Blower motor fuse

[kervinmervin]

I daren’t really post this as I know its well covered, but, after spending the last week with the dash out replacing the heater blower motor and putting in new resistors (should of put mod in I realise) but using it today, the fuse blew again, well nearly blew and smelled of heated plastic.

The wiring looked in ok condition and it seemed to blow at the top speed, so I understand that this is direct power without stepping down. Is there likely any other causes that would create this issue, its a red 10amp fuse (early DG fuse board) its not actually blown just the holder is slightly warped and melted, is this normal assume the wire strip would go first on the fuse.

I did a quick test on the motor when the heater box was open, and all seemed ok, didnt run it for long though.

Looking forward to opening up the dash again, but hoping for simpler fix!!!

Thanks

K

[kervinmervin]

Ok to answer my question, I guess the suggestions will be - possibly the switch or the earthing... the switch looked ok, but I guess worth a go first off.

K

[marlinowner]

Sounds like a high resistance at the fuse to me.

[kervinmervin]

Ok, for an electrical novice what would be the likely cause of the high resistance

Thanks

K

[syncroandy]

I don't follow that. A higher resistance would result in a lower current. But for the fuse to blow, a *higher* current would be required.

Was the replacement motor the same spec ? Was it tested prior to fitment ?

E D I T. Just checked the diagram, on the late wiring the blower fuse (S13) is *20A*.

[bigbadbob76]

Mine does that too, the seized motor has caused sparking/heating at the fuse holder terminals which corrodes them and causes high resistance, the fuse then gets hot and melts.
If you use the older style hard plastic fuse (steal one from another slot in the fuse box) rather than the softer semi transparent type it doesn't melt.
This is not the best solution as the fuse is still getting hot but short of replacing the fuse box there's not much else you can do.
I try not to use my blower unless I really have to.

Andy-The current through the resistance means power is lost as heat in that resistance. P=I^2R

[syncroandy]

I suspect we're talking at cross purposes. For a given voltage current flow is inversely proportional to resistance, therefore as resistance increases current will decrease.

I understand whats been referred to here is the heating effect of power dissipation in a contact resistance caused by corrosion, *adjacent* to the plastic fuse, causing it to melt.

In any case I think the OP may not have the correct rating fuse fitted...

[kervinmervin]

Pretty sure its the right fuse - 16a (red) slot 10. Stand to be corrected. Actually think some of the others are wrong..

[marlinowner]

Looks like it has been arcing at the top of the fuse?
Clean up the fuse holder with emery paper and fit a new fuse, check it is held firmly, maybe bend the contacts together slightly.

[syncroandy]

Try measuring the current with a multimeter on its 20A range, using croc clips on those two terminals. Might be interesting..

E D I T. FWIW an early fuseboard I have here as a 25A fuse fitted at S10. Bentley does show a 16A.

[kervinmervin]

Its been on my shopping list for a while, so I'll get a multimeter and check.

It does however seem odd to me that the plastic holder will melt prior to the strip.

K

[CovKid]

Ah, thats cos some are plastic and some are ceramic.

This is the other thing to bear in mind with PWM. You're not passing anywhere near as many amps (except when flat out). With resistors, you'll draw pretty much the same regardless of switch position but its being shunted.

[kervinmervin]

Sounds like a high resistance at the fuse to me.

....going right back to the start, I see what you're saying - its not that there is too much current being drawn and therefore blowing the fuse, its the holders heating up because of bad contact somewhere or through possible corrosion and melting the plastic part of the fuse and around the holder. Hence why the wire hasn't blown.

Will check and report back..

Thanks

K

[syncroandy]

Well to me it looks like the fuse rating of 16A is simply marginal for the load resulting in it getting warm and melting the plastic. Later vans have a 20A blower fuse, and an early fuseboard I have here has a 25A blower fuse fitted. I've never had a fuse problem on either of my 'late' vans.

[CovKid]

25amp with a blower still in its youth about right - by the time its a pensioner, 75amp lol