"NOT GEN" dash light MCI 102a3

Singing Land Cruiser · July 05, 2009, 08:36:18 PM

"NOT GEN" dash light of our MCI 102a3 is coming on. I can't find anything in the manual about it.

What does it mean? Anyone. M&C

Sean · July 05, 2009, 09:31:03 PM

What it "means" is that your alternator is not charging.

I put that in quotes, because lots of things can happen to make this light come on even if the alternator is charging, so you have some troubleshooting ahead of you.

Are you running a battery charger (or inverter/charger) off a generator while you are driving by any chance? This can make the light come on.

-Sean
http://ourodyssey.blogspot.com

MacGyver · July 05, 2009, 09:31:33 PM

It means that it's no longer sensing a charge coming from your alternator... Check your connections and ground on the alternator. Could be anything from a loose connection to blown diodes.

My 60 footer had to have one of it's alternators rebuilt. Bearings went bad, blew some diodes, cracked the housing, all kinds of nasty stuff from non-use and lack of maintenance over the years (before I bought it, and after)... The complete rebuild with almost ALL new parts (except the housing, which was repaired) only cost me $170 at the local truck shop. Quite reasonable I think...

Get out your voltmeter and start checking.

-Mac

Singing Land Cruiser · July 06, 2009, 07:40:49 AM

I'll start checking. Thank you very much. M&C

Busted Knuckle · July 06, 2009, 07:47:47 AM

It could also be a lack of air to the sensor that tells the light to switch off! Many of these coaches were changed over to charge anytime the engine was running long ago, but the air warning light system left intact!

Get "Da Book" it'll explain the way the system works in detail! It's been so long since I fooled with an MCI I've forgotten how exactly it is set up!
Good Luck!

BK

Sean · July 06, 2009, 08:40:55 AM

Quote from: Busted Knuckle on July 06, 2009, 07:47:47 AM
It could also be a lack of air to the sensor that tells the light to switch off! Many of these coaches were changed over to charge anytime the engine was running long ago, but the air warning light system left intact!

Bryce,

The air pressure switch does not tell the light to switch off -- it tells the alternator to start charging (by controlling the field). So if the air pressure switch is what is causing the "not gen" light, it really does mean that the alternator is not charging.

The reason for the pressure switch is that many coaches had pneumatic belt tensioners, and you didn't want a load on the alternator until there was enough air pressure to tension the belt.

-Sean
http://ourodyssey.blogspot.com

luvrbus · July 06, 2009, 08:51:51 AM

On my MCI I owned the air opened a switch to tell the alternator to start charging had nothing to do with the belts because it had a manual adjuster.I know on the one I owned there was a relay in the panel at the rear that went bad causing the light to remain on. good luck

Busted Knuckle · July 06, 2009, 09:40:14 AM

Quote
Bryce,

The air pressure switch does not tell the light to switch off -- it tells the alternator to start charging (by controlling the field). So if the air pressure switch is what is causing the "not gen" light, it really does mean that the alternator is not charging.

The reason for the pressure switch is that many coaches had pneumatic belt tensioners, and you didn't want a load on the alternator until there was enough air pressure to tension the belt.

-Sean
http://ourodyssey.blogspot.com

Sean,
I agree with some of the older ones having belt driven alternators. But by the time they built the 102A3 and all the way up to C's if it had a Detroit in it, it had a gear driven alternator off the back of the engine from MCI on the Cummin's, & CAT powered 102A3's, B3's, & C3's they had belt driven units as do all D and up models!
Our MCI's have all had the gear driven alternators, and all would start charging the instant the engine was running. All would show "Not gen" lights lit up until 100 psi was reached, but a test with a voltmeter would actually show 28-29 volts coming out of them while the light showed "No Gen". FWIW.
But on one we had a problem once with the light staying on all the time even after air was built up to 123 psi. It turned out to be a bad relay or switch. (I don't recall and dad is not here to ask, since he does the electrical trouble shooting most of the time)

Also I am not arguing with you on the air needed to actuate the field for charging, just pointing out how ours had been modified or changed over prior to our ownership of them! (could have been due to the fact that all of ours had been changed to "solid state regulators" too!)

BK

~~Also Sean I sent you a PM on a different subject that I believe you may be able to help me with.~~ opps I just got your reply! LOL!

Sean · July 06, 2009, 10:13:29 AM

Quote from: Busted Knuckle on July 06, 2009, 09:40:14 AM
... Our MCI's have all had the gear driven alternators, and all would start charging the instant the engine was running. All would show "Not gen" lights lit up until 100 psi was reached, but a test with a voltmeter would actually show 28-29 volts coming out of them while the light showed "No Gen".

OK, now I am out of my depth, because I don't own an MCI nor do I work on a lot of them, although I have done some charging system diagnosis on a couple. It is quite possible that MCI or some previous operator had wired the light to stay on until air pressure built, even though the field itself was no longer wired through the pressure switch. If that's true, I would speculate that the reason for it was as simple as drivers had gotten used to using the Not Gen tell-tale as the indicator that air was up to operating pressure. IOTW, they made it work the "old" way to avoid confusing coach operators. There is otherwise no real reason for the Not Gen light to be on when the alternator is actually charging.

But the original reason for the air pressure switch was to avoid loading any belt-driven alternator before belt tension was applied.

Quote
But on one we had a problem once with the light staying on all the time even after air was built up to 123 psi. It turned out to be a bad relay or switch. (I don't recall and dad is not here to ask, since he does the electrical trouble shooting most of the time)

Well, as I said in my first post here, there are many potential reasons for the Not Gen light to come on even though the alternator may be charging. You've already touched on one, which is that, apparently, the light itself is being illuminated by a pressure switch, separate from the actual charging indicator.

The other is the way the charge indicator works. The alternator has an auxilliary terminal called the "Relay" terminal, which should be labeled "R" or "Relay." This terminal produces half-rectified DC from the stator (output) windings; the voltage varies with alternator speed, but will be roughly half the rated voltage of the alternator. So for the 24-volt units we are talking about, this terminal produces roughly 12 volts, when and only when the alternator is actually producing output.

Somewhere, and on most MCI's I think this is in the rear J-box, there is a 12-volt relay connected to this terminal. When the alternator is charging, this relay closes, and connects the 24-volt blower relay to a 24-volt supply. This is to ensure that the blowers, which use a ton of power, are only on when the alternator is charging. The Not Gen light is connected between +24-volts and the blower relay. When the blower relay is energized, the Not Gen light goes out.

If the 12-volt relay goes bad, and they sometimes do (especially because they are driven by half-rectified voltage), the Not Gen light will come on and stay on (and the blowers, if any, or whatever else might be on that circuit, will shut off), even though the alternator is getting good field and charging properly.

So the first troubleshooting step here is to check the actual output voltage with the engine running on high idle. If the charge voltage is present (27+ volts), then the problem is strictly with the circuit that lights the light. If no charge is present (~24 volts), then there is a regulator, field, or alternator problem.

However, my original question still stands, about an alternate charging source. If there is a battery charger connected, say from a generator, then the charge voltage from that can actually convince the Delco regulator that no charging is needed, and it will then shut off the field. That will cause a Not Gen light, even though the batteries (and everything else) are getting plenty of juice.

-Sean
http://ourodyssey.blogspot.com

Busted Knuckle · July 06, 2009, 10:20:37 AM

Again Sean I applaud your explanation of the way things are supposed to work!
Once again it shows you have a much better understanding of the way electrical items work than I do. (Which is why dad does most of our electrical work as my trial and error ways tend to let the smoke out, then things don't work at all!)

BK

gumpy · July 06, 2009, 10:50:38 AM

What Bryce said is correct for the MCI. My MC9 with gear driven alternator is exactly this way. It starts charging immediately, but the light stays on till the pressure reaches 85 lbs.

Chopper Scott · July 06, 2009, 12:57:14 PM

I changed my old generator and air tensioning system over to a one wire 24 volt alternator and a turnbuckle tightener. I lost a couple hundred pounds, an air leak and 2 of the 4 belts in the process. The light still wouldn't go off until the air hit something like 70 lbs but the alternator was still kicking out the 28 volts on start up on the gauge that I installed in the dash. The "not Gen" light got a piece of black tape over it. If I pull the dash out someday I'll remove the bulb. Later

Tim Strommen · July 06, 2009, 03:18:55 PM

I wouldn't take out the Not-Gen light... It's supposed to be a serious warning - it's about as bad as if you run out of fuel out in the middle of nowhere. You could lose power right in the middle of an unlit one lane tunnel with little or no power for people to see your lights coming up from behind

.

I would recommend reconfiguring (i.e. fixing) the light so that it indicates what it is supposed to... (low air should be called out by a LOW AIR light and/or buzzer below 85PSI anyway).

I'm kind of a fan of getting something to work as it should, not just getting used to it being broken - that's how people get hurt.

-Tim

Sean · July 06, 2009, 03:49:06 PM

Quote from: Tim Strommen on July 06, 2009, 03:18:55 PM
I wouldn't take out the Not-Gen light... It's supposed to be a serious warning - it's about as bad as if you run out of fuel out in the middle of nowhere.

Tim, he said he switched to a one-wire alternator. Unfortunately, the Not Gen warning system will not work at all with a one-wire. You'd have to make up some kind of voltage-detector circuit to drive the light -- easier just to put a voltmeter in the dash.

-Sean
http://ourodyssey.blogspot.com

JackConrad · July 06, 2009, 04:09:39 PM

Quote from: Sean on July 06, 2009, 03:49:06 PM
Tim, he said he switched to a one-wire alternator. Unfortunately, the Not Gen warning system will not work at all with a one-wire. You'd have to make up some kind of voltage-detector circuit to drive the light -- easier just to put a voltmeter in the dash.
-Sean
http://ourodyssey.blogspot.com

Sean,
I added a 24 volt 1 wire alternator on our MC-8 and still use the No Gen light. Light comes on when master switch is turned on and goes out as soon as engine starts (and generator starts charging). I did have to add an IGN wire on the alternator to get it to charge without revving the engine. Jack