Inertia switch

[Steve-B]

Hi all,

As part of my ongoing thread on the mystery cut outs, today I've pulled the Inertia switch and was planning just to connect the 2 wires (which I have now done). Manually pushing/pulling the cylinder had quite a bit of resistence to moving. Then testing resistance, I had 1 ohm then 1.3, 1.5, 2.3 which was a bit weird however when popped the switch open I had the expected open circuit. This made me more suspicious of the actual switch as just sitting on the bench, it shouldn't be flaky resistence readings depending on which axis I was holding it.

I took the switch apart and found the plunger in a very sorry state with the top part of the plunger looking slightly melted and at a 30 degree angle to the shaft. I'm therefore thinking either leave it off, with the leads connected or replace with a new one based on these pictures which gives me pause that when hot in the engine bay, it could be intermittently tripping:

 

I would love to hear any opposing views to my thinking and am grateful for your knowledge/advise!

 

Edited July 2, 2023 by Steve-B

[Kiwifrog]

Don’t delete it completely, it is an essential safety feature on PI cars. Replace it with a modern equivalent 

[trchris]

Yes you need to keep a cut out in case of a collision check out eBay for a inertia fuel pump switch Peugeot 206 Land Rover etc there not expensive and easy to replace with original 

Chris

[BlueTR3A-5EKT]

 

buy and fit a modern replacement

Then - clean and tidy your old one and sell it as ‘unknown - spares only’ to a Jag owner. As part number C41220.  That should cover the cost of a tank of fuel and the modern switch………

Edited July 2, 2023 by BlueTR3A-5EKT

[Steve-B]

Thank you guys for the validation , I’ll try to see if I can find the right replacement.

PS> I kind of figure this might not be the cause of the cut outs, but it’s certainly time for a replacement after 50 years.

[Steve-B]

Is this a suitable replacement? 

[john.r.davies]

KiTrchris is quite right, and this inertia switch is fitted by multiple manufacturers, so one hopes it's effective.

It comes in two types, two wires an three wires.  The second will ignite a warning light to tell you why your engine has died, as any inertia switch may be triggered by a heavy bump that isn't a crash, such as mounting the kerb. In that case you can reset the switch and carry on.

John

[RobH]

2 minutes ago, Steve-B said:

I kind of figure this might not be the cause of the cut outs

The melting of the plastic plunger must be due to a poor resistive connection, as there is no other way it could get hot enough.  That must have had an effect on the operation both of the fuel pump and the ignition.  It is highly likely that was indeed the cause of your problem .

[Steve-B]

21 minutes ago, RobH said:

The melting of the plastic plunger must be due to a poor resistive connection, as there is no other way it could get hot enough.  That must have had an effect on the operation both of the fuel pump and the ignition.  It is highly likely that was indeed the cause of your problem .

Agreed in principle as that plunger not aligned could cause weirdness at temperature. Odd it doesn’t do so when cool which is another reason I decided to take it apart.

 fingers crossed 

[MilesA]

44 minutes ago, Steve-B said:

Is this a suitable replacement? 

Steve

That is the two wire version mentioned by John and will do the job. Fitted one to my 3A and have one ready to fit to my 6 as it does not have one (original or modern).

On the casing somewhere the two cables may be marked "C" and "NC". C is power in and NC goes to the pump. 

If you want a warning light you can wire one from the plug between the two wires shown in the picture. A 'nice to have' but not essential.

Hope this solves your cut out problem!

Miles

[Steve-B]

20 minutes ago, MilesA said:

Steve

That is the two wire version mentioned by John and will do the job. Fitted one to my 3A and have one ready to fit to my 6 as it does not have one (original or modern).

On the casing somewhere the two cables may be marked "C" and "NC". C is power in and NC goes to the pump. 

If you want a warning light you can wire one from the plug between the two wires shown in the picture. A 'nice to have' but not essential.

Hope this solves your cut out problem!

Miles

Cheers Miles, ordered it. Much appreciate your confirmation!

[Martin50]

I have looked high & low on my TR6 (early PI) and cannot find an inertia cut off switch. I also looked on the wiring diagram and can’t see any switch shown in the diagram. What is the history to these switches?

It does seem to be sensible safety item but were these always a dealer addition or an after market addition. I just wanted to check if I’m not looking in right places, or if the car was sold originally without it? I intend to add one - I’m pretty sure (99.9%) it is not fitted so I’m guessing it was sold without one.

Regards Martin.

[Jonny TR6]

Cream coloured cylinder to the right of the battery (looking from the front). Approx 2.5” high.

[Martin50]

Thanks Jonny but nothing there. Not even fixing or bolt holes. The pump feed cable has no break in engine bay and disappears into wiring loom. Pretty sure I don’t have one which triggers my question.

[PodOne]

 

8 minutes ago, Martin50 said:

Thanks Jonny but nothing there. Not even fixing or bolt holes. The pump feed cable has no break in engine bay and disappears into wiring loom. Pretty sure I don’t have one which triggers my question.

Hi Martin 

My 1969 didn't have one fitted.

There were no wires in the loom either when I stripped to use as a pattern while making a new loom so I suspect they were a later safety addition. I added one when I made the new loom linked to the low current side of the relay which runs the fuel pump.

Andy

[Tom B]

I am not sure that the old inertia switches can carry the current for the "Bosch" replacements. Do you have a replacement pump? If you do then fit a relay and put a  modern inertia switch in the feed to the relay. I posted a circuit diagram a while back. The inertia switch on mine was in a similar sorry state.

[PodOne]

21 hours ago, Tom B said:

I am not sure that the old inertia switches can carry the current for the "Bosch" replacements. Do you have a replacement pump? If you do then fit a relay and put a  modern inertia switch in the feed to the relay. I posted a circuit diagram a while back. The inertia switch on mine was in a similar sorry state.

Hi Tom

My reasoning is that the switch doesn't need to carry any high current any impact will kill the current by switching the relay off instead and kill the pump it that makes sense. Any inertia switch should then last longer.

Andy

Edited July 3, 2023 by PodOne
Grammer

[Mike C]

1 hour ago, PodOne said:

 

Hi Martin 

My 1969 didn't have one fitted.

There were no wires in the loom either when I stripped to use as a pattern while making a new loom so I suspect they were a later safety addition. I added one when I made the new loom linked to the low current side of the relay which runs the fuel pump.

Andy

Same with my 1969 model. I later fitted an inertia switch when I installed a direct feed to the Bosch pump.

[saffrontr]

Inertia switches were fitted from body no 51399CP on RHD cars  and body no 52328CP on LHD cars.

Derek

[Martin50]

Andy, Tom, Thanks. Useful to know. I still have the original Lucas pump and will install a new inertia switch. Not had any problems with the pump to date.

Derek again good to know. Thank you.

[Peter S]

Hi Steve,

when your 2 wire switch arrives check the connections are correct. I had one (2 wired but 3 pin plug & socket) which had the output wire in the warning light position in the plug. Net result was the switch would only work in the ‘post-accident’ position.

Pete

[Steve-B]

3 hours ago, Peter S said:

Hi Steve,

when your 2 wire switch arrives check the connections are correct. I had one (2 wired but 3 pin plug & socket) which had the output wire in the warning light position in the plug. Net result was the switch would only work in the ‘post-accident’ position.

Pete

Good advice Pete! I noticed it comes with the wiring connector with just 2 leads so hopefully they’re on the right pins

[john.r.davies]

4 hours ago, Peter S said:

Hi Steve,

when your 2 wire switch arrives check the connections are correct. I had one (2 wired but 3 pin plug & socket) which had the output wire in the warning light position in the plug. Net result was the switch would only work in the ‘post-accident’ position.

Pete

I bought one of those!  From the 'budget' end of Chinese market.  Sound lesson in ' buy cheap, buy rubbish'

John

[Tom B]

21 hours ago, PodOne said:

Hi Tom

My reasoning is that the switch doesn't need to carry any high current any impact will kill the current by switching the relay off instead and kill the pump it that makes sense. Any inertia switch should then last longer.

Andy

Andy - yes that is what I meant. The reasoning for replacing the old one in this case is because it is u/s! Here is what I did:

 

All the best

Tom

[Andy Moltu]

Some of the eBay ones are wired wrong.

Helped of of the group wire one in and had us puzzled but one we had the meter on it and pulled the connector apart and swapped around it was working fine.