Hotwire conundrum

DaveEFI · Post by **DaveEFI** » Tue Mar 27, 2012 10:55 am

A pal has fitted a hotwire setup to his SD1, and had initial problems with the fuel pump not running. It's a fairly common mistake to replace the EFI relays with the common 5 pin types (SPDT) instead of the correct twin output ones, but looking at the diagrams I have, there would appear to be two different ones used on the hotwire - one with a diode, one without.

Anyone know why the difference?

Other thing is BL didn't appear to do much quality control of their diagrams.
87 and 87a only refer to the outputs of a SPDT relay - on the twin output ones they should both be labelled 87.

ChrisJC · Post by **ChrisJC** » Tue Mar 27, 2012 12:25 pm

I suppose the addition of the diode means that the polarity must be right for the relay.

I wonder if adding the diode was a convenience thing - and it's presence only makes sense in the context of the rest of the circuitry. I am thinking of the diode pack in the Flapper EFi - maybe they moved the diodes into the other relays....

Chris.

SimpleSimon · Post by **SimpleSimon** » Tue Mar 27, 2012 9:02 pm

If your using a Cat loom 92on then both relays should have diodes and are bespoke although earlier BMW cars use the same spec relays too.

DEVONMAN · Post by **DEVONMAN** » Tue Mar 27, 2012 10:13 pm

The diodes are sometimes introduced to protect sensitive components in the circuit/ecu from back EMF when the relay coil switches off.

Denis

DaveEFI · Post by **DaveEFI** » Tue Mar 27, 2012 10:47 pm

SimpleSimon wrote:If your using a Cat loom 92on then both relays should have diodes and are bespoke although earlier BMW cars use the same spec relays too.

It's early Hotwire I'm more interested in. The Hotwire3 circuit clearly shows the diode in the relay.

If the relays are expensive/difficult to get it would be easy to just add an external diode.

ChrisJC · Post by **ChrisJC** » Wed Mar 28, 2012 6:57 am

DEVONMAN wrote:The diodes are sometimes introduced to protect sensitive components in the circuit/ecu from back EMF when the relay coil switches off.

Denis

I thought that too, but discounted it because for that use, the diode is connectec across the coil connections, rather than in series with it.

Chris.

SimpleSimon · Post by **SimpleSimon** » Wed Mar 28, 2012 8:50 pm

Its not like the correct relays are hard to get anyways here http://www.ebay.co.uk/itm/LAND-ROVER-DI ... %26ps%3D54 or alternative Euro car parts 450110050. BMW Part 12631729

DaveEFI · Post by **DaveEFI** » Wed Mar 28, 2012 10:30 pm

Think I'd go for the R30BT from

http://www.vehicle-wiring-products.eu/V ... relays.php

and add a tuppeny diode.