Pertronix & Tachometer Query - New member with 1973 450sl

[DJet Tragic]

Hi - I am Peter from Melbourne in Australia, I have a 73 450SL Djet FI (its an early one with the clock in the dash)

This is my first thread after signing up a while ago, then watching, reading & searching.

What have been peoples experiences with their Tachometer after a Pertronix installation?

I have just installed a Pertronix igniter & flame thrower coil (including specially grounding the distributor base plate) the engine started first time and ran well. But the tachometer is now reading about double what it should. It was working perfectly beforehand. It is also a bit 'bouncy'.

Has anyone had this problem and 'fixed' it?
Any advice would be appreciated.

Peter - Melbourne Aus.
450 sl - will eventually receive a new MegaSquirt system.
+ some old Eng. French & Swedish cars

[Graham]

Originally Posted by DJet Tragic

Hi - I am Peter from Melbourne in Australia, I have a 73 450SL Djet FI (its an early one with the clock in the dash)

This is my first thread after signing up a while ago, then watching, reading & searching.

What have been peoples experiences with their Tachometer after a Pertronix installation?

I have just installed a Pertronix igniter & flame thrower coil (including specially grounding the distributor base plate) the engine started first time and ran well. But the tachometer is now reading about double what it should. It was working perfectly beforehand. It is also a bit 'bouncy'.

Has anyone had this problem and 'fixed' it?
Any advice would be appreciated.

Peter - Melbourne Aus.
450 sl - will eventually receive a new MegaSquirt system.
+ some old Eng. French & Swedish cars

I just installed an Ignitor on my '72 350SL 4.5. Same problem. Tach swung over to about 4000 rpm as soon as I turned key on and then bounced around.

As a first step, I installed the unit just as a replacement for the points. Wired as shown below (sketch provided by Pertronix)

Does anyone have suggestions as to how to get the tach to work?

Only wiring changes I made were to run red to point shown, run black to point shown (which is where green wire from points previously connected). I also ran a ground wire from distributor casing to battery negative.

[Graham]

Got it fixed - just moved red to 12v side of other resistor and it works.

[DJet Tragic]

Originally Posted by Graham

Got it fixed - just moved red to 12v side of other resistor and it works.

Hi Graham - that's good news. Can I ask a few questions?

In your installation, was the ignitor unit the only part installed - meaning you kept the transistorised ignition unit and the original coil operational?

Do you intend to eventually use a Pertonix coil and bypass the transistorised ignition, ballast resistors & original coil? (this is what I have done)?

Where does your Tach wire (green/black on mine) presently connect to? PO of my car had an immobiliser installed so there is non-standard wiring from the ignition switch to the ballast resistors.

I had an issue with the rotating magnet in the distributor - as it wouldn't seat properly on the points base-plate & the rotor button was too high. I filed away some of the plate and the advance/retard arm to give it clearance to sit fully down.

PS: One last question - what sort of rotor button are you using - is it a normal one or a rev-limiter one?

regards: Peter in Melbourne Aus.

[Graham]

Originally Posted by DJet Tragic

Hi Graham - that's good news. Can I ask a few questions?

OK - I will try and answer below:

In your installation, was the ignitor unit the only part installed - meaning you kept the transistorised ignition unit and the original coil operational?

Yes - Just the Ignitor

Do you intend to eventually use a Pertonix coil and bypass the transistorised ignition, ballast resistors & original coil? (this is what I have done)?

Yes - Ignitor itself is only minor benefit. I will see how it goes for this season and add probably add new coil next year.

Where does your Tach wire (green/black on mine) presently connect to? PO of my car had an immobiliser installed so there is non-standard wiring from the ignition switch to the ballast resistors.

It connects to same terminal that green wire from distributor used to connect to and where black wire now connects.

I had an issue with the rotating magnet in the distributor - as it wouldn't seat properly on the points base-plate & the rotor button was too high. I filed away some of the plate and the advance/retard arm to give it clearance to sit fully down.

Mine seemed to go in OK, but I will double check. I just pushed it on! I only have vacuum retard on my early SL, so perhaps the later distributors are different.

PS: One last question - what sort of rotor button are you using - is it a normal one or a rev-limiter one?

I put in a regular new style Bosch one without the rev-limiter.

Some more...

I had a response from Retrorockets and from another owner. Both of them said that solution to tach problem was to add a resistor on the line feeding the tach. Did not tell me what value should be because I had already resolved problem. I may need to know if I add coil down the road! Pertronics should know.

One thing I am still not sure about. If I go to high output conventional coil like Flame Thrower, will new Bosch wires and resistor plugs recently installed work properly with the new spark characteristics?

Final step might be to ADD MSD ignition unit and - sounds like a straight forward add-on and provides better spark and therefore more complete combustion. Under $200 it seems. Don't know what it would do for me, but it's fun to mess with these old cars!

[DJet Tragic]

Originally Posted by Graham

Mine seemed to go in OK, but I will double check. I just pushed it on! I only have vacuum retard on my early SL, so perhaps the later distributors are different.

Thanks Graham
An important thing to check with Pertronix installation is that the rotating magnet over the cam in the distributor is seating right down on the base plate. If it fouls with the vacuum advance arm &/or the raised LIP on the base plate - then the rotor button sits too high & can push the shaft down into the guts of the distributor and pop the little circlip off (thats under the felt pad in the shaft)

[jtbenzman1]

There is a lot of confusion out there and even the Petronic Instructions are not as clear as they could be. A couple of points of note follow that may say someone time get a test light or meter to check for hot wires. Wiring color may be slightly different than described. Use the test light to see what is hot when the ignition is on and what is only hot during cranking. IMPORTANT OBSERVATIONS

1. MAKE SURE THE DISTRIBUTOR BODY IS GROUDED TO CHAISSES. PETRONIC DOES NOT SPELL THAT OUT STRONGLY OR LOUDLY ENOUGH. THE DISTRIBUTOR BODY MUST BE GROUDED.

2. THE RED IGNTION WIRE IS A THICKER WIRE THAN THE STARTER WIRE WHICH WILL HAVE A STIPE ON IT ALSO THE STARTER WIRE WILL ONLY BE HOT DURING CRANKING. WARNING-WARNING! THE RED IGNTIRON WIRE MAY BE VERY TIGHLY COVERED IN A BLACK PLASTIC SLEEVE AND MAY APPEAR AS A THICK BALCK WIRE IT IS ALWAYS HOT WITH THE IGNTION ON

THE 4 OHM BALLAST RESISTOR IS THE BLUE ONE. THE 6 OHM IS THE SILVER ONE.

(HERE IS THE WIRING WITH SWITCH GEAR RETAINED)

1. DOUBEL TERMINAL CONNECTOR:

(USUALLY LOCATED BELOW THE BALLAST RESISTORS)
BOTTOM OF CONNECTOR PUT GREEN AND YELLOW STRIPED WIRE LEADING TO SWITCH GEAR ALONG WITH GREEN AND BLACK STRIPED WIRE TO TACK ON THE CABIN SIDE OF THE BLUE 4 OHM CONNECTOR. ON THE OTHER SIDE OF BLUE 4 OHM BALLAST RESISTOR PLACE THE NEGATIVE BLACK WIRE TO THE PETRONIC UNIT. THE COLOR MAY BE OFF BUT BASICALLY YOU WILL HAVE A GREE WIRE OF SOME TYPE TO SWITHC GEAR AND A GREEN WIRE OF SOME TYPE TO THE TACH.

TOP OF TERMINAL ALL BROWN WIRES TOGETHER AS GROUNDS. ONE BROWN WIRE FROM SWITHC GEAR AND ONE BROWN WIRE FROM COIL OR FROM COIL THROUGH CHAISSIS. IF THER ARE ANY OTHE BROWN WIRES PLACE THEM HERE>

2. BLUE 4 OHM BALLAST RESISTOR:

THE RED STARTER HOT WIRE (WHICH MAY HAVE A VIOLET STRIPE) AND WILL ONLY BE HOT DURING STARTING AND THE RED WIRE WITH THE BLACK STIPE LEADING INTO SWITCH GEAR ARE MOUTED TOGETHER ON THE CABIN SIDE OF THE BLUE 4 OHM RESISTOR. NOTE: THE RED WIRES LEADING INTO THE SWITCH GEAR AND THE RED WIRE TO THE STARTER MAY OR MAY NOT HAVE A STRIPE. ON THE OTHER SIDE OF THIS RESISTOR IS THE RED IGNTION WIRE. THIS WIRE MAY BE WRAPPED IN BLACK PLASTIC AND APPEAR BLACK, IT WILL BE HOT ANY TIME THE IGNITION IS ON! YOU CAN PEEL THE BALCK PLASTIC SLEEVE BACK TO SEE THE RED OR CHECK WITH A LIGHT TO ENSURE IT IS HOT WITH THE IGNITION ON

3. SILVER 6 OHM BALLAST RESISTOR:

BLACK WIRE FROM SWITCH GEAR TO CABIN SIDE OF 6 OHM SILVER BALLAST RESISTOR. ON THE OTHER SIDE USE A THICK BLACK WIRE TO CONNECT THE RESISTOR TO THE POSITIVE SIDE OF THE COIL.
CAR SHOULD START RIGHT UP

SWITCH GEAR TOTALLY ELIMINATED (GROUND DISTRIBUOR BODY!

DISCONNECT AND TAPE OFF ALL WIRING LEAING TO SWITHC GEAR

1. RED 12 V IGNITON WIRE (COULD BE WRAPPED IN A BLACK PLASTIC SLEEVE AND APPEAR TO BE BLACK) CONNECTED TO POSITIVE SIDE OF COIL ALONG WITH RED WIRE LEADING TO PETRONIC UNIT.

2. BLACK WIRE FROM PETRONIC UNIT TO NEGATIVE SIDE OF COIL, THEM TO TERMINAL CONNECTOR TO TIE TO GREEN ABD BLACK TACH WIRE.

THIS IS THE BEST SET UP FOR RUNNING DEPENDABLE DRIVING AND PERFORMANCE. MAKE SURE TO GROUND THE DISTRIBUTOR BODY TO CHASSIS. MAKE GOOD TIGHT CONNECTIONS AND HAPPY MOTORING