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Wiring from car to engine
Slimy38 - 13/5/25 at 02:27 PM

This might be a basic question, I just want to make sure my understanding is correct. I've got most of my loom sat in the engine bay along the firewall, all bound and secured at suitable points. I then have a couple of runs of bound wiring going to the engine, but essentially they're in free air until they get to the engine. In one case it goes straight to the plug that is on the engine.

Is it a problem that the wiring isn't lying on anything or secured to anything? I was trying to figure out how I could secure it but then the engine would move anyway, so anything I would use to secure it would bend/break after a while. I reckon it's about six inches from the bulkhead shelf to the engine.

adithorp - 13/5/25 at 06:35 PM

Not a problem and unavoidable. As long as the free length is fairly short. Under 30cm is the accepted max' distance between wiring fixings.

Slimy38 - 14/5/25 at 07:13 AM

Excellent, thanks. I did wonder how it would be possible to sort it, I'm obviously overthinking things once again!

coyoteboy - 14/5/25 at 11:17 AM

I'm going to go against the grain here a little. It is unavoidable but there are definite improvements to the situation that you can make. Keep unsuspended sections short, but if possible you should try to get the cable to land on the engine as close to the centre of rotation as possible, and inline with the crankshaft. Reason being that you're a) minimising the movement of the part (because the engine will give an angle change under load, and the further you are from the centre of rotation, the bigger the linear movement will be) and b) if you run the harness in the direction of the crankshaft, the harness is gently bending when the engine moves, instead of being pushed and pulled which can damage the mounting points or cause bending in a different section you were not intending (this is harder to judge to be honest). I'd never ever have a wire span a gap and then go straight into a connector without a stress relief tie-down, it's a recipe for failure but maybe only after a while.

Slimy38 - 14/5/25 at 01:49 PM

quote:
Originally posted by coyoteboy
I'd never ever have a wire span a gap and then go straight into a connector without a stress relief tie-down, it's a recipe for failure but maybe only after a while.


thanks for the detailed response, but in particular this line here. This is what I have at the moment, and it's what got me asking the question. However the connector itself has a fixing on the engine. This for me puts a stress riser on the back of the connector, which is a natural weak point for wiring anyway.

The wires are bending/twisting rather than being pulled/pushed, but they are near the cam cover so they're taking the biggest movement action.

I think I need to look again at this particular one. Thanks again for the advice.

gremlin1234 - 14/5/25 at 03:58 PM

a point to remember, is to use flex, not cable.
cable is generally for infrastructure - ie fixed sockets in house walls etc., and usually single strand.
whereas flex is more 'flexible' using multi-strand conductors.

cliftyhanger - 15/5/25 at 06:09 AM

quote:
Originally posted by Slimy38
quote:
Originally posted by coyoteboy
I'd never ever have a wire span a gap and then go straight into a connector without a stress relief tie-down, it's a recipe for failure but maybe only after a while.


thanks for the detailed response, but in particular this line here. This is what I have at the moment, and it's what got me asking the question. However the connector itself has a fixing on the engine. This for me puts a stress riser on the back of the connector, which is a natural weak point for wiring anyway.

The wires are bending/twisting rather than being pulled/pushed, but they are near the cam cover so they're taking the biggest movement action.

I think I need to look again at this particular one. Thanks again for the advice.


Yes, fix the cable to the engine, and a bit of a loop between engine and body to allow movement. No soldered joints either, they are prone to fracture if there is any movement. Wrapping a large number of cables in tape can make them rigid, it may be better to use a corrugated plastic sleeve so they can move individually (this is me thinking about this more than I should!)

My old Triumph Vitesse belonged to a chap who was a mechanic in the 70/80s. There are loads of places he has used pig-tails on wiring to avoid stresses. Always good to see.

Slimy38 - 15/5/25 at 07:06 AM

quote:
Originally posted by gremlin1234
a point to remember, is to use flex, not cable.
cable is generally for infrastructure - ie fixed sockets in house walls etc., and usually single strand.
whereas flex is more 'flexible' using multi-strand conductors.


I'm reusing the MX5 loom, so while I may have issues with age, I'm hoping they're already fit for the movement involved in a car.

Slimy38 - 15/5/25 at 07:15 AM

quote:
Originally posted by cliftyhanger

Yes, fix the cable to the engine, and a bit of a loop between engine and body to allow movement. No soldered joints either, they are prone to fracture if there is any movement. Wrapping a large number of cables in tape can make them rigid, it may be better to use a corrugated plastic sleeve so they can move individually (this is me thinking about this more than I should!)

My old Triumph Vitesse belonged to a chap who was a mechanic in the 70/80s. There are loads of places he has used pig-tails on wiring to avoid stresses. Always good to see.


That's a thought, I've bound the cable in tape to deliberately keep them rigid on the fixed runs, but I've done the same on the short hop to the engine. It's so rigid I can unplug the loom from the engine and it'll stay pretty much exactly where it is!!

I think I'll reverse that decision, I've got a couple of short lengths of plastic sleeve that would work better.

I will admit I have done some solder joints, but they're enclosed in heat shrink tubing so the only movement is in the flexible part of the wire. They're then bound and secured. That's only for lighting runs though, I kept engine wiring as is.

nick205 - 15/5/25 at 10:23 AM

My Pinto engined MK Indy was SVA (pre IVA). The loom was in corrugated plastic and followed the guides suggested.

Try and sttached the loom to the engine before it went into the connector

No issues at SVA.

3k miles road use for me with no problems. Another 3k miles road use for the next owner without issue. Lost touch with the car after that.

I used a Vicki Green purpose made loom. Designed for a 7 type car with a Pinto engine.