC/etc) and get a better feel for loaded capability of course. Few do. Mine showed 13+ under that condition at 1500 RPM, but it wasn't enough to keep up with loads and recharging duty at idle. New one at 13.9 does.

tmpusfugit

I think dr bob has a number of good points. I decided to upgrade from a 95 amp to a 115 amp, rebuilt. Even after cleaning all the grounds it performed much like the one dr bob describes. Looked like it was working, just not working well. So I then upgraded to a rebuilt 130 amp Bosch. It worked well for about 50 miles...behaved like I thought it should....then it's output dropped to 11.85 vdc..... I opened it up and found the slip rings to be deeply groved (but clean). So, the rebuilder thought that to be good enough, and it was good enough to pass their testing. Just not done right, and IMO expensive junk. I have spent nearly enough on these two "rebuilts" as to buy a proper OEM.

LT Texan

ouchey - ouch - ouch.

dr bob, I defer to you and promise no more smart *** comments