I have a 79 T/A with olds 403. I have a few questions about wiring. im wondering what wire is for the tach? What color is it and where is it leading from in the engine bay. I have a msd 6al box now and i know there is a tach terminal on the side of the msd box. My next question is the heavy red wire that goes to the alternator has another red wire about 3 inches long coming off of it and im wondering where that goes? The alternator seems to be working. Is it needed? My last question is how long should it take for the temp needle to jump up? I dont want to overheat the new engine because the gauge isnt working properly. thanks

 

Hi.
I'm not looking at it now, but I believe my '79 403 T/A's tach wire comes through the firewall by itself in its own dedicated grommet... look for it close to where the accelerator cable passes through. It should connect to a little capacitor can screwed into the firewall..the wire that connects to the distributor should be white. I've seen many folks bypass the capacitor but I probably would not recommend that.

I can verify later tonight when I get home but someone else will probably chime in before then.

A GREAT investment is one of those handheld infrared thermometers -- just point it at your target, pull the trigger, & read the digital readout. It's excellent for testing temperature senders & I think many of them can read high temps such as exhaust. They are a lot cheaper than they used to be and well, they are kind of fun too.

 

Well I plugged that white wire into the msd tach output and started the car and then I immediately shut it down because the tach still wasnt working. I went to start it again and now I have no spark. I checked wiring. I tested the box like it says in the instructions and i even tried a new coil. Still no spark. I think it fried the box but I find it very hard to believe. That white wire goes through the firewall in between the brake booster and fender and then runs up with a red wire to the tach. The red wire goes to the fuse box where it says ignition. I spliced the ignition wire to the msd box in with that red wire. Everything worked fine until I hooked up that white wire to the box. If that isnt the right wire then where the heck does that one go?

 

Most (if not all) MSD boxes will require a separate module to operate that hood tach.

 

I dont have a hood tach. Just stock gauge in dash. I believe there is a separate harness I need to make it work. Im sending my msd in to get fixed. No spark

 

generally a white wire on a MSD box is for points. if you have it hooked to something and it is finding a ground the box wont work because it is looking for the points to open. the gray I believe is the tach wire. you should have a big red and big black wire that should hook direct to the battery. I would not hook it to the alternator. the small red should be the "key on" signal wire that tells the box to turn on. the only other wires should be purple and green which plug directly to the distributor.

 

Yes i have that white wire cut and isnt being used because im using magnetic pickup not points. I have the heavy red wire running from battery to starter and heavy black from battery to ground on engine. I have a ground strap from engine to chassis. There is another red wire branching off of the alternator wire. No idea what it is for. There is a white wire though running from the tach out the firewall right next to the wiring harness block. Ive been told that is for the tach but a harness is needed to connect it properly to the msd box. Everything was working properly and I had started many times. It was after I started it with the white tach wire connected to the msd box that It acted up. After that I couldnt get any spark. I double checked all connections and wiring, the few fuses I have, and tested the box the way msd says to and still no spark. I know the wiring has to be right because it ran fine. It would be different if I just hooked it up and didnt get spark.

 

http://www.summitracing.com/parts/MSD-8920/