Generator warning lamp

[mothertrucker]

Bone stock '97 3500 ...160,000 miles

For the first time since I owned the truck {20 years} the generator warning lamp came on today.

The ammeter gauge usually rides in the center of the gauge.
Does not seem to want to go any higher than 1/4 of the way up the gauge.

Shut the truck down & it started right up & the warning lamp is no longer showing.

I should mention that is was over 100 degrees today while idling the truck for about 1/2 hour just before the warning lamp came on.

It's telling me that something is wrong, or is about to go wrong.

I am thinkin' of replacing both batteries & the alternator.

Is there anything else that I should address while working on this issue ??

 

[GAmes]

Don't just start throwing parts at it. First, now that the light is off is the battery gauge needle in the middle? Was the tach working when the gen light came on? If the PCM doesn't know the engine is running it won't command the alternator to work. Is the serp belt in good shape? More questions to follow based on your answers.

 

[mothertrucker]

Thanks for the fast reply.

Just started it up, Gen. lamp is not on.
Tach works.
Gauge is still stuck around the 1/4 mark.
Belt is a Genos green belt that was replaced about 20,000 miles ago & still looks nice.

I can't remember if the tach was working or not when the lamp was on.

 

[GAmes]

With the needle that low the next thing I would check is the grid heater relays. If they fail they will pull a significant amount of amps until the PCM cries uncle and kicks the alternator off line. The easiest way to check them is to disconnect the wires from the grid heater terminals (be sure to isolate them so they don't ground out on something) and see if the gauge reads normal. If it does the relay(s) is bad. If it does not the next thing to check is the battery temp sensor under the drivers side battery. It is a round disk at the bottom of the battery tray. Pull it out and check it for ohms. The specs are listed in the factory service manual.

 

[mothertrucker]

Thanks. Will check those out first.

I found a listing for the grid relays with Geno's but, I could not find a listing for the battery temp sensor.
Any recommendations on the best place to locate the battery temp. sensor ?

 

[GAmes]

I bought mine at AutoZone, but don't get ahead of yourself. There is a possibility the alternator has failed, I'm just giving you some alternative failure items that are easier to check. If it is, in fact, the alternator I recommend you spend the extra $$$ for a new OE Denso. Lifetime warranty ones from the box parts houses will leave you changing alternators in parking lots. Mine did.

 

[mothertrucker]

Grid heater relays are good. Went ahead & replaced the battery temp. sensor.... fired it up & GEN warning lamp is still illuminated.

The shop I talked to said that I am lookin' at an alternator replacement & if the charging circuit in the PCM is fried.... he would ad an internal regulator to the alternator....... or I would have to start the search for a new PCM.

http://www.fostertruck.com/alternator-internal-regulator-conversion-kit.html

There are lots of "refurbished" Denso alternators out there, but I was unable to locate a NEW one anywhere so I think I will have my original unit rebuilt locally.

Looks like I am back to my original assessment.... 2 new batteries & an alternator replacement.

 

[HHhuntitall]

If you have a local shop, they should be able to test the alternator before they tear it down to see if it is malfunctioning. It'd be rare for the ECM to have quit, being a '97, and given the mileage, IMO. I have chronic problems with the dust eating up mine. For what it's worth, I've had good luck with NAPA rebuilds.... not so much with anyone else. Orielly's needs a good slap upside the head.... with a pillow case.... with an alternator in it.....

 

[GAmes]

If in fact the PCM charging circuit is kaput an external voltage regulator costs a lot less and is easy to wire, maybe easier than Larry B's. I carry a https://www.oreillyauto.com/detail/...93f/bwd-select-voltage-regulator/r400/2619358 in my spares box when I'm on the road along with the wires. It takes four. First remove the black plastic wire box from the alternator and secure it out of the way. You'll need it if you get a new PCM. F terminal connects to one of the small poles on the back of the alternator, either one it doesn't matter. S terminal connects to a key on constant 12 volts wire, I use the hold wire to the FSS. A terminal connects to the large pole on the back of the alternator. The fourth wire connects to the other small pole on the back of the alternator and goes to a good ground. The I terminal on the VR is not used.

BTW, you never said the tach is working when the gen light is on.

 

[Bill Lins]

Here's what I used when rebuilding mine- same part but MUCH cheaper than LarryB's: https://www.motorcityreman.com/chrysler-jeep-dodge-12v-conversion-voltage-regulator.html

 

[mothertrucker]

Update.

PCM charging circuit was OK.

Replaced both Optima red top batteries.
Replaced the original Denso alternator with a reman. Denso unit.
Went ahead & paid the extra $35 core charge on my original unit so I can bank it , have it rebuilt , on the shelf .... & ready to go when the reman. unit fails .

As long as I had the alternator off.... I went ahead & replaced the original water pump & thermostat with genuine Cummins parts & I was surprised at how much cooler the engine runs now.

 

[mothertrucker]

Thanks for the help fellas.