JohanvdWalt

Is it necessary to use a resistor when bridging the RDK and if so what value resister should be used?

Johan

Antoine928

Did mine a few days ago and did not use one. IIRC you have to bridge pin 2 & 13 on the RDK connector and one of these pin is ground. I dont think you should need a resistor in that case

borland

Before I bridged mine, I measured the current across those pins with the ignition key removed. I don't recall the current in mili-amps, but it was considerable.

My car is a garage queen, so I wanted to reduce the battery drain as much as possible. I installed a resistor.

If you have a daily driver, then you shouldn't be concerned.

JohanvdWalt

Thanks Borland, as mine is a daily driver I'm going ahead.

Johan

TRUESCOTSMAN

This is an interesting and poinient topic you have raised. Over the months i have read various posts on battery drain and poor starting. Various suggestions have been put forward, cleaning connections and fuses, checking regulator level etc. This could be a vital answer to all our questions. I would recon that the vast majority of us 928 owners have aftermarket wheels , all without the appropriate sensors, most having done the suggested bypass proscedure. Giving that most would not have included this resistor, as i cannot recall it being mentioned in the past, This could be the answer to the battery drain/poor starting problem. I follow any links like this with interest because i myself cannot leave the car for a week, or it will not start. In the past i have put this down to the alarm (aftermarket to allow remote door opening) drawing too much current.
So, what is the highest value of resistor that can be put in to minimise the current drain , but still allow the bypass? Any techies out there have an answer...Good post Borland.

SharkSkin

Measure the current, then multiply by 12 to find watts. Get a potentiometer, say a 5K or 10K with an adequate watt rating for the load. Adjust the potentiometer to get the desired result(I'm guessing some light will go out). Remove the potentiometer from the circuit and measure the resistance. Round down to the nearest common resistor size in the needed watt rating. Then post your findings here.

D

JohanvdWalt

Hi Sharkskin

I'm not sure are you insulting or complimenting us or perhaps swearing at us?

I don't have a clue what you're talking about but hopefully someone else does.

Johan

SharkSkin

Johan,

I'm offering a way to determine what value resistor to use for RDK bypass that will get the job done with minimal current drain. When I said "measure the current" I am talking about basically using an ammeter in place of the jumper.

If you want to jumper this with absolutely zero battery drain, place a miniature relay in the jumper line and wire it up so that there is no connection across those RDK connector pins until you turn the ignition on.



D

TRUESCOTSMAN

Tried to measure the draw current through this today, but either the current is so low my meter is not picking it up, or my s4 does not power it when the ignition is off. 2.5 amps when ignition is on.. Regardless, i wired up the relay so it was definately not connected when off.

TRUESCOTSMAN

rough wiring diagram

borland

That should work! I checked my notes....

I did check the drain current across this jumper and the current
was measured at 20 mA. That's 20 mA with the ignition key removed. Not
bad, but I thought 5 mA would be better, so instead of the simple jumper
wire, I jumpered the terminals with a 1.5K ohm resistor.

SharkSkin

Well, 20mA would take 2500 hours to drain a 50 Amp-Hour battery, which is IIRC the capacity of the OEM equivalent battery. 3-1/2 months.

Given that size battery, a 300mA drain should kill it in a week. 70mA would kill it in about a month.

It's basically H = (AH / D)

Where:
H = Hours to kill battery
AH = Amp-Hour capacity of battery
D = Drain current in Amps, measured between battery post and vehicle

So, if you have a dying battery, it's very handy to have a multimeter that can measure in the milliamps. There's really no excuse not to have one. Yeah, Fluke meters are great, but if you're on a budget something like this $20 Chicken-Shack meter will get the job done and probably save you many times the purchase price.

D

borland

Dave,

Good stuff, but it's not a perfect world out there.

You still need to keep plenty of reserve capacity for cranking over and starting the engine.

Then there's the problem of the car not being driven until the battery was completely charged before shutting it down?

Also, that battery rating is dependent on temperature, age, number of deep cycles, and other factors.

So your H formula is for ideal conditions.

When you add up all the other loads like the alarm and clocks, the instrument cluster load from the RDK jumper, that 20mA won't seem like a big concern, but it's still running the clock that says "dead battery"

SharkSkin

Of course, other factors come into play, such as discharge rate. At low discharge rates, the Amp-Hour capacity of a battery is increased significantly, which sort of acts as a built-in "fudge-factor". You can measure the actual AH capacity of a battery easily enough, just charge it fully put a known load on it and find out how long it takes to reach 10.5 volts, then divide load(in amps) by hours. Still, if someone says "my battery goes flat overnight" then the first thing to check is if there is a 1-3 amp drain with everything shut off(including the light in the hatch, duh). If there is, the problem is not the battery. If there isn't, the battery is probably shot.

Anyway, What I set out to do is offer some general info that might be helpful to those inclined to try their own electrical troubleshooting. Of course there are variations, and different people will strive for different levels of perfection. But when you're troubleshooting a problem, often times you are looking for things that are an order of magnitude out of whack on the first pass, then you go looking for the little stuff. Chasing a 20mA drain would be pretty pointless if the overall system drain was an amp!

D

TRUESCOTSMAN

I myself am happy to locate and "sort" any problem that might be causing battery drain. When you have possibly 20 ma from the rdk + whatever the door flashers for the alarm are using+ the flashing led for the aftermarket alarm+ coming and going to work in the dark , the wipers and heater to deal with the scottish weather. Any little saving helps.