Motorcraft Distributor Upgrade for the AMC 258

by Keith Hardt

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No sooner did I finish my HEI Upgrade on the "MIGHTY EIGHT" (thanks hobbs) did I start to see threads on the BBS about upgrading the stock Motorcraft distributor and coil to newer technology.   After alot of threads and maybe a little tears on shirt sleeves, a few of the folks on the BBS took Aaron's (a.k.a. TeamRush) advice and did the upgrade.


After reading the reams and reams of threads on the BBS I thought it would be good to compare both distributors side-by-side in a non-technical test.  I really just want to find out what works best for me.


I have decided to jump right in and try it myself.



The distributor does not have to come out of the engine to perform this upgrade.  I happen to have my stock distributor laying around since the HEI Upgrade, so I will start from there.



I could have used the stock coil, but I figured that if I was going to compare this new setup with the HEI Upgrade that I would go ahead and install the Motorcraft TFI coil instead of using my stock coil. The coil I used is listed for a 1984 Ford F-150 Pickup with the 300, I-6.  I got this from my local Advance Auto store.  This is basically a raw coil with a plug wire post on the top.  Here are pictures of the coil and the angle bracket holder mounted to the passenger side fender well.


coil_1_small.JPG (18753 bytes)           coil_2_small.JPG (21944 bytes)


I had already made a bracket from 4" angle brackets bolted to the inside fender for the HEI coil.  I just removed the HEI coil and installed the TFI coil in it's place.



The existing stock spark plug wires will not work with the Motorcraft Large cap.  When I did the HEI Upgrade  I went to my local parts house and got plug wires for a 1980 Ford F-150 with the I-6, 300 engine.  I got these from the NAPA store.  Make sure that you go with at least 8mm plug wires.  The smaller wires will not carry the energy from the TFI setup.  This Ford engine has a distributor on the center of one side of the engine similar to the AMC 258 and the plug wires work real well.


tfi_new_plug_wires_small.jpg (6340 bytes)



The cap and rotor I used is listed for a 1984 Ford F-150 Pickup with the 300, I-6.  Make sure that you use the premium cap with brass terminals.   Otherwise, you can run into problems down the road.  These came from Advance Auto as well.  Below are pictures of the larger cap as well as the new Cap Base Adapter.


distributor_2_small.JPG (19124 bytes)           distributor_1_small.JPG (18240 bytes)



The new large Motorcraft cap will not fit on the stock distributor.   To make the match I needed a cap base adaptor.  This will allow me to fit the larger cap on the stock unit.  The adaptor I used came from a 1982 Ford F-150 Pickup with the 300, I-6.  Make sure that you get the adaptor that SCREWS down to the stock distributor and CLIPS to the new cap.  If your local parts house has the bottom clip type try the same adaptor for a 1981 P/U.  This came from Advance Auto as well.



I have been told that this is not necessary, but does help the performance of the ignition setup.  Of all of the parts this was the hardest to find.   The only place that I could get this was from one of the local parts houses that carry's Mr. Gasket parts.  This advance spring kit is listed as a number 925D for 1965 and up Ford Distributors.  I have seen them in a Jeg's Catalog, but I had already ordered it locally.



I wanted to have a clean wiring setup without splicing into the stock harness.  Although I am planning on ripping out all of my vacuum and air lines, I was going to keep the computer and harness in the Jeep just in case North Carolina ever get emissions testing.  That way I can go back and put the emissions stock to pass the tests.  Otherwise, I would just rip it all out and start from the bottom up.


To do this I needed an extra wiring harness to go from the stock ignition module to the stock / upgraded distributor.  My 1985 CJ-8 uses an on board Electronic Engine Control Unit (EECU) to control many things, including the ignition.  Subsequently, the Motorcraft Ignition Module (located in the driver's side engine side firewall) has four wires coming out for ignition control, but my Jeep wiring harness only uses three of the wires.  I did not want to splice the wires straight through or have two ignition modules so that I could go back to stock if needed (se above) so I decided to change the connector plugs on the module and make a new harness.  I purchased 4-conductor waterproof connectors from Centech Wiring.

Here are some examples where I used these connectors:


connectors_2_small.JPG (17806 bytes)        connectors_1_small.JPG (17596 bytes)           connectors_3_small.JPG (18232 bytes)


These will allow me to change out the ignition module as I need to.


Also, I needed a pigtail for the new Motorcraft TFI coil.  I also this fom Glenn at   He has the new pigtails available and he was great to work with.



There will be a few other things that you will need.  This list does not include everything, but I tried to include the major items:

         Vacuum Hose - Now is a great time to install a new length of vacuum hose for the advance unit.

         Heat shrink tubing and wire connectors.




Cleaning the Distributor

I used spray carburetor cleaner and a rag to clean the inside of the distributor.  I kept close attention to the wires so as not to get too much cleaner on them or they could get damaged.  I used the old cap to keep the old outside dirt from getting into the interior of the distributor while I got the years of crud and dirt from the outside distributor.  The spray carburetor cleaner and a wire brush work well for this. 


Installing the Advance Springs

The first installation was the advance spring kit.  There is a small set pin that holds the distibutor trigger wheel to the shaft.  Remove this pin with a punch and remove the trigger wheel.  Then, remove the base plate attached to the distributor housing.  There are two advance springs attached to the advance lever.  Replace these with the springs from the Mr. Gasket #925D kit.


Attaching the Cap Base Adaptor

The cap base adaptor attaches just like the old distributor cap, except the hold-down screws are on the inside of the housing.  Just screw down the adaptor tight.


Assembly of the Harness and Coil Pigtail

I cut the original connector off of the stock ignition module and the distributor.  I installed new Centech Wiring waterproof connectors on the ignition module, the distributor, and on the stock wiring harness.  The connectors on the wiring harness are so that I can still use my stock setup if I would ever need to.


Here is a drawing of the STOCK ignition schematic:


stock_ignition_schematic.jpg (35164 bytes)



Here is a drawing of the MODIFIED STOCK ignition schematic:


tfi_schematic.jpg (38443 bytes)



I used color coded 16 gauge stranded copper wire.  I used the color codes shown in the schematic above.  If I had to go out and buy some wire I probably would have used 14 or 12 gauge, but I had plenty of 16 gauge laying around.   The only connection left to make is the splice into the positive wire from the stock harness that fed the original coil to feed the new TFI coil. (as shown in the schematic above).


Installing the Distributor – Aligning the Engine Up with Top Dead Center (0o TDC)

I scribed a mark on the distributor housing (with the cap installed) where I wanted the position to be for the number 1 plug wire.  This will make it easier for me to line up the rotor on 0 deg. TDC when I install the distributor.


I marked on the existing distributor where the rotor makes contact with the number 1 spark plug position of the cap.  I removed the low side and high side wires from the stock coil.  I used a screwdriver to engage the starter switch to pulse the engine until the rotor approached the number 1 plug position and lined the timing mark with 0 deg. TDC.   I had to do this a few times to get the timing mark on 0 deg. TDC.


Another way to find 0 deg. TDC is to place a wine cork (or the finger of a second person) in the number 1 spark plug hole.  Don't force the cork into the hole, we want it to be able to come out when the piston compresses.  Place a wrench or socket on the vibration damper's bolt, rotate the engine clockwise until the wine cork pops out so you now know this is the compression stroke of the number 1 cylinder.  Continue turning until the timing mark shows 0 deg. TDC.  Make sure you have not made a complete 360 degree turn.  If you did go past 0 deg. TDC, this will be the exhaust stroke and you will need to start over.  Thanks to Larry Maggio for this paragraph.


Removing the Old Distributor

Loosen the distributor hold down clamp and lift the stock distributor straight out.  If you have some problems or it seems to be stuck give it a little tap with a hammer to loosen it.


Installing the Distributor – Determining the Position of Plug Number 1

As I mentioned above I scribed a mark in the distributor housing where I wanted position Number 1 to be.  It really does not matter at this point since the engine is at Number 1, 0 deg. TDC. Whatever position on the cap that the rotor points to will be Number 1 plug position.  Turn the rotor to line up with the scribe mark.  Rotate the rotor one tooth counter-clockwise to compensate (this will be about 1/8 of a turn) for the rotation of the shaft (due to the helix on the gear) when the distributor is installed.  When the distributor is installed in the engine the rotor will rotate back to the scribe mark.


The shaft of the distributor is also what turns the oil pump.  Look at the position of the end of the shaft and use a screwdriver to rotate the oil pump (in the engine distributor housing) to line up with the end of the new distributor.  Insert the new distributor.  It should go in fairly smooth and easy.  If you are having problems you probably do not have the oil pump or the drive gear properly lined up.  After the new distributor is installed tighten the hold down clamp.


Note: Remember to mark the plug wire positioning on the new cap... often they are marked from the factory... by rotating the distributor and using a position not factory, it could get very confusing, especially if you take the Jeep to another mechanic who spends some time thinking position 5 is position 1... and then tears apart your timing case to “cure” the problem!  Thanks (again) to Larry Maggio for this paragraph.  My new cap had a "1" for the number one position.  Others that I have seen did not have anything, do what you need.


Putting It All Back Together

I gapped the spark plugs to 0.045" and re-installed.  Re-install or replace the vacuum hose from the vacuum port off the manifold to the distributor advance unit.  You will probably need to use a tee in the manifold vacuum line.



After all of this is finished, start up the engine and adjust the timing.  Have everything ready the adjust the timing since the distributor is probably not lined up exact and you want to keep the engine from running bad for an extended period of time.


When I started my Jeep the timing was pretty off.  You should be able to turn the distributor to where the engine sounds good and let it warm up before you try to fine tune the timing.  It is now set at 9 deg. BTDC at 1600 RPM.  This is the setting on the emissions label for a 1985 Jeep with Automatic Transmission.


My Jeep starts and runs great.  I have not found any hesitation or dead spots.