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  #1  
Old 07-25-2012, 01:43 PM
33Coupe 33Coupe is offline
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Default Low Tachometer Reading

My tach is reading low by about 2,000 rpm at upper end and 200 low at idle. I noticed that the rpm limiter was kicking in (Had a 6,000 rpm pill in the #6420 box), but the Autometer tach was reading about 4,000 rpm. Checked the rpm with my analyzer tach and it was reading 6,000 when the limiter kicked in. Then checked the MSD#6420 box with no pill (default limit) and it was working ok but the tach still read incorrectly. Decided it was not the pill. Installed an MSD#8671 adjustable limiter box. Tach still not reading right. Switched to a SunPro SST802R, thinking the tach was bad, but it had same result. Bought an MSD##8920 Tach Adapter and installed it. Same result - low rpm reading on tach. My Sunpro tach is switch is set to 8 cylinders. I have attached the wiring diagram that I used. Can you please help?
331 Hemi engine (1955)
MSD 8391 Distributor (early version with no gray tach wire)
MSD 6AL #6420 Ignition Box (SER#211881)
MSD 8671 RPM Limiter Box
MSD Blaster 2 Coil
Autometer Street Tach and Sunpro SST802R Tach (both have same problem with reading low)

Forgot to mention that I verified all power leads at 12volts and all grounds and all connections (< 1 ohm).
Attached Files
File Type: pdf Tach Wiring Setup - Hemi 331.pdf (755.6 KB, 10 views)

Last edited by 33Coupe; 07-25-2012 at 01:48 PM. Reason: left out part
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Old 07-25-2012, 02:11 PM
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msdtech1955 msdtech1955 is online now
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just a quick basic check....look at the long side of the box.
you will see a black cover the size of a dime with a screw in the middle.

remove the cover...are all the loops not cut?

also..what plug wires are we running ? and how old?
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Old 07-25-2012, 07:43 PM
33Coupe 33Coupe is offline
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I removed the black side piece cover and there are two red wires that I see, not three. It looks undisturbed. I am including a picture of what's there.

Taylor SpiroPro 8mm Silicone Plug Wires - 2 years old

I appreciate any help - I am stumped!

Dave
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  #4  
Old 07-25-2012, 07:45 PM
33Coupe 33Coupe is offline
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Here's the picture of the loops. They look intact but I only see two red wires.
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File Type: jpg 2012-07-25_21-31-13_111.jpg (850.2 KB, 11 views)
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  #5  
Old 07-25-2012, 07:48 PM
33Coupe 33Coupe is offline
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Default Plug Wires

Here's the link to the plug wires.

http://www.jegs.com/i/Taylor/895/752...rentProductId=

http://www.jegs.com/i/Taylor/895/752...rentProductId=
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  #6  
Old 07-26-2012, 06:48 AM
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get a small screw driver...hook it under those red loops and see if they pull out about an inch or so.
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  #7  
Old 07-27-2012, 11:27 AM
33Coupe 33Coupe is offline
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Default Low Tach Reading

One wire is connected and one wire is not connected.

If this is it, I gotta say you're pretty d***ned good!

What do I do next?
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File Type: jpg 2012-07-27_13-20-33_585.jpg (864.4 KB, 11 views)

Last edited by 33Coupe; 07-27-2012 at 12:13 PM.
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  #8  
Old 07-27-2012, 12:18 PM
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drill out 3 rivets ( 1/4 ) slide open silver cover...look where those loops are at .

do you see any blue loops?

if so...connect them together ( this way ..the 6al knows it is being used on a 8 cylinder)
must have red and green loops conected.
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Old 07-27-2012, 12:23 PM
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if it is not ...and if you dont want to mess with it.

send it to me for repair,
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  #10  
Old 07-27-2012, 02:27 PM
33Coupe 33Coupe is offline
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Default Low Tach Reading

I will give it a try.

Thanks again.

I will update the result.
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