Thread: Relay to fix starter voltage drop?

Originally Posted by DiscoMick

Has anyone else had this experience?
Recently the Disco (1995) wouldn't start despite trying all the usual tricks, so the RACQ guy diagnosed a voltage drop to the starter motor and recommended fitting a relay, which I had done. The auto sparky, who trained on Landies a while back, said it was common practice now to fit such relays to vehicles generally to overcome this. I don't really understand auto electricky, but as I understand it if there isn't a relay there is a constant charge which can drop, but with a relay the contact is broken unless you go to start when a big charge goes down the line. Is that right?
Anyway, the auto elec. also said this was a common problem with Toyotas.
Since it was fitted the Disco has started perfectly every time. And its no longer slowly draining the battery.
Is this a common thing? Anyone else had it?

There wasn't ever a battery drain........a relay fitted to the starter is like a remote starter switch thus lessening power losses through a long run of thin wiring - it provides a shorter route usually with a heavy power supply link.

It's a common problem with most Lucas ignition switches.

As the contacts get old and corroded the resistance increases and doesn't send the whole 12v to the igniter on the starter solonoid.

The voltage is burned up as heat which damages the contacts even more as well as drawing more current as as resistace increases so does current thus also damaging wiring to the ignition switch.

The best way to fix this problem is to replace the ignition switch as well as install a relay between the activator wire and the starter to divert power directly from the battery to the starter .

This way only enough current is drawn through the wiring to and the switch to trip the relay which only takes about 5 amps with most common relays.

If anyone needs a diagram im happy to draw one up.

Originally Posted by mcrover

which only takes about 5 amps with most common relays.

POINT 5 of an amp more like it!!!

500 milliamps your right my bad...

OK so glad to know this is a common thing. Thanks.

Originally Posted by byron

There wasn't ever a battery drain........a relay fitted to the starter is like a remote starter switch thus lessening power losses through a long run of thin wiring - it provides a shorter route usually with a heavy power supply link.

Generally the voltage drop is caused by the coil, gas solenoid and all sorts of things that come on when you crank, usually nothing to do with the run length as you won't get this drop from the starter solenoid alone, hence why it works properly with a relay