Mk 2, 50 amp fuse blowing

[Volvo]

Can anyone suggest a cause for the 50 amp fuse in my Mk2 blowing? Recently, I replaced it on the road and it blew again within a mile or two - as I disengaged overdrive. As well as all gauges and heater fan, the brake lights, indicators and wipers go through this fuse so I am anxious to trace the cause.

Richard

[piman]

Hello Richrad,

if it is not an instant blow, i.e. it blows as soon as you replace the fuse, the quickest way of narrowing it down is to split all the outgoing circuits using in line fuses then reconnect to the main fuse. What should happen is that one of those fuses then blows and you can see what circuit is causing the problem. Then it's a bit of detective work paying attention to where the loom might be chafing on the body or similar? As it is a 50 amp fuse it's less likely to be an overload more than likely it's a short to earth.

Alec

[RichardSEL]

Can anyone suggest a cause for the 50 amp fuse in my Mk2 blowing? Recently, I replaced it on the road and it blew again within a mile or two - as I disengaged overdrive. As well as all gauges and heater fan, the brake lights, indicators and wipers go through this fuse so I am anxious to trace the cause. Richard

What were the weather conditions? Day or night? "Within a mile or two" could be making discovery worrying as it's an intermittant. Yet that could also help you.
Leave on tickover for five to ten with all electricals switched off. If you've no blow by the time full warm up occurs and then some you can start adding in the loads.
You've identified what circuits are fed via this switch so let's assume that the overdrive solenoid is not the culprit for the moment. And that your faulty circuit is one you've mentioned. Switch in one by one (for gauges I only had a Mk I many years back but seem to recall gauge lights only came on with sidelights (?), brake lights (probable as together with side lights they're available for water leaks to spoil), then indicators (ditto), heater fan, wipers.
Repeat when driving. If you can get this far with full loads (had a Mk IX that was glove box light -- they've only got four fuses Then use your overdrive a few times. If there fault still doesn't show itself then it's time for waggling
looms around those circuits.

You might like to clean the fuse holder and the fuse ends. I had a Citroen CX that had 1¼" glass fuses fitted to spade holders.
The holder cover had been missing for years so each of the fuse ends and the spades were deeply corroded. Rust being an insulator and the current flow high the Watts of heat generated at each end of the fuse was such that the fuse wire would melt... nothing whatever to do with a bad circuit

[piman]

Hello Richard,

I'm a little intrigued by your last paragraph. When there is a bad connection as you describe, it is very difficult to get a high current flow? Normally the circuit in question will not operate due to the high resistance?

Alec

[RichardSEL]

Hi Alec

>The holder cover had been missing for years so each of the fuse ends and the spades were deeply corroded. Rust being an insulator and the current flow high the Watts of heat generated at each end of the fuse was such that the fuse wire would melt... nothing whatever to do with a bad circuit

With a high load (in them thar olden day cars) the volts drop as you say across the bad connection is high too.
So, with the old W=IV the Watts of heat is generated at the bad connection and conducted to the fuse wire. x2 that (for each end's volts' drop) and there's a source of heat for the fuse wire inside the cartridge fuse to melt

[piman]

Hello Richard,

using the watts formula is going to give an error. You should use ohms law and I = V/R, high R gives low I and hence low watts. However theory and practice is sometimes different.

Alec

[RichardSEL]

Hello Richard, using the watts formula is going to give an error. You should use ohms law and I = V/R, high R gives low I and hence low watts. However theory and practice is sometimes different. Alec

That's good for the total current flowing around the circuit. Yes, the current is restricted by say a Ohm at each "U" terminal. What we're considering here is the heat generated by a bad connection at each of the fuse holder's snap-in connectors. While it's true that there's a volts drop across the bad connection that still doesn't change the load (just restricts the current a bit) but does generate heat at each of the "U"s

The number of times I've been to ships (as a Safety Radio Surveyor) only to find the emergency transmitters' rotary transformer (24V) is spinning slowly and the radio operator (many flags just had a morse / wireless operator without any electronics training) says: "We've changed the batteries and had all new so they must be alright".
I find that the new Exides fitted by a local Exide agent all have their cables of the clamp type where the wire is just crushed into the terminal spade. You run up the system, get the key pressed, and the emergency light dims, and the rotary spins down. Feel each spade and some are warm. Duh! You spend the next couple of hours filling up each spade wire clamp with solder

Automotive wiring is like that -- so many times (with or without warmth) bad electrics are traced back to a bad / corroded chassis to battery negative connection or similar further up the high current tree.

Anyway's I hope what we've outlined here has given Richard a plan of action to avoid a shop that'll just change out things just to sell some thing and the time to change it, irrespective of whether that fixes the fault