Voltage drops

[sr20truck]

So your S chassis is getting older by the day and is at least 20 years old today. Example 240sx, and not to exclude 180sx, 200sx, Silvia, and more. Your chassis harness is not what it used to be. Your key switch may be very tired and loose.

These are my findings. Alternator puts out 14.2 volts. Sends voltage to fuse box under the hood. Through that maxi fuse (75amps) and then back out of the fuse box straight to the key switch via a thick white wire, then back out to the reverse fuse (lower right of under dash fuse box which controls voltage at the consult port for connectivity) and on back to the under hood fuse box for the ignition power fuse and the fuel pump relay fuse via a black wire with red stripe (culprit wire), then onto the chassis harness via black wire with white stripe which ends up in the F8 plug better known as the thick wire that is ecu power and coil power (sr20det in 240sx will have these bound together for install).

So, a voltage drop test is conducted by selecting dc volts on your multimeter and probing positive to positive points while a circuit is energized. I back probe positive point on a coilpack to the alternator. Your reading is how many dropped volts are in the circuit. 0.500 and below is generous and nothing to worry about. As many S chassis as I've worked on, every single one has had voltage drops from 1.5 volts upwards of 3.7 volts.

Incomplete burn? Wasted fuel? Fix just this and the car comes alive with power with this fix alone. Fuel economy even goes up by as much as 3mpg. Forget tuning for the moment.

Now you could remedy this by replacing the black wire with red strip in the engine bay harness from the key switch to the fuse box but that is super tedious if you know just how much work it will be. 10 hours plus for a tidy job. Or do the quick fix. An inline relay right near the under hood fuse box.

Here's how. Cut the black wire with white stripe on your efi harness and insert a relay there. 85 ground, 30 battery power, 87 Black/White going into efi harness, 86 Black/White going into F8 plug. Start engine and do another voltage drop test. You will be within values given (0.5 volts or below) and have the most powerful spark. This same principal applies to the fuel pump resulting in a lazy pump at high load. (white wire with purple stripe near blue connector right at fuel pump) This I know will undoubtedly help every last one of you have ease in tuning.

My personal goal is to teach about lost power through faulty wiring and how to fix it. Apply this to any vehicle you tinker with. Comments would be great. Tell me if this helped you. Possible addition to pre-tuning checklist?

[Matt]

This is great. I'll forward to Pete to put into his tuning notes document. I spaced out the notes into paragraphs to improve readability. So many issues (mainly disconnects with consult) start from poor wiring and then all the other associated issues with voltage drops

[PL]

Yeah for sure. I've been doing wiring conversion jobs for SR20 into older cars for years so I'm intimately familiar with those wiring looms. Although just the engine loom - obviously the body loom doesn't feature with these jobs. Over the years I've seen S13 looms that people send me go from minty fresh to grubby unusable rubbish. They're just getting old. Some of the wire in S13 looms is now getting grey and tarnished inside instead of bright copper.

I was speaking to a local workshop recently and they were saying that these cars run excessive current through the IGN switch and so switch replacements are becoming commonplace. The old cars I do conversions for never had switch contacts rated anywhere near what the S-chassis cars require, so I have always added a relay for all the circuits that come from the IGN ON terminal.

There are variations across the model range (S13, 14, 15, turbo, non-turbo) on what circuits are powered from the IGN switch. Some are more likely to fail than others. Looks like S13 would be the worst. By the time they got to S15 they'd improved things somewhat and the power for the coils no longer came from the poor overworked IGN switch!

As you mentioned - the fuel pump wiring is a problem. And a very common problem. The classic I see here is to upgrade the fuel pump to something that draws a lot more current - without upgrading the factory wiring. Often the battery has been relocated to the boot so it's simply a matter of fitting a relay from battery to fuel pump. And of course upgrade the earth wire too. It's amazing just how much a small voltage drop affects pump performance. I did some tests many years ago and was staggered by the results. Since then I've used 5sqmm wire to the fuel pump on all my wiring jobs!

Pete L

[sssbassil]

so are these relays 14v 30a 4 pin's? and for the battery power supply is this a new line straight from the battery(full time power)?

[sr20truck]

yes the 4 pin relay of a 30 amp nature will suit things fine. wire number 30 straight to the battery with an inline fuse rated for the original circuit value will be fine. given your battery is relocated to the trunk you may wire 30 to your battery distribution block in the engine bay.

[sssbassil]

thanks bud, I have just done the relay on f8 plug and now have 14.1v. I have gained 0.9v and it was very easy thanks for the great info

just one more thing about the fuel pump rewire, I rewired my pump along time ago and I had only done the power and not the earth so I will be redoing the earth wire this week end,

so once I rewire the earth on the fuel pump, do I just cut the harness earth and leave it or do I need to earth it out so in make a circuit for the ecu??? sorry for being a wiring noob,