Courtesy lights dimmer module - parts info/tech tip here

[Stingraynut]

The courtesy lights dimmer module is no longer available as a replacement part - with some help from an electronics guru, my 88 coupe has a working module again.

The part that had failed was in a T220 case, it gets hot when the lights are on and I would guess this is the major cause of the module failure.

It is 047 GE 8708 which is no longer available - after analysis of the circuit, the electronics guru found the current replacement for it - MJE 3055.

Mine has been working perfectly now for 5 weeks.

I've detailed a list of parts and as much info as I could find, in case it helps anyone else.

I think this tech tip need pics to show the parts and the module. The Corvettes Down Under Club website, here in Australia, has hosted this tech tip for me.

http://www.corvettesdownunder.com.au/pages/Dimmer.htm

Hope this helps someone

S'nut

[Cameron 77C3]

My friend that would have to be the most brillant technical post I have seen.  Well done.

No wonder the transistor failed.  I checked the specs in my transistor manual.  It was under rated, by my standards, for that amp draw.  The heat sink was no way big enough in that closed environment.

Hats off to both you and your electronics mate.:}

[Stingraynut]

Thanks Cameron 77C3

Several people told me not to bother trying to fix the dimmer, there's a Dick smith kit for under $20 - but it doesn't perform all the functions of the dimmer, but once again I had my 'stubborn' head on!! (although I prefer the word 'determined' -lol)

I'm amazed you found the 047 GR 8708 part listed anywhere - how do you mean it was under rated, it lasted 15+ years - I'd be very interested, just to tie up loose ends.

S'nut

[Cameron 77C3]

It's the way I spec up systems when I'm designing my circuits.  Basically I over engineer my circuits.  If a heat sink is hot to touch then it ain't big enough in my book.  But that's just me.  I have always found that automotive industry run it to the wire when it comes to electronics.

Me, when a unit goes out my factory door I don't want to see it again just  because a $1.00 component failed

On the specs, I have some real OLD spec books.  I keep them for occasions like this.  You just never know when you want specs on something that's not made any more.  And in the world of electronics that can happen over night.

[Stingraynut]

I wish I'd know that you had old spec books when we were trying to identify a replacement!!

How do you think the replacement part - MJE 3055 will hold up?

How does it compare with the original ?

Is there a heavier duty part that would be better ? I would include it in the tech tip.

Quote "Me, when a unit goes out my factory door I don't want to see it again just because a $1.00 component failed"

- that's very good to hear, that kind of thinking seems to have almost disappeared - our microwave lost it's display after only 8 months, was repaired under warrantl but lost the display again after about 16 months! now out of warranty - everything's use it and chuck it now.

S'nut

[Cameron 77C3]

Yeah it's a throw away world.  But I'm from the old school.  My test is "now what would the old man say if I did it that way."  That always keeps me on the straight and narrow.

I relation to your parts to clarify my statement in more detail.  What I posted was not quite true.  I just couldn't be bothered going right into it

The part 047 GE 8708 is not actually listed.  That number is an in house part number.  [companies often do this so that other companies can't copy what they are doing. I do it my self by rubbing the part numbers off IC's] Knowing how that circuit would work by your schematic circuit diagram I took an educated guess as to what it would be.  MJE 3055's, an NPN transistor, are as common as sand on the beach.  It's a safe bet that the 047 GE 8708  is actually an MJE 3055.  These have been around for decades.

I made the assumption that the unit is taped to the wiring loom or stuck up under the dash some where, where there is not a lot of air movement.  This is what I was actually referring to about over engineering.  If this is the case I would have put on a larger heat sink in the original design.

When a circuit is designed one also has to look at where it is going to be mounted.. Heat sinks work on being able to dissipate heat from the transistor.  If you could keep a transistor cool enough you could pull many, many amps through it.  [This is why computer fans are used.  To reduce the size of heat sinks.  A heat sink can be much smaller when used in conjunction with a fan.]

Any time I build a circuit to go in a vehicle I usually over spec the heat sink as the customer does not always mount them where he said he was.

So if you want to ensure your unit is OK, make sure there is plenty of "breathing" space around it.  It should last for a long long time.

Your electronics man did well.  Not many guys these days want to reverse engineer to work a circuit out.  Most just want to throw it out and build a new one.  [now where is the fun in that]

NOTE:-  Putting higher wattage light bulbs in than originally speced can cause the unit to over load the transistor [MJ 3055] again.

[I now have writers cramp] :22

[Stingraynut]

Quote "[I now have writers cramp]"

Ha ha - thanks v much, I understand what you'r saying.

In fact the heat sink is the same size as the circuit board, about 50mm square, it clips onto the board and that assemble slides into an open ended plastic case - which prevents airflow, then it's taped by GM to the wiring harness, behind the centre of the dash above the transmission tunnel - could they ensure it'd get any hotter ?

I'm planning to fit a switch to it, so that I can disable the interior lights for any reason, like working on the car with the door open, which happens very often - I pull the fuse but then also loose the memories in the CD/radio.

I checked out your pics this morning - looks like you did a lot of good work to that shark - what are you doing over here, with the 'modern' cars ?

BTW my favourite vette would be a C3 body on a C4, although now the C6's are out I'm beginning to like the C5!

S'nut

[Cameron 77C3]

I'm always trolling the different forums.

On the list for the C3 is the composit type suspension front and back.  Gota get the slug to handle.  But thats going to have to wait 6 months or so till my bank account recovers.

It would actually be better if the transistor was directly bolted to the heat sink.

Anyhow I'm sure if it lasted 15 years, it'll last another 15 years.  Catch ya.

[Stingraynut]

Quote "Gota get the slug to handle"

don't be fooled into thinking the C4 handles Aussie roads- on a dry smooth surface Oh yeah - with 11" wide tyres what wouldn't ?

But the suspension travel is way to short and I've done a heap of research into the handling problems - not helped at all as our Far North Qld roads are often described as 'Moonscape' etc.

S'nut