Intermittent fuse blowing - Sprint (late 1977)

[gmsclassics]

I've been unable to resolve this which has been going on for some months now. It is the lower of the two glass 35A fuses in the fuse box, the one that protects the 'ignition on' circuits.

I have checked and rechecked all these circuits, can't trace any shorting nor have ever been able to replicate the fuse blowing. I have also used 35A fuses from different suppliers. Every time I go through the checking process, everything on the car works fine - have never blown a fuse while checking. Same if I replace a blown fuse, works fine after that.

It does appear to be a problem that occurs most often when starting the car from cold, rather than when restarting a warm/hot engine. I usually notice the issue as various gauges & lights are not working on the dash. It is winter here NZ at the moment, but car is kept in a modern barn and doesn't get that cold.

Happened today, went to start car, ignition on but noticed fuel gauge not working. Replaced blown fuse, all okay, reversed out of barn and no further issues while out. Made three stops/ restarts without issue. Tried restarting engine after arriving home and again, no issues.

Car starts easily on choke, just takes pump usual time to get fuel up to carbs if left for a few weeks.

Any suggestions how I tackle sorting this issue out would be much appreciated.

Geoff

[ClassicFan]

Is the starter motor starting to fail causing excessive current drain?

[yorkshire_spam]

Bit of a tangent... do you have an overdrive? No experience of wiring issues on Dolomites, but my Spitfire used to intermittently blow the fuse associated with the OD feed due to frayed/cracked insulation on the wires that feed the switch on top of the gearstick - happened most when I "waggled" the stick to check for neutral before starting.

[marshman]

Wires to the overdrive switch would be my first port of call as well. Mine had "chafed" where they exited the gear lever gaiter and were shorting onto the top of the gear box. Physical examination of the full length of both wires is the only way to properly check as the fault is intermittent.

Replaced mine with some silicone coated extra flexible cable - silicone insulation was quite thick and it was a squeeze getting them threaded up the gear lever. Took care with the routing to ensure that they didn't chafe on anything as the gear lever is moved.

[xvivalve]

Third vote for overdrive switch harness.

My test each time it happened was did the wipers work!

[MIG Wielder]

Hi Geoff, Given your situation I would perhaps suggest the following course of action...

Yes, the wires to the overdrive circuit are the #1 problem but do try the following diagnostics.

Make up 4 x 6 in long electrical leads with two female Lucar connectors at each end.
Crimps are fine for this exercise.
Get 4 x 10A blade fuses from a modern car.
Disconnect the 4 green output cables from the fuse box. Reconnect each one in series with the wire links as above and insert the 10A fuse in series with each one before reconnecting each one to a green wire.

You will then have four separately fused circuits to each output from the 17A fuse.
Wait until one of the 10A fuses blows. That will tell you which circuit the fault is in.

At least the main 17A fuse won't blow and kill all the circuits.
Let us know how you get on,
Cheers for now,
Tony.
( Edit to the correct connector types.)

[jackharper]

Hi Geoff, Given your situation I would perhaps suggest the following course of action...

Yes, the wires to the overdrive circuit are the #1 problem but do try the following diagnostics.

Make up 4 x 6 in long electrical leads with two female Lucar connectors at each end.
Crimps are fine for this exercise.
Get 4 x 10A blade fuses from a modern car.
Disconnect the 4 green output cables from the fuse box. Reconnect each one in series with the wire links as above and insert the 10A fuse in series with each one before reconnecting each one to a green wire.

You will then have four separately fused circuits to each output from the 17A fuse.
Wait until one of the 10A fuses blows. That will tell you which circuit the fault is in.

At least the main 17A fuse won't blow and kill all the circuits.
Let us know how you get on,
Cheers for now,
Tony.
( Edit to the correct connector types.)

Was going to suggest exactly the same as I've just done the same to my Toledo. You can buy cheap fusebox holders on ebay which come with the crimps that slot in the holder. I adapted these to the existing spade terminals so I didn;'t have to modify the existing loom!