Upgrading Renix Computer

I'm an electrical engineer. I have run across a deficiency in the Renix computer. I went with very large battery cables and this give me better cranking speed with the starter.
You would think that better cranking speed would be good. Well it is but it then shows up another problem. I let my XJ sit for about 6 weeks without starting it. I hop in and it cranks nicely but doesn't start. I look at the battery voltage and it is around 9v. The bigger cables allow the starter to still crank nicely at 9 volts, but 9 volts isn't enough for the Renix computer to wake up and provide spark.

The problem lies in the use of a 3 terminal regulator to generate the 5v used by the processor and sensors. The old regulators needed a fair amount of voltage above 5v to regulate correctly. I am switching the old school 3 terminal regulator with a pin compatible low drop out regulator based on newer technology. This will allow the computer to work even when the battery voltage drops.

I will be doing a writeup and posting the part numbers in a day or two. I still need to disect the circuitry to see if there is a low voltage trigger circuit as well.

or you could buy a booster pak and keep it with you.....:rof:

Tom, was your battery voltage measured under load (200Amps), or static?

You are correct, the newer Monolithic regulators do better than the old ones However if static, there may not have been enough voltage and current left over for the monolithic regulator to put out 5 volts under load.

Perhaps a UPS kind of system is what you are aiming for if you want the vehicle to stat with minimal starting voltage. The UPS would run the comp, and the main battery would run the starter and accys. Both would be recharged by the main alternator.

Or you could buy a trickle charger and stop trying to fix a problem that was never there

...engineers

Well, you have to crank the Renix up to 300+ RPMs to get enough of a signal from the crank sensor for the ECU to react.

9 volts is borderline for any ECU/PCM engine management system. Why so low after only 6 weeks?

My RENIX does this as well but I blame it on the OPTIMA red top. I put a battery tender on it and all is well now, just unplug it and she cranks right up and goes.

I have not played with any yet, but their are a lot of new super capacitors, and hybrid battery - super capacitors on the market, like Maxwell and Axion Power to name just a few, that have huge cranking reserves compared to regular lead acid batteries. Can't help but wonder if they have a drop in solution yet. They are aiming for the new start stop hybrid car market in the US and Europe.

Every RENIX I have had always takes a few seconds of crank before starting, I have even had dead cells in the battery causing under 8V on start and it will always start.

I just live with it and love when people freak out for the long cranking

What'd they use for the +5VDC regulator - a forerunner of the venerable 7805?

It is a 7805 type. The new LDO's only need about 5.6v to still give a regulated output, where as the old ones need 7-8v minimum.