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Hotwire conundrum
Posted: Tue Mar 27, 2012 10:55 am
by DaveEFI
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.
Posted: Tue Mar 27, 2012 12:25 pm
by ChrisJC
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.
Posted: Tue Mar 27, 2012 9:02 pm
by SimpleSimon
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.
Posted: Tue Mar 27, 2012 10:13 pm
by DEVONMAN
The diodes are sometimes introduced to protect sensitive components in the circuit/ecu from back EMF when the relay coil switches off.
Denis
Posted: Tue Mar 27, 2012 10:47 pm
by DaveEFI
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.
Posted: Wed Mar 28, 2012 6:57 am
by ChrisJC
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.
Posted: Wed Mar 28, 2012 8:50 pm
by SimpleSimon
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
Posted: Wed Mar 28, 2012 10:30 pm
by DaveEFI
Think I'd go for the R30BT from
http://www.vehicle-wiring-products.eu/V ... relays.php
and add a tuppeny diode.