b.lee

Well the GEN light came home last night on my way home from work. After that came on i noticed the volts where right at 10-11v. Normally the truck is at 14v (straight up on the gauge)

What is the problem and how do i diagnose it so i can fix it right the first time?

Alternator?
Batteries?
Cables?
Grid Heater stuck on?

bent valves

I had to troubleshoot my charging system recently and came up with a fairly simple way to isolate the problem. Here is a step-by-step way.

First you will need a DVM to measure voltage. Also the batteries need enough charge to start the engine, so use a battery charger to get them charged enough.

The charging system is made up of the PCM mounted on the firewall, alternator, and don’t forget the 120 amp fuse in the PDC on the driver’s side fender well.

1) Start the engine and measure the battery voltage at one of the batteries. It should be more than 13 volts but probably less with a charging problem and that is why you are here.

2) Measure the voltage at the alternator output terminal (B+) leaving the DVM negative connected to the battery negative post. If the measurement is greater than 13 volts then the fuse in the PDC should be checked or the nut holding the heavy gage wire (B+) on the alternator is loose.

3) If you get this far then the next step is to measure the voltage coming from the PCM. The two small nuts on the back of the alternator are the ones to measure with the lower one positive and the upper one negative. These are the field terminals on the picture. If the voltage reading is greater than 8 volts the PCM is calling for full charge and your alternator is bad and not putting out. If the voltage is less than 3 volts your PCM is toast or the wiring between the PCM and alternator is broken. The normal voltage reading at the field terminals will vary from 3 to 6 volts when working correctly.

It’s always a good idea to clean battery terminals and check for bad grounds before spending money on replacement parts.

My alternator turned out to be bad and I had 9 volts at the field terminals. It was the diodes inside that were bad.

patdaly

Bent valves, that was a very clear, concise explanation, thanks, and I have saved that to my repair file.

That should be in the wikki!

b.lee

patdaly - what are you talking about?

anyways,

I put a new alternator in it tonight and no more GEN Light.
volt gauge is reading normal again (14v+)

BUT the grid heater is cycling ALOT in park/neutral and anywhere under 20 mph,
after the truck hits 20mph you can stop or go under 20mph and it will stay 14v+ until you shut off and restart the truck, then you have to go 20mph again to get the grid heaters to stop cycling. This happens everytime, regardless if the truck has been drove for 30min or not.

Seems like there is a grid heater temp sensor turn on switch thats not working correctly?

edit: I also cleaned the snot out of the battery terminals, cable ends, and put new ends on all the little accessories going to positive terminal

patdaly

Look at the bottom of the main forum page, the DTR Wikki is a area where much good info is stored.

That writeup deserves to be there.

GAmes

If your grid heaters stay on long enough the PCM will kick the alternator off line until you shut it off and restart. That may have been the problem to start with. There is an outside temp sensor, but I don't know where it is located.

After I had a failure that cooked both batteries I disconnected the ground wires to my relays and have them wired to a toggle switch in the cab. The hots are always hot is why I chose the ground sides. It also did away with the annoying cycling after starting the engine. Once it is running the grid heaters are not needed. Plus on super cold days I can manually cycle the heaters if I want to.

b.lee

Thanks patdaly and GAmes

bent valves

Thanks patdaly, I cleaned up the typos in my post to make it readable. Maybe it will be put in wikki.

b.lee, what you describe is the normal workings of the grid heaters except they should not come on with a warm engine. I believe it is the IAT sensor that controls the grids.

b.lee

Can anyone else second that the IAT sensor controls the grids?

Thanks

Dr. Evil

You dont have an IAT sensor. 24V trucks.

bent valves

Thanks Dr. Evil. I forgot we were talking about a 12 valve.

b.lee

grrr, what would cause the grid heater to cycle even when the truck is hot / has been driven for an hour ?

GAmes

The intake manifold air temp sensor is mounted on top of the intake manifold, right behind the air horn. It has a two wire plug going into it. I'm thinking that if you unplug it, that the relays will not energize at all. I can't tell you from mine because it is bypassed. FWIW, my manual says the heaters should de-energize above 10 mph. If this helps you owe me a beer