MSD RPM Modules

[Guest Anonymous]

Hey guys, I have a 6AL msd. Had a 6600 rev module in it and it seems to kick in at 6K (sputtering/black smoke). Could it be detonation due to hot weather?

 

This is a new condition. I've reved it over 6K with the 6600 module and had no problems, though that's always been in the cool of the evening.

 

1) Could it be detonation due to the hot weather????

2) Could a RPM module go bad?

3) Could timing have slipped?

3) Bad gas? (Opps excuse me couldn't resist.) It's been sitting for about a month with a 3/4 tank.

 

Thanks for any input.

 

Jon

[Guest Anonymous]

I answered my own question.

 

Jon

[Owen]

It was the gas?

 

Owen

[DavyZ]

Jon, I'm curious to know what the problem was--please let us know since the car I'll be buying here shortly has the same MSD 6AL.

 

Davy

[pparaska]

There used to be a page on the web that had info on reprogramming MSD RPM pills. It's gone now, but luckily I printed it out!

 

Anyway, if you remove any pill from the rev limiter, you won't have a rev limiter! How about taking the pill out and see if it still breaks down? That way you'd know it's not the rev limiter.

 

The page mentioned above showed a few things about the pills:

 

1) They consists of a plastic housing and two pins that you can see and inside is simply a precision resistor.

 

2) The relationship (within 10% error supposedly) between Resistance of the pill and the rpm limit is (screwed up in my printout - see below).

 

3) Use a quality metal film resistor, not a carbon resistor like you find at Radio Shack.

 

4) To open a pill, peel back the label on the pill, pull out any hot melt glue with tweezers, and use a desoldering tool to remove all the solder from the inside of the pins. The pull the resistor out of the plastic housing. Add your own resistor and solder the leads to the pins.

 

Here's a table of values that I reproduced from his list (I check against my printout from the page as well):

 

code:

---

From the original web page entitled "Reprogramming MSD RPM pills"							

(The formula given was hosed up, so I fit this one to it:)

Ohms = 0.044569*(RPM/1000)^5 - 1.2032*(RPM/1000)^4 + 14.865*(RPM/1000)^3

- 49.539*(RPM/1000)^2 + 780.6*(RPM/1000) - 313.2

 

RPM R(ohms) RPM R(ohms) RPM R(ohms) RPM R(ohms)

100 2600 1593 5100 3691 7600 6399

200 2700 1668 5200 3786 7700 6523

300 2800 1744 5300 3882 7800 6649

400 2900 1821 5400 3980 7900 6776

500 3000 1897 5500 4078 8000 6904

600 3100 1975 5600 4177 8100 7034

700 3200 2053 5700 4278 8200 7165

800 3300 2132 5800 4379 8300 7298

900 3400 2212 5900 4482 8400 7432

1000 3500 2292 6000 4585 8500 7568

1100 3600 2373 6100 4690 8600 7706

1200 3700 2455 6200 4795 8700 7845

1300 3800 2538 6300 4902 8800 7986

1400 3900 2621 6400 5010 8900 8129

1500 4000 2706 6500 5119 9000 8274

1600 4100 2791 6600 5230 9100 8420

1700 4200 2877 6700 5341 9200 8568

1800 4300 2963 6800 5454 9300 8719

1900 4400 3051 6900 5567 9400 8871

2000 1151 4500 3140 7000 5682 9500 9025

2100 1224 4600 3229 7100 5799 9600 9181

2200 1297 4700 3320 7200 5916 9700 9340

2300 1370 4800 3411 7300 6035 9800 9501

2400 1444 4900 3503 7400 6155 9900 9663

2500 1518 5000 3597 7500 6277 10000 9829

---

So that's the gist of that page. I just found out tonight that the guy had the equation all screwed up, so I broke-out the Matlab program and did a curve fit to the data he had in his table. My answers above are within 10 ohms or less of his and should be plenty close enough to give RPM results within the tolerance needed. Heck, 1 ohm is about 1 rpm so it's not worth worrying about getting the resistances just right anyway.

 

Way more than you wanted to know, I'm sure .

[Guest Anonymous]

Thanks for the info Pete! Checked my plugs and found my number 1 cylinder was cold. Been busy so I haven't had the chance to get into it yet. It's nice to know those "modules" are accurate!

 

Thanks again.

 

Jon

[katman]

Pete's info jives with a graph I made a few years ago, just from measuring a pile of the MSD rpm modules. An equation, cool. Don't forget most resistors are +/- 2 percent or so, so at 7000 rpm that's like 140 rpm one way or another. Not too accurate really, and I don't think the MSD pills are anything special in that regard.

[pparaska]

katman, the resistors in the pills look like metal film precision resistors to me. But I agree, buy some carbon resistors, measure them, and you'll know within a hundred rpm or so what the limit will be.

 

Oh yeah, plug in one with infinite resistance (like leave the pill out) and you get an infinite rpm rev limiter .

[Guest Anonymous]

A potentiameter? Sweet, you could then mark your own rev limit knob faceplate (checked against your tach). (maybe label the last RPM on the knob GOING FOR BROKE!!

 

Regards,

 

Lone

[katman]

Originally posted by pparaska:

katman, the resistors in the pills look like metal film precision resistors to me.

Could be. My old measurements are within 1% or better from your chart in the 3-8k range. That would make more sense. The "pills" never matched very well with what our data acquisition system was telling us which is why I thought they were standard. Probably closer than most people's tach's, anyway. Funny why they didn't make it linear....