TR2/3/3A TR3 Fuse Question

[TR3-Stu]

I've found other helpful posts on TR3 fuses, including the difference between British fuse values and US AGC fuse values, but I have not been able to find out which fuse (the 35 amp and the 50 amp) go into which fuse box position (the A1-A2 and A3-A4 positions). I checked my service manual hard copies, and the Moss and Rimmer websites, but neither point out which fuse goes where. Can someone point me in the right direction.
Thanks,
Stu

 

[DrEntropy]

From original wiring diagrams, the only thing A2-A3 has for load is the horns. A3-A4 is handling all else. My WAG (Wild Arse Guess) is that should be the 50 amp.

The Advance Auto Wire schematic for the TR3 (page 3) seems to be the same as original.

 

[TR3-Stu]

Thanks for the reply. That was my assumption also, but I'm old enough to know what happens when you make assumptions.

 

[Geo Hahn]

Off the top of my head, I'm thinking the 50a was for the horns... but also thinking that they eventually went to 35a for both.

So an early wiring diagram may show a 50a fuse but your car may take a pair of 35s.

 

[DrEntropy]

You're more than likely right, George. The horns are basically a near-dead short. I hadn't thought it thru.

 

[karls59tr]

You're more than likely right, George. The horns are basically a near-dead short. I hadn't thought it thru.

 

[karls59tr]

I don't see "35AMP" fuses in local stores? Is the US equivalent a 17amp or 25 amp and what are the brand names. I'm battling a mysterious intermittent fuse that blows maybe maybe every 25 miles?

 

[DrEntropy]

AGC 17 for the 35 Brit replacement, AGC 25 for the 50.

Old brands:

 

[karls59tr]

remember those but you never seem to see them for sale anymore. I never see AG 17's at all? Maybe I have to order a bunch online?

 

[TRopic6]

remember those but you never seem to see them for sale anymore. I never see AG 17's at all? Maybe I have to order a bunch online?

Geo is right; according to some handbooks I have, early cars had a 50A for the horn fuse & 35 for the other. I don't know when it changed, but the last driver handbook shows 2x 35A fuses. I'll check them again.

The Lucas 400e reference shows the TR2 used WT614 horns and in 1956 (TR3?) went to WT618s. Our MG brethren have a nice website; Jerry Austin wrote a great technical article on them. Basically the 614 takes ~5.5 amps each and the 618 about 3, so maybe the higher-rate fuse wasn't needed later:

http://vintagemg.com/articlepdfs/thorns1-2.pdf

But if not working properly, they could easily pull more amps. Like Dr E says, they are basically a short until the diaphragm is pulled up to break the contact and the horn actually starts to sound.

 

[Geo Hahn]

The usuals sell a British 35 amp fuse... or you can get these from Amazon:

https://www.amazon.com/gp/product/B07S9BVFTC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

That is what I use. 15a SAE is quite adequate for TR loads.

I open them up and insert repro vintage Lucas papers but certainly not necessary.

 

[karls59tr]

The usuals sell a British 35 amp fuse... or you can get these from Amazon:

https://www.amazon.com/gp/product/B07S9BVFTC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

That is what I use. 15a SAE is quite adequate for TR loads.

I open them up and insert repro vintage Lucas papers but certainly not necessary.

so far I have seen in various write ups that the 35 amp Brit fuse is equal to US 20 amp.....17 amp....and now 15 amp? I was told that 17 AGC was the equivalent to the Bfir 35 amp?

 

[DrEntropy]

Someone posted a PDF recently here, listing British to American fuse equivalents.

An excerpt: "As an example, the 35 Amp fuse called out for many 1950’s/ 60’s/ 70’s vintage Britcars (Lotus Elan & Europa for sure) is roughly equivalent to a US-spec BUSS AGC 17 Amp fuse. Install a US 35 Amp fuse into your LBC fuse box and the wiring harness will melt down before the fuse blows."

 

[Sarastro]

I once measured an LBC horn. This was on my Healey Sprite, not a TR, but it was still a Lucas horn and probably similar, maybe even identical. I measured an average current of 10 amps. It conducts in pulses, though, of about 20 amps, That's nice to know, but probably not relevant; I'd base the fuse on the 10 amp average current. So, a bit over 25 amps for the two horns seems right to me. I think a US-spec fuse of 25 amps is right. If you blow the fuse for no obvious reason, maybe increase it to 30.

 

[Geo Hahn]

I have seen in various write ups that the 35 amp Brit fuse is equal to US 20 amp.....17 amp....and now 15 amp? I was told that 17 AGC was the equivalent to the Bfir 35 amp?

I wasn't saying that 15 amps was equivalent... I was saying that 15 amps is adequate (i.e. doesn't blow in any of my TRs).

The advantage of the 15 amp fuse is that they are usually much easier to find.

I think every automotive fuse I have ever blown has been a dead short, not a component that put a bit too much load on the circuit.