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Grounding the battery/engine/frame/body - best practice?


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Folks, the current issue of TR-Action (326) has a letter from a gentleman who suffered a car fire as a result of questionable electrical grounding under heavy loads from an electric fan. I do not have an electric fan, but I do run an electric fuel pump, so I took this article to heart. I did some searches on the forum about grounding the electrical system, but I’m still not clear on what’s considered best practice. My TR4A (negative ground) has a  ground strap from the engine to the frame, and the negative post of the battery is attached to a bolt holding the bell housing to the block. Is this configuration ok, or are there better ways to insure proper grounding?

thanks

Jim

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Hi Jim,

the standard wiring has an earth strap form Battery Neg to the body at the front bulkhead (make sure it makes a good contact)

There is also the engine to chassis strap at the front lefthand suspension area.

The chassis will get to the bodywork via many attachment points - however these tend to be painted.   But it appears to work.

I also add an extra strap from Battery Neg direct tot he engine.

Make sure all connections are on clean metal to metal.

Roger

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The 6 that had an under bonnet fire was lacking the correct earth strap. On the 6 it is an “all in one” that goes from battery to body shell to the engine at the bell housing. His just earthed battery to body and no bonding of block to body/ chassis. Cranked over reasonably well so started. Presumably short bursts of high current were not too much of a problem however a long spell with heater, lights, fuel pump and fan were enough to heat up something enough to start a fire. He had got through a number of throttle cables over the years and with hindsight they may have been getting hot with the current flowing through it.

The 4s have separate earth straps one from battery to body and the other linking the front of the block to the chassis near at the font. These should be fine unless they have frayed.

 

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Thanks gents! As my car has the negative post of the battery going to to the bell housing/engine rather than to the bulkhead, I’ll correct that. As Roger states, the frame-to-body bolts should provide a grounding path between the frame and the body. However, not knowing how good those connections are, is it prudent to run a strap from the frame to the body as well?

cheers

Jim

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16 hours ago, Tr4aJim said:

 However, not knowing how good those connections are, is it prudent to run a strap from the frame to the body as well?

cheers

Jim

Of course, Dont forget there should be an engine to frame earth strap too at  the front left side of the engine from the lifting eye to the cross tube bolts, The thing about the bolts that hold the body down is on fully restored and painted cars theres often no earth through due to all the paint!

Stuart.

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Thanks Stuart. Yes it does have the strap from the engine to the frame at the cross tube. So I’ll move the battery strap from the bell housing to the bulkhead, and add one someplace between the frame and the body. Belts and braces!!

Jim

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26 minutes ago, Tr4aJim said:

So I’ll move the battery strap from the bell housing to the bulkhead,

Better to leave that where it is and fit a separate lead to the bulkhead.  The direct connection from the engine to the battery is a good thing which saves the starter current from having to take the long way round through various 'iffy' joints. 

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22 hours ago, Andy Moltu said:

The 6 that had an under bonnet fire was lacking the correct earth strap. On the 6 it is an “all in one” that goes from battery to body shell to the engine at the bell housing. His just earthed battery to body and no bonding of block to body/ chassis. Cranked over reasonably well so started. Presumably short bursts of high current were not too much of a problem however a long spell with heater, lights, fuel pump and fan were enough to heat up something enough to start a fire. He had got through a number of throttle cables over the years and with hindsight they may have been getting hot with the current flowing through it.

 

 

just had this on my 5

starter cranked slow and smoke from the throttle cable . it seems it liked this as an earth.

connected a jump cable from battery earth to engine and starter  zoomed .

Cleaned the earth to body connection and all is well.  With a new throttle cable that is.

Why do i always read these tips after a problem / expense 

Roy

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Wasn't it Kenneth Williams, in "Round the Horne" (or another of Kenneth Horne's programmes?) who said, "the answer lies in the soil". 

One cannot have too much earth - just needs to be in the correct places.

Ian Cornish

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On 12/30/2020 at 3:08 PM, roy53 said:

just had this on my 5

starter cranked slow and smoke from the throttle cable . it seems it liked this as an earth.

connected a jump cable from battery earth to engine and starter  zoomed .

Cleaned the earth to body connection and all is well.  With a new throttle cable that is.

Why do i always read these tips after a problem / expense 

Roy

This is what you would expect from a poor engine earth, choke cables are another earthing point (starter motors draw 60+ amps). I would not expect an electric cooling fan to cause a fire. The fan itself should have a connection to earth and the relay it is fed through should also be earthed. there should also be a fuse somewhere in the system, usually between the relay and electic thermostat any heavy current would cause the fuse to blow. The maximum a fan should draw is @35 amps, normally must lower so all being well a poor engine earth would have little to do with it.

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