Another tale of Rangerizing.
Cliff notes: Master relay and engine wiring harness bypassed for DME power.
Details. It became clear this past year that the DME is sensitive to small voltage drops. This was apparent at Mid-O when my high rpm @ WOT (but only at WOT) miss became about 50% fixed when I tightened my alternator down a bit more. The alternator tries to put out 14-14.5v, IIRC, and the battery wants to hold ~12.6v. The interaction between the two creates a system voltage of slightly better than 13V depending on a number of “real-life” factors because grounds aren’t perfect and wires and connectors have resistance.
The lesson learned there was that at some voltage between ~13.3 and 12.6, the DME craps out.
In pursuit of better system voltage I replaced the alternator’s voltage regulator and gave the alternator another ground. Then I pulled apart every important electrical connection, abraded the tarnish off and put them back together with conductive grease. There’s another thread that details all this, and it probably wasn’t worth the effort. It especially isn’t worth the effort if you nearly burn a finger off at the root.
A handful of us have had problems at or near the master relay. Vignettes from different folks:
- Racer 1. Replace master relay because it fell out at CMP turn 14.
- Racer 2. Replace a perfectly serviceable master relay and high rpm missing goes away for a couple sessions, which of course makes little sense.
- Racer 3. After significant troubleshooting of intermittant engine cut-outs, figure out that a female connector in the master relay’s socket isn’t in good shape and isn’t mating well with the male connector on the relay. Fix by replacing engine harness.
So I got to thinking, not only is the master relay, it’s connector and the 12’ of wiring to/from it a problem child, but the sheer length of wire is causing a voltage drop to the DME. What if I could supplement that with a better source of voltage?
Here’s how to do it. The DME has 2 power wires. A thin red one that is always on and a thicker red/blue wire that is switched on. My guess is that the thin one can be ignored, but I don’t know that for certain. Connect a fairly beefy wire to the cold side of your kill switch and then bring it to a switch on your dash. From there run it to both the red and the red/blue wires a couple inches before the DME.
Once you have the car started, flick your Master Relay Bypass switch. This gives your DME to separate sources of power so if the Master Relay was only providing 13.1, but you had 13.3 at the kill switch, the DME will get 13.3v. And if your Master Relay entirely hiccups, your DME won’t know it.
Because this is wired into the cold side of your kill switch, your kill switch will still operate. You wouldn’t want to leave the switch on with your car off tho. The key will still kill your car because it will kill injector power, but there’s no upside in feeding your DME power while your car is off so I’d hit the dash switch on the out lap and then turn it off after the checker.
The next logical step. My goal was to ensure optimum voltage to the DME. But if a person really wanted to proof their car against a Master Relay problem, then they’d need to wire the dash switch to the injector harness also.