Unilite problems?

[Bonnewagon]

A friend is going through Mallory Unilite modules like crazy, so bad he is putting a points distributor in for now just to get up and running. It's a crate Chevy 350 with a tunnel ram intake that replaced a broken 350 and he is using a complete Unilite distributor/coil/wiring system that ran well prior to the swap. A lot of chassis wiring and stuff was changed at the time of the swap so he is presently going through it all looking for problems. I read up on the system and it seems it is very susceptible to voltage spikes from high amp alternators, bad grounds, wrong ballast resistors, etc. I have read lots of negative things and my only personal experience is a Unilite retrofit into a '68 Cadillac distributor that ran great even after the car was junked. So let's say if you follow the directions exactly, use only Mallory parts, and are otherwise smart about your wiring practices, how good or bad are the Unilite systems?

 

[pontiacgp]

I had a bad coil once which fried a module

 

[Bonnewagon]

Coil is new.

 

[pontiacgp]

I found this on the line..

"For all MALLORY UNILITE distributors, and Conversion Kits. Requires a 1.4ohms Ballast resistor, Loom resistor, or internally resisted coil."

 

[Bonnewagon]

Steve, I went to the Mallory site and saw a pdf. of the installation instructions- very detailed with warnings and operational tests. As far as I know the install was good since it worked well on the original motor. Unfortunately with the motor swap came a lot of upgrades and changes to the car itself that may be playing a role here. Worse, he had a "git-er'-done" mentality so I 'm convinced he screwed up somewhere. What I was asking was, all things being equal, with a correct installation, is the Unilite a reliable unit or is it prone to failure no matter how correct everything is. I personally considered it the gold standard of ignition products especially as the drop in replacement for a points system.

 

[pontiacgp]

I have used a Mallory coil and never had an issue with it. I know some guys running Mallory distributors and never heard a complaint from any of them. The reviews on Mallory products are always great...I think you right about maybe something got missed installing it.

 

[drogg1]

I plan to run one cause I got it for cheap. As an electrical engineer I would say if they call for a ballast, it is not something you can leave out. Nor should you half-hazardly connect things. Electronics do not generally allow for recklessness.

 

[Bonnewagon]

Drogg1, how would a high amp alternator create a voltage spike? Is it because they don't put out until a certain rpm is reached, then it blasts in? It might be one of those chrome plated one wire jobs. Also, I remember that points ran on 6 volts and only got 12 volts when the starter engaged, for better fire up. Mallory stated not to exceed 9.5 volts, so I'm guessing the factory resistance wire should be fine, yes? And if you used the Mallory ballast, that should be even better?

 

[Ribbedroof]

Back a million years ago (OK, just 29 years ago) when I was selling these, we ALWAYS stressed that the voltage requirement was 7 to 9 volts max. Simply adding a ballast resistor is not always enough, you have to check voltage to the coil.

We never really had issues with them, aside from people that would not follow the instructions.

 

[drogg1]

There could be some transient spike when the alternator reaches the RPM where it begins to supply current. Seems unlikely as I would think they would have clamps to prevent spikes but I'd have to read into it more.

Reading the instructions, it seems the ballast resistor is to limit current through or, equivalently, limit the voltage across the coil. So, you should be able to use the factory loom resistance provided it limits the current adequately (or drops the desired amount of voltage). Or just use the ballast resistor.

It would be helpful if they have specs on how much the current should be limited to. Perhaps a suitable cheaper alternative to the "official" ballast resistor could be sourced. Maybe if I dig around more I can find them....

From Ribbedroof's '7-9 V max', we can take 8 V as a safe value. Let's say the alternator is running (~14 V to the system) and so we have a voltage drop across the resistor of:

System - Coil Voltage = 14 V - 8V = 6 V

However, that doesn't tell us much as we now either need to know the current limit or the ballast resistance.

Typical ballast resistqnce looks to be from 1-2 Ohms and so the current is limited to 3-6 A (6 V / 1 Ohm or 6 V / 2 Ohms).

This large (relatively) amount of current is only drawn at each ignition event and so the voltage at the coil terminal may fluctuate between 14 V and 8 V. I imagine the multimeter being used has a fairly slow sampling rate and so you should be able to at least periodically read this voltage.