Check this please, possible fire risk.

Errm.

I've been looking at the part-throttle channel heater on my carburettor, and find it's not doing as well as I thought, but that's by the by.

I've looked at the wiring diagrams (Diagrams 52 and 53 in haynes 1081, for my car) and have found something that worries me.

It seems to be a reasonably common problem that the wire that takes the 12V feed into this heater may suffer insulation failure, or actual complete breakage of the wire, at the point where it enters the cylindrical housing of the heater itself. This heater is fitted low down on the front of the carb (Pierburg) or just below the carb, at the front of the flange/adaptor, I think (Weber). If failure occurs here, leading to a 12V-to-chassis short, large currents may flow through this wiring, large enough to burn stuff, not to mention the chance of sparking.

The trouble is that where you might expect a wiring short such as this to simply blow a fuse; there isn't a fuse. According to my Haynes book, this heater (and the TTV in Pierburg installations) are fed by a 1.0mm black wire from connector G1/4 on the back of the fusebox, which is directly fed from the 12V rail "15", ignition on. Edit 17/03/09 I think if your electrics are CE1, pre-1990 there will be a 10A fuse in the fusebox for this heater.

Please have a look at this heater, and especially the wire where it comes out, right next to the metal mounting bracket for insulation breakage etc. A small mirror and torch may be required, as well as moving the airbox out of the way.

Being so close to the fuel supply apparatus/pipelines makes me think this is quite an important safety check. I hope I have made a mistake somewhere, and that this is fused somewhere...If not, it could at best, fry your ignition switch; at worst, well it's a worry tbh. Piccies to follow.

 

mines ok,
Been looking at everything tonight anyway as i thought i would strip my carb and give it a really good clean outside and in. About the only thing i found mas that one or two of the vac pipes have started to perish but other than that its ok.

If the chanel heater isnt fused? Maybe it would be worth seeing what sort of amperage draw it takes normally and put an in-line fuse in just before it as a precaution.

Funnily enough tho i was thinking virtually the same thing about the age of the wiring and what would happen if it broke, when i was taking everything apart and inspecting it.

 

I agree, I dont think there is a fuse.
When I fitted my weber the 12V live feed to the heater touched the ground.
This resulted in ALL the car electrics not working (nothing at all).
I then proceded to turn the car off and remove this heater (was so fecking hot I burnt my fingers). After removing the heater, the car started and worked fine - I guess I was lucky.
It may be an idea to fit a fuse here, for safety.

 

Yes, I have encountered similar failure myself. I've replaced a few of these heaters in the past due to the connecting wire starting to deteriorate. When I had my Driver I'm pretty sure I inserted a fuse also.

 

For the time being, I'll borrow a picture that mark25 kindly posted a while back. This is the heater in question in a Weber context. It's the cylindrical black thing right in the middle of the pic, with the wire looking like it might have some insulation 'crackage' just near where it emerges from the housing. Not quite so easy to eyeball in the Pierburg set-up.



I would guesstimate that a 10A inline fuse would be ample (and the 1mm wire should cope with that) but I'll do some measurements on the 'best' heater I can find in a recently-acquired bucket of Pierburg spares, later.

 

just to wander off the subject a tad, is the manifold preheater fused?, as i have a feeling that mine is causing my fuel to vaporise, so even though my engine was hot, i unpluged it (big blue spark) its taking a fair bit of power as the revvs picked up dramitacally when i unplugged it.

 

The preheater isn't fused, but fed from a relay, which is, in turn controlled by a coolant-heated thermoswitch. If yours is still powered up when the coolant is warm, your thermoswitch must have failed, or (less likely IMO) the relay may have welded itself 'on'. See the FAQ for which thermoswitch, easily tested by measuring its resistance cold and warm, should be 0 and infinity respectively. If that's OK, check out the relay, posn 12, bottom right hand corner of a CE2 f/b.

By the way, when you say the hedgehog is vaporising your fuel, that's actually what it's meant to do AFAIK . Can't see why unplugging it would do anything to the revs, except by a reduction of load on the alternator, which could explain it.
Once the manifold is up to temperature with coolant warming, it is no longer required to be electrically heated, hence the thermoswitch control.

 

So here we have a part-throttle channel heater on a Pierburg 2e2. As you may be able to see through the blurriness of the image, the wire is bare where the insulation has failed, perilously close to the metal mounting bracket, which is at chassis ground potential...



I've just added an inline fuse between the feed wire connectors, although the heater I've just fitted doesn't have this insulation problem. It would be a good idea to tyrap/strain-relieve this wire in a position where it can't waggle about and touch anything if insulation failure does happen, or the wire parts completely at this point.

If your heater still has a healthy resistance (<4 ohms is great, an ohm or two more won't matter much) but the wire is dodgy, I would urge you to fit an inline fuse, as a precaution, or consider refurbishing the heater (ask me nicely by PM). Or if the resistance is much higher (>10 ohms) consider replacement if you suffer from icing problems and have ruled out all other factors.

 

this no-fuse circuit was in Steve Cresswell's 'Crasher's Column' in the Golf a few years ago (before I stopped buying it!) and I think it he said one earthed and caused a fire which fried the circuits. I quickly added an in-line fuse!