M1009 Gen 1 issues- im confused

Hi crew,ive got a strange problem with a M1009 picked up. When i got it the #2 altenator's two wire plug was melted at the alternator and the wires were not connected. The alternator also was smoking whenever current was put to the truck running or not. The Gen1 light lights up when the key is turned on and stays on when its running.

I replaced the #2 generator and fixed the plug and am not having any issues with it anymore i think.

Gen1 wasnt engaging and had no power going to the 2 wire plug. I fixed it by wiring it directly to the firewall plug block where the wiring diagram says it goes.

Before i found the wire not being hot I had replaced the Gen1 altenator. i ran a jumper wire from the Gen 1 alt hot post to the plug which was probaly pretty dumb looking back on it. Once i figured out that the altenator wasnt the problem i replaced the old alt and tested t with thejumper wire. The battery gauge never read in the green but just barely. Later duringthe test (still on the jumper wire) the Gen1 light came back on. Bu it worked for a half hour before coming back on and stopping charging.

Did I burn out the alt with my jumper wire stunt (likely)? Is something else going on. I am going to get another alt tomorrow without my stupid stunt with a jumper wire.

I am not very good with electrical stuff and could use some help.

Dave - Jumping the hot lead to the #2 position on the plug should not have done any harm, jumping it to the #1 position might have.

I’m not real clear on what exactly you did, you say you replaced the #1 alt, then tested the circuit, and found the feed not connected, then put you old alt back in and this is the one that failed after a half hour? So then it sounds like you still have a new one laying around to swap out, why are you going to get another one?

Sorry for being thick headed, just trying to get a grasp.

The Alt1 #1 plug feed comes directly from the Gen 1 light with only the bulkhead connector in the circuit. The # 2 feed comes from the hot side of the 12V for the truck. Make sense?

To further my EWAG, if you did hurt the alt, it would probably be the voltage regulator that took the brunt of the damage, testing and replacing the components in the alt is WAY cheaper than replacing the alt.

Very low batteries will sometimes make the gauge read in the yellow, as well as poor/dirty/loose connections anywhere in the circuit.

Hope any of this helps.

Post back when you get back to it.


---John

The volunteer Fire Dept. that i am on has a 1985 m1009 and the gen2 light comes on when you turn the swith on but not the gen1. Once you start the truck the gen2 light cuts off but the batteries are still not charging. Is the Gen1 light supposed to come on when you first cut on the switch and could this be the charging problem? Any suggestions will be greatly appreciated.

Both GEN lights should come on when you have "key on, engine off" and go out once the engine is running and the alts have reached the proper rpm to start charging, sometimes requiring a blip on the throttle. If the GEN bulbs are bad, the associated alt will not charge. Remember that the bulbs are different, as the GEN 2 bulb is 24v while GEN 1 is 12v.

Read the alt troubleshooting section of the TM's, and you will probably find the problem.

Could be a lot of things. Yes, the light is supposed to be on when you start. Check the bulb first, it is part of the circuit, then read the sticky alternator thread up top and start checking voltages.

Thank You both for the quick reply. I will check the bulb in the morning.

cpf240 said:

the GEN 2 bulb is 24v while GEN 1 is 12v.

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CFP240 Are you sure? I thought that Gen 2 is standard 194 bulb and gen 1 is standard 168 bulb?

The generators only put out 12v, not 24.

Gen2 is a 194 and the GEN1 bulb is a 168. The difference is the wattage.

The voltage thru the GEN2 bulb is 24v. The diffence in the bulb wattage allows them to have the same brightness.

Ken_86gt said:

CFP240 Are you sure? I thought that Gen 2 is standard 194 bulb and gen 1 is standard 168 bulb?

The generators only put out 12v, not 24.

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I should have been more specific, as Warthog was. Yes, both alts put out only 12v, but GEN 2 is using the 12v output of GEN 1 as its ground reference. Which is why GEN 2 *MUST* be an isolated-ground alternator. So, when measured from chassis ground to output of GEN 2, you get 24v. GEN 2 using the output of GEN 1 as its ground reference is why GEN 2's exciter circuit needs 24v, not 12v like GEN 1.

This is a 5 year old thread but the information is still good to know. Very helpful information here. I was looking at it because of my current issues with a customers truck. I was never aware of the cluster bulbs being different. I used 194 bulbs on all the bulbs. Never knew it would matter. Learn something new every day. Just never bothered to look. 194 are abundant bulbs and I have many as that is the predecessor of the LED clearance lamp that we do not use anymore. I removed them from discontinued clearance lamps and recycled the lamps base and lenses. I know the 194 bulb is 12 volt but it must also be able to run on 24 volt. That explains the heat in that socket when the key is left on a long time. Merry Christmas.

Thank you for resurrecting this. I have something else to check, now.

Warthog said:

Gen2 is a 194 and the GEN1 bulb is a 168. The difference is the wattage.

The voltage thru the GEN2 bulb is 24v. The diffence in the bulb wattage allows them to have the same brightness.

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Now that this thread has come back to life, I will ask the question. I plan to change all my dash bulbs to red LED, so does the GEN2 bulb still need to be different? Also, since LEDs can withstand varying voltage, can the dash light circuit be increased to 24 volt to increase brightness?

From what I read here, you cannot replace the GEN bulbs with LEDs as it interrupts the charging circuit.

NO LED's. I don't think I can take anymore of this. LED's are the way of the new world. Embrace change but keep your incandescent bulbs in the Gen 1 & Gen 2 idiot light sockets. LED's do not excite the circuit.

Sorry, Rick, that's the way it is. The design requires the current to flow through the GEN 1&2 bulbs, and they have to allow the correct amount of current to pass. LEDs won't, and that will break your charging circuit.

It's a brain-dead stupid design. I can't imagine why anybody thought that was a good idea, really. It's just crazy. But, that's the way it is.

One of these days I may find the time to measure the current flow through those bulbs and design a workaround so they are no longer in the circuit. It should be possible to replace them in the charging circuit with the correct resistor, and then use LEDs in place of the bulbs.

Maybe when I retire.....

MarcusOReallyus said:

Sorry, Rick, that's the way it is. The design requires the current to flow through the GEN 1&2 bulbs, and they have to allow the correct amount of current to pass. LEDs won't, and that will break your charging circuit.

It's a brain-dead stupid design. I can't imagine why anybody thought that was a good idea, really. It's just crazy. But, that's the way it is.

One of these days I may find the time to measure the current flow through those bulbs and design a workaround so they are no longer in the circuit. It should be possible to replace them in the charging circuit with the correct resistor, and then use LEDs in place of the bulbs.

Maybe when I retire.....

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Yes, you can. No need to measure current, just measure resistance of a bulb and swap in resistors over the cathode and anode of the LED. Better yet, use two LEDs, reverse biased to prevent polarity issues. One LED will illuminate regardless of the installation. But realize you need at least 5 Watt resistors to replicate the draw so the energy conserving advantage of the LED is gone. The resistor replicates the inefficiency of the incandescent bulb.

Who designed this system? The same caliber of engineers that won WWII. The same ones that put us on the moon. It is a KISS system which worked.

Keith_J said:

Yes, you can. No need to measure current, just measure resistance of a bulb

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Nope. Incandescent light bulbs change resistance when they are lit due to the heat of the filament. The only way to get the correct resistance is to measure the current flow and do the math.

Keith_J said:

Who designed this system? The same caliber of engineers that won WWII.

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You need to get better acquainted with the non-Hollywood version of history. We put out a lot of badly engineered crap in WWII. We won on PRODUCTION, not engineering. The Germans were far better engineers.

Keith_J said:

It is a KISS system which worked.

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It's an unnecessarily complicated system which depends on two rather fragile bulbs and is unnecessarily difficult to repair. It worked for GM because it's CHEAP. That is what drove the design of the electrical system on these vehicles, NOT simplicity. It's just whatever worked that was low buck.

I have been reading this thread and it came to me that I don’t ever remember seeing my gen lights on! I went out and turned the key on and sure enough, no lights. I have had the blazer for about 10 years and the volt meter doesn’t work or the lights, but I know at least one alternator is charging. I took out both bulbs and the filaments look good. I wiggled them in their sockets and put them back in still no lights?

Now the fun begins. You must clean and check everything.

johnjr said:

I took out both bulbs and the filaments look good. I wiggled them in their sockets and put them back in still no lights?

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Filaments sometimes look good, but aren't. Use some jumpers to apply 12v to them and see what they do. Other than that, it's what Rick said. Time to check and clean everything. The first thing I would do is to measure voltage on each battery with the engine running to see what you really have.