[PontGuy]

Think I know the answer to this, but hoping..

New-to-me 74 Trans AM with factory A/C. Car is very nice but still has the original wiring, and probably the orginal A/C fan motor.

With just the engine running voltage is very stable at 14.3. And with headlights on then a very small drop. But with the A/C fan running voltage drops down to around 13 (with A/C on or off). And with both headlights and A/C fan running then even lower than that- enough to stall the car at idle when I flip the fan on.

New battery, and just replaced the alternator (63 amp) which helped a bit but did not solve the problem.

Is the old fan motor just drawing way too much current? Has anyone seen this before? Hate to go through all the trouble replacing it unless confident that the fan motor is the problem.

[TedRamAirII]

IDK. Be sure to verify a good ground. Measure the voltage between the engine and the battery NEG while cranking. Measure the voltage between the body and NEG terminal. Measure voltage between ALT body and NEG Terminal. Measure voltage between Alt Batt wire terminal and POS Batt Terminal. Anything more than .5 volts is too much. Is the idle speed high enough with a/c on? I would think if the blower was pulling too many amps the fuse would blow. Usually normal amp draw is half of the fuse rating.

[PontGuy]

I expected these sort of questions. The car cranks right up so the ground to the block should be OK. There is a big ground wire between the trans and frame. And I confirmed that the ground strap from the block to the firewall is getting a good connection. But I guess its worth pulling all the wire connections and cleaning them up just to be sure. And measuring the voltage drops as you suggest.

Agree that the fuse should blow if the fan motor is really pulling that much. I'll add checking the fuse to the list to make sure that it hasn't been by-passed.

Another factor- there is a 200+watt RMS power amp in the trunk. It is fused and should not pull that much, but I'll try disconnecting it at the battery.

Just seems that with everything turned on a 63A alternator can't keep up, and the A/C fan is the biggest electrical load.

[chrisp]

Is the idle speed high enough with a/c on? [/QUOTE] Turn all systems on then adjust to proper idle .

[Blued and Painted]

Blower circuit is normally protected by 30 amp fuse, but a fan should normally draw around 10 amps. The 40 year old inline fuse holder was burnt on my car so i replaced it with the modern blade style.

Check the voltage at 1500 rpm's. Should be 13.4 or better with blower and lights on.

Make sure the belt is high in the V pulley. If it's running in the bottom of the V, slippage without sequel will occur

[george kujanski]

All of the above; at idle with heavy electrical loads, the alt can't provide enuf current so the volatge drops, 9if the belt is not slipping).

Increase the idle speed a bit with the A/C on or add a idle speed solenoid such as found on some A/C cars to bump up the idle speed when the A/C is on.

George

[PontGuy]

Check the voltage at 1500 rpm's. Should be 13.4 or better with blower and lights on

This is what is so odd. Its not even close, even with the A/C compressor cut off. The dash gauge is pretty accurate (compared to my good multimeter) and it reads well under 13V driving down the road when both the blower and lights are on.

The engine stall is either due to a sudden electrical load on the system that is killing power to the ignition, or the drag of the alternator working as hard as it can to keep up. If the lights are off then no problem. But if the lights are on and then the fan is cut on then it goes south. Again this is with the compressor cut off so no A/C drag on the engine.

Could there be something else that is drawing a ton of current? Maybe a bad coil? I tried disconnecting the audio amp and that did not make any difference.

I cleaned all the grounds, added another ground strap, and checked the volts from the alt to the firewall to the battery, etc. Not more than a few mV difference so a bad ground does not seem to be the cause. I really wouldn't expect this to be a ground issue anyway.

Another odd thing with this car. When I connect the battery I get a pretty good spark at the connection even with everything cut off. But it only sparks once, and it does not seem to be killing the battery.

[GOLDOHC]

I would take the alt. back and have it load tested

[george kujanski]

Speed up your idle a bit. The loading on the alt slows the engine down more and if your base idle speed is slow to begin, with may cause the stall.

If it still stalls with the idle speed higher, more diagnosis is required. Check for vacuum leaks also...with vac leaks, slowing the engine by adding electrical loads causes poor running and eventual stall.

george

[PontGuy]

The idle on this engine is rock solid. It goes in and out of gear easily, etc. And I just re-ran the vacuum lines when I installed the new carb. I know you guys are having a tough time believing, but increasing the idle speed does not help the voltage "dive".

A bit more history here- the car had a Holley EFI system on it when I bought it and wouldn't run right at all with the lights and blower on. It would die even at a light cruise. I pulled the EFI mainly because I didn't want to deal with it, but thinking now that the low voltage to the EFI controller was a big part of the problem.

I've owned plenty of old cars that were affected by the A/C compressor load, but can't think of one that wanted to die just turning on the lights and heater blower. My Lemans is sitting in the garage right next to the TA, and turning on the blower and lights has no impact on the idle. It doesn't have a volt meter, but tonight I'll fire it off and report the numbers on both cars measured at the battery just to compare.

I had the original alternator load-tested and it checked out good. I bought another just to be sure and it hasn't solved the problem. No doubt in my mind that something is pulling a bunch more load than it should. But it is not blowing any fuses so no obvious place to look. The blower motor is suspect because I can see the big impact on the volt meter when I cut it on. I still need to check the fuse to make sure that it is rated properly- another task for tonight.

For those that have not run across my threads in the street or TA sections here is a picture of the car so you can see what I am working on. Got to get it right!

[stellar]

Test the amp draw of the fan motor. I suspect the spark when connecting the battery is the power amp loading up and should not be a problem.

[george kujanski]

Is the alt wiring original? What alt is in there...10SI with ext reg or a 10SI with internal reg?

Regardless, if the sense point for the reg is not connected correctl y, the system voltage under load may be lower than the voltage at the alt. For a 10 DN, the wire on terminal 3 of the reg should be connected to the battery pos. For a 10SI alt, terminal 2 on the alt needs to be connected to the bat pos also.

Some people connect term 2 of a 10SI directly to the alt output stud........that does not regulate the bat nor the system voltage well.

George

[Blued and Painted]

Get another alternator.

[george kujanski]

I thought of another possibility....more probable with a 10SI.....if the sense wire from the alt terminal 2 is in a harness where there is ignition noise, Or too close to spark plug wires, for example, the xtra electrical noise may drive the regulator crazy and it won't regulate well.

Just another possibility since your system wiring was modified.

George

[TedRamAirII]

You may need to check at the beginning of all electrical issues. Battery good? Get it load tested and/or check voltage while cranking it over for 10 seconds. If it goes below 10-9.6 volts, thats not good. Does it do it for any blower speed or just high speed? I think there is a "High Speed" relay.

[PontGuy]

Just spent a few minutes measuring voltage at the battery. All readings taken with car in park at fast idle:

Before startup- 12.55
Startup, fast idle- 14.30
Lights on- 14.24
Blower on- 14.00
Lights and blower on- 13.70
Lights and blower on after warmup (still fast idle)- 12.80

All that looks OK, but here is what happened next.

Shut the blower and lights off- voltage back at 14.30
Blower on- immediately killed engine
Restarted engine- could not turn blower on without killing it. Tried several times.

Just to round things out, responses to questions and suggestions:

Test the amp draw of the fan motor. I suspect the spark when connecting the battery is the power amp loading up and should not be a problem.

I just realized that they make DC clampmeters for under $100 these days. Don't know if I can find one locally but I could order one from Amazon and have it here by Fri. Sounds like a good investment. I disconnected the power amp and still got the spark, so still do not understand what is up with that.

Is the alt wiring original? What alt is in there...10SI with ext reg or a 10SI with internal reg?

Internal regulator that uses the stock plug-in. Admit I am stuck in the 60's and much more practiced at debugging external voltage regulator systems. And I don't remember ever having a charging problem with the few cars I've owned with one of these.

Regardless, if the sense point for the reg is not connected correctly, the system voltage under load may be lower than the voltage at the alt.

All the wiring under the hood is original. As much money that went into the car I would have replaced all the wiring during the restore, but it is what it is for now. I would need to unwrap all the black sticky tape to check it out, but probably worth doing. The GM chassis wiring diagram does not show a separate lead to the battery but I can unplug and jumper it over there to see what happens. Except that would not explain why the blower is killing the engine.

Get another alternator.

On alternator number 2 now, and the one I pulled checked good. Same exact problem.

Battery good? Get it load tested and/or check voltage while cranking it over for 10 seconds. If it goes below 10-9.6 volts, thats not good. Does it do it for any blower speed or just high speed? I think there is a "High Speed" relay.

The battery it came with was only a couple of years old but would not charge right and checked bad. Getting a new one was the first thing I did. Also worth mentioning- the voltage drop happens at all blower speeds. I unplugged the resistor pack to keep it from running except in high position since it was dragging the battery down. Unlike earlier cars the blower is wired so that it is always running.

The blower motor is now very suspect...

[TedRamAirII]

Well OK. I would not think a blower motor could do that, but it looks like it may be the case here. It would be easy to get a new blower motor, connect wires and check it like you did, BEFORE you took the trouble to install it. One more thing, I dont know if you ran the blower motor with or without the a/c compressor on, that would be good to see also. Most DVOM meters will check up to 10 amps, but the motor may pull more than that, and blow the internal fuse. Most any garage could check that pretty easy for you. No Offense, but when it comes to people doing DIY repairs, I find they spend a lot of money on parts that didnt need to be replaced. Usually its just one problem. I had a guy that put 3 alternators, 2 batteries and 2 starters on a car. Finally he brought it to me. 10 minutes later I found a loose ground cable mounted to the engine block. FIXED. He was pissed that he spent all that time and money replacing parts that didnt fix his problem. Good thing his car wasnt a "classic" and he swapped good OEM parts for aftermarket trash that he got a Otto Zoo.

[george kujanski]

As you say the unusual thing is the engine dies with heavy load, also the bat voltage tends to droop also.

I suspect a bad fusible link in the harness that gets stressed at higher load currents to the point where the circuit opens, killing power to the ignition.

If that's repeatable, put a voltmeter on the ignition power (coil +) and monitor the voltage when the engine dies. If the voltage drops very low or to zero, the battery feed (fusible link possibly or bulkhead connector) is the next thing to check. Connections at the ignition switch should be cnfirmed also.

George

[PontGuy]

Take your point at throwing money at the problem. So far I am into this "fix" one alternator that probably wasn't needed. A blower motor doesn't cost much, but the aggravation of installing it is another matter.

I ran it with the compressor both on and off last night and it really didn't make much difference. The compressor was off when the voltage started really dropping. I disconnected the power lead at the compressor near the end (when switching the fan on started killing the engine.) Didn't make any difference.

So yes the voltage drooped as always, and after a while turning on the blower caused the engine to quit. I like the theory of a fusible link at the harness cutting out the main power feed. But it shouldn't be pulling that much.

I ordered an AC/DC clamp meter that is supposed to be good to 400A and it should arrive Thurs. I'll wait to do any more testing until I have it. If current to the blower motor is high and gradually gets higher then we will know there is a problem with it. And I should also be able to measure the amps in the main power feed(s) from the alternator.

[connman]

If memory serves me correctly there are 2? fuseable links on this car. Both near the the battery. Take the time and replace the links and look for corrosion in wiring near the battery and also peal a little bit of the battery cables back from the underside and if they are not bright copper color replace them. The low voltage would not let the EFI run right. Have you had the battery checked? Load checked? A sulfated battery will act full charge but the longer it runs the hotter it gets and the lower the alt output is cause the battery says it is fully charged.