One of VIM fuses keeps blowing

[CountryFit]

Allison MD3066 coupled with Cummins ISC350 engine.

The ignition fuse inside the VIM keeps blowing whenever the ignition key is turned to ignition position (preheat count-down). But if I turn the key from off to start quickly, the same fuse doesn't blow. What could be the gremlin? Appreciate all the pointers.

[dons2346]

Possibly a shorted heater grid???

[CountryFit]

Hi Don, this morning moved from one park to another. When tried to cold start engine, the shifter was out of power again, exactly like what happened a week ago. The good thing was - I was able to find the fuse in VIM and replaced it.

I'll flip up the bed and check the heater grid out.

[Jon_C]

Do you know if you have Wtec-2 transmission controls?

Is your shifter lighting up then going off when the fuse blows?

- Edit: I see you just posted that it is.-

looking in the Wtec-2 manual (see pic) the ignition input to the TCM (transmission control module) comes thru the VIM fuse. So if that fuse is blowing seems you either have a short in the wiring or bad TCM.

I would unplug the TCM and see if the fuse still blows or not. If not that points to bad TCM - Note its called ECU in the pic

And if there is anything plugged into the diagnostic connector unplug it also as the same fuse feeds power to it. I think my connector is in the bay with the VIM.

[Skip426]

Jon , Steve's 2000 should be a WTEC III control. My 1999 chassis has the III.

Steve , I don't know if examining the wiring schematic for my Freightliner chassis would be of any use to try and figure out your issue .
I can't think of anything in the VIM ignition side wiring that would be tied to the engine during a wait to start cycle .

Do you have any wiring info on your chassis ?

The grid heater activation should be a function of the engine ECM and it has fuses and relays and a 12 volt circuit breaker in the actual power supply to the heaters.

To eliminate the grid heater , if your in a cool enough area to have it cycle .
Remove the fuse that blows , turn the key on and note the volt gauge reading well the wait to start light is on .
If the gauge jumps up when the light goes out then the grid heater operation is normal .

[Jon_C]

Ok heres the Wtec-3 schematic, looks same as the -2.

I know a common failure in the -2 TCM is a capacitor that either goes bad or maybe develops a cold soldier joint. The symptom is no power to the tcm. I wondering if that cap in his is starting to short out. Theres a video on YouTube that shows which one it is. I carry a spare cap in case mine decides to give it up. Not sure if this applies to the -3 or not.

Either way transmission instruments.com can fix it.

[Jon_C]

One other thought is the grid heaters pull a lot of current and drop the battery voltage significantly.

This causes other devices like the TCM to pull more amps to start and if the TCM is also beginning to have issues it might be enough to blow the fuse in the vim.

It sounds like at times it powers up and works so maybe if the heater happens to be off it works ok due to the higher voltage.

Temporarily disabling the heater may also be a good test to try.

And maybe his batteries are just getting old ? I wonder what the voltage drops to when heaters on?

If its anywhere near 10.x volts that may be the problem.

And check for codes on the keypad, hold up /down arrows together and release, do twice if it has an oil level sensor then mode button to go to next code, up to 4 total

[CountryFit]

Just got back from outing... Wow, lot of great info! Skip was correct - mine is WTEC III.

My battery is a big pack of lithium 900ah, and full. That can be ruled out.

Lemme work on it and test out all of your described scenarios, and get back with you. Big thanks!!

[CountryFit]

Was tied up with a few things so my tweaking/shooting was slow. But at the end of the day, I was able to pin down the problem. Here're the steps I have gone through -

(1) Tested the heating grid - the voltage dropped about 0.2v whenever it's on (for example from 14.2v to 14v). Not much drop for the size of the battery, but it did drop, meaning it was not short and doing its job.
(2) Opened up the ECU box, visually didn't see bulging/burned capacitors.
(3) Chased power source. This was where I found the problem - I have a 300a continuous duty relay on the positive lead in the battery bay. Over the years the connection bolt was shaken loose a bit. Tightened it up and tested the starting, the fuse was no longer blown.

(I didn't read the status codes on the shifter pad, as I was aware that the trans fluid has to be in the normal operating temperature to be accurate. I was thinking wait until next time driving.)

Thank you all who have shared the knowledge and experience with me. I really appreciate that.

Happy travels!

[Jon_C]

That’s great thanks for letting us know!

[CountryFit]

Somehow I missed Skip426's question in the beginning, blaming my old glasses - about my chassis wirings... Yes I have them but no info on tranny. Sorry for not been able to get you back quickly...