Converting a TBI coil to a cylindrical coil

[Dan Brown]

I am currently putting a TBI unit off a 1995 Chevy pickup on a 1974 Chevy pickup. I would like to use an old style cylindrical coil instead of the TBI coil. Does anyone have any experience doing this, and if so, how do you wire it.

 

[bucket]

Just guessing here, I believe you can wire up the old coil just the same as the modern coil. The two wire harness going to the ignition module would get the connector cut off (coil end) and replaced with eyelets. Then there's just the ignition power wire, but it needs to be a resistance wire. About 8 volts, circ. I'm completely unsure if a condenser would be needed.

 

[1987 GMC Jimmy]

What about voltage disparity between points and HEI? I’d think you’d need a blaster coil or whatever high voltage options are available. It seems like a high voltage points coil is close enough to an HEI/TBI coil that the loss of performance would be negligible.

 

[chengny]

GM points style coils are not designed to run full time on 12 volts - like HEI distributors. The pre HEI ignition systems used a special wire (PNK/BLK) with an intrinsic resistance to power the coil. That built in resistance (about 1.35 ohms for a V8 engine) reduces the voltage – supplied from the ignition switch to the coil positive – from 12 V down to around 8.5 to 9 V. That reduced voltage is used for normal engine operation.


But the engineers also realized that the engines would start more easily if full battery voltage was temporarily available while cranking. That’s simply because increased voltage produces a hotter spark. To provide the temporary 12 volts they decided to run an extra power supply from the starter solenoid. A wire (yellow) was run from the “R” terminal at the solenoid directly to the coil positive.


When the key is moved to the START position, power is supplied (from the SOL terminal on the ignition switch) to the solenoid on the PPL wire. The solenoid actuates and closes a set of contacts that then provide a full 12 volts to the “R” terminal – and ultimately to the coil positive.


When the key is released from the START position, the contacts open and the 12 volt supply from the YEL is cut off. From then on, power to the coil is only supplied by the PNK/BLK wire.


But the above is just about power supply to the coil. I am not really sure what your situation is. Are you currently set up with a points style distributor and a remote cylindrical coil on a 1974 motor? It appears you are going to attempt to install a throttle body injection system into a stock 1974 truck.

I assume you have an applicable ECM and the various sensors it requires to run the engine. But regardless, the conversion will be extremely involved.

Just one example, the ECM needs to see a pulsed signal from the distributor before it will energize the injectors. If you intend to run a points style ignition system, it will require a pre-HEI type distributor. If you can even get an old style distributor installed, it won’t have any capability to generate the pulsed signal the ECM requires.

 

[bucket]

That's a good point about having a full 12v to the coil at startup. Most starters don't have the "R" terminal these days.

I took the original post as a question about using the old style coil with a late model, small cap HEI distributor.

 

[C10MixMaster]

To the best of my recollection. The bosch coils used on 240-740 volvos and probably a lot of european stuff looked like the traditional round coil but were used on electronic systems that ran a full 12 volts. might be worth looking into.

 

[Dan Brown]

I am using the TBI distributor off the 1995 truck, which had a new module in it. I want to use the old style coil and understand what chengy is telling me. It sounds like if I attach the two pink wires (one of which has the proper resistance in it) from the harness (mated connector) off the 95 truck to the positive side of the coil then ground the negative side, and use your suggested method of getting a full 12 volts to the coil while starting, it will work. Is this correct.

 

[C10MixMaster]

I am using the TBI distributor off the 1995 truck, which had a new module in it. I want to use the old style coil and understand what chengy is telling me. It sounds like if I attach the two pink wires (one of which has the proper resistance in it) from the harness (mated connector) off the 95 truck to the positive side of the coil then ground the negative side, and use your suggested method of getting a full 12 volts to the coil while starting, it will work. Is this correct.

Resistors were used because point will burn up when run on a full 12 volts. If you run a resistor on a solid state ignition system it might work however the spark will be weaker than it should be. Like I mentioned earlier The europeans used traditional looking cylindrical coils that ran on a full 12 volt solid state system. think that's your best bet if going for a retro look.