Diagnosis Time - what's failing that causing this?

[steved]

The obvious...



If you shut off the truck, you shut off all the electronics... so how do you isolate them from the alternator?

 

[Prairie Dog]

I got it. . Coast down a hill with 0% throttle. All the inj activity is dead and so that entire circuit is out of the loop. If it still does it then at least you've isolated it (slightly).
EDIT: Oop's, sorry just read your post 2 up. Glad you found it. Funny, some people don't think about grounds as being important but this shows how critical they can be.

 

[steved]

I'm not convinced its the alternator... this doesn't change with RPM, it changes with accelerator pedal position and load.

Being I have a manual, I can be cruising 2k rpm, and change the load based on the pedal without the RPM changing much... the noise will completely go away with zero load, even though the engine is still turning 2k RPM. So, I'm thinking its injector or injector circuit related?

I did find I have a grounding issue on my trailer wiring, but that's another issue...

 

[zzman]

When I started driving big truck in the 70s, we use to have something simular to what you are explaining.

It was the bearings going out on the turbo. As strange as that sounds, it happened to my dad and I twice. He knew exactly what it was when the CB started the noise.

Sure enough, strange,

Just a thought

 

[Wingate]

The obvious...



If you shut off the truck, you shut off all the electronics... so how do you isolate them from the alternator?

with the key on, the electronics would be powered up, but the engine not runing the alternator would not be turning... .

If no noise there, press the accellerator pedal, if the noise appears, it could be coming from the APPS. (TPS)

 

[steved]

Wimgate... I will give that a try.