An open mind....

[Jim Warman]

Recently, one of our less experienced techs was having a fit with the cargo lights on an 08 SooperDoodee. The lights would always work with the thumbwheel but would occasionally (but usually not) work with the doors.

 

And here is where some of it gets interesting.

 

First.. some all important background.... the SJB had a code - sorry I can't remember the exact designation but it was a code indicating the affected circuit(s) were seen to have a short to ground. The truck is an early 08 so there weren't many branches to the cargo lamp circuit.

 

Since the lights actually worked sometimes, we could verify that wiring and lamps must be OK... Yet, after clearing, the code would return like clockwork....

 

Looking at the wiring diagrams, it is apparent that this circuit is controlled by a MOS-FET... MOS-FET circuits are current sensitive. A clue....

 

With no obvious wiring concerns.. and the truck is low mileage, you are left to scratch your head....

 

Think sideways....

 

The spec bulb for cargo lights on an 08 SD is a 906. This bulb is a 9 watt bulb... you can do the current draw computation - it will be good practice. As a rule, we install 912 bulbs... being as our main partsmonger is a cheap c*nt and any time he can reduce his part number count is something he will celebrate. 912s are a 13 watt bulb and appear to have the ability to fly under the SJBs radar.... However, somewhere along the line, some enterprising soul managed to install 921 bulbs in the cargo lamps... (21s are an 18 watt bulb. All three of these bulbs share the same physical dimensions yet one of them causes grief.

 

The notion that the wrong bulbs might be installed was an absolute shot in the dark...

 

The important thing to remember... the shape of the modern automobile is changing... We need to change with it...

 

Sidebar.... the second car I ever owned was a 1961 Ford Falcon.... In the early 60s, the park lights would extinguish when the headlights were turned on (tail light bulbs were an 1157 while front park lights were a 1034). One of the simple things we would do to "pimp our rides" "back in the day" was to jumper the tail lights to the park lights at the headlight switch. Voila (or, for you Yanks 'walla') we modernized our cars. Back then, we had one fuse box with maybe eight fuses in it....

[Bruce Amacker]

Good catch. I've seen the wrong wattage bulb cause problems on other cars too, but not something as new as an '08. I wonder why they were changed? Or wrong from the factory?

 

[Jim Warman]

We see a lot of burned out hi-mount brakelight bulbs, but rarely burned out cargo light bulbs. They wouldn't be from the factory (MicroCat calls for 906s as I'd mentioned) so one can only assume that they were changed "for some reason".

 

What I don't understand is that you could still turn on the cargo lamps using the panel dimmer switch. As far as I can see, door switch input and panel dimmer switch input both tell the SJB that you want interior and cargo lights to illuminate. Could there be, for some unexplained reason, different logic based on the source of the input?

[ktmlew]

We see a lot of burned out hi-mount brakelight bulbs, but rarely burned out cargo light bulbs. They wouldn't be from the factory (MicroCat calls for 906s as I'd mentioned) so one can only assume that they were changed "for some reason".

 

What I don't understand is that you could still turn on the cargo lamps using the panel dimmer switch. As far as I can see, door switch input and panel dimmer switch input both tell the SJB that you want interior and cargo lights to illuminate. Could there be, for some unexplained reason, different logic based on the source of the input?

Wouldn't the "extra" length of wiring running to & from the switches at the doors pull more current than the shorter distance from the panel switch? Apparently doesn't take much curent difference to kick a code...