Remote Ford solenoid for GM- no hot start

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by: Alittle1, Cobalt327, Jon
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[edit] Introduction

Sometimes a modified vehicle- especially one that uses headers- will experience starter "drag" when the engine is hot, which is akin to having a low battery even though it's fully charged.

[edit] Heat soak

If the ignition (especially the timing), electrical, starting and charging systems are otherwise adjusted and working like they should, the problem may be "heat soak". Heat soak is a condition where the heat from the engine and exhaust system heats up the starter, solenoid, and wiring which causes an increase in resistance.

[edit] Remote solenoid

The following changes will help this situation by removing the relay function of the starter solenoid to a cooler part of the engine compartment.

[edit] Electrical connection diagram

Ford sol on gm strtr blue.jpg

[edit] Using a remote solenoid

Also when the battery is trunk-mounted, it's a good idea to use a remote Ford solenoid/relay. This will keep the run of heavy cable from the battery up to the starter from being always "hot".

Basic wiring diagram for remote solenoid

Warning Note: If for some reason you want to keep your vehicle all GM, use a Standard p/n U939 solenoid like the one pictured below. They were used in some GM powered motor homes:

Standard PN U939.jpg

[edit] Original GM wiring configuration

The battery wire to the solenoid in the stock configuration is hot at all times. The sequence of events that happens in the standard GM when you hit the starter is as follows;

  1. The electricity has to go from the starter switch through the loom to the neutral safety switch.
  2. If you are in park (or neutral) the electricity then goes through the loom to the stock solenoid "S" terminal.
  3. When the "S" terminal goes hot, it energizes the solenoid. This causes the plunger to throw the starter gear out to the ring gear (flywheel). It also closes an internal set of contacts that sends electricity to the starter motor and also to the "I" terminal.
  4. The starter motor doesn't get power until the solenoid contacts are closed. The wire from the "I" terminal is a ballast resistor bypass for the coil. It gives full battery voltage to the coil during cranking.

Here is where a problem is encountered. It takes a fair amount of current to energize the stock solenoid when hot. Due to the amount of resistance in the wiring and starter motor from heat soak, the battery may not be able to supply the required current to the starter through the stock solenoid and wiring. This is where the Ford solenoid comes into play.

[edit] Modified wiring with the remote Ford solenoid

If you are running an GM HEI distributor, don't worry about wiring to "I" terminals.

  1. The wire that goes to the "S" terminal on the GM solenoid (closest to the block on GM engines having the starter on the passenger side of the engine) is removed and wired to the "S" terminal on the Ford solenoid.
  2. The wire that goes to the "I" terminal on the GM solenoid (farthest from the block) is removed and wired to the "I" terminal on the Ford solenoid. The "I" terminal on the GM solenoid will not be used. When you purchase the Ford solenoid, make sure that it has the "I" terminal needed for non-HEI/points-type distributors.
  3. The positive battery cable is removed from the GM solenoid and is put on one of the large terminals on the Ford solenoid.
  4. Another cable is run from the other large terminal on the Ford solenoid to the large terminal on the GM solenoid where the hot battery cable was just removed.
  5. Also on the large terminal on the GM solenoid, you have to add a heavy gauge (10 gauge) wire to the "S" terminal on the GM solenoid, or use a shunt. This can be bought or easily made.
Solenoid shunt for GM starter solenoid

[edit] This is what happens when you hit the start switch:

  1. The electricity from the starter switch only has to energize the Ford solenoid (that is mounted in a relatively cool location).
  2. The "battery" cable to the GM solenoid goes hot. The Ford "I" terminal also goes hot, providing non-ballast/full battery voltage to the coil.
  3. The GM solenoid now gets a solid "high capacity" current supply and works the plunger and contacts. The internal contacts send voltage to the starter motor.
  4. This set-up DOES NOT make the GM solenoid any cooler. It just gives it a better current supply. Anything that can be done to shield the starter from the header heat would be beneficial. There are several heat shields on the market including those from GM.
  5. Always be sure that no wiring comes in contact with the headers/exhaust manifolds and pipes and use heat shielding tubing or "socks", available from Jegs or Summit, just in case contact may occur.

[edit] Other possible cures

The wiring has to be in good condition. This means no frayed wiring, tight high quality terminals, no wires of insufficient size, no parts store clamp-on replacement battery terminals (the #1 cause of "bad starters").

Throughout the electrical system there needs to be tight connections, a properly working charging system including the battery, and a good starting system. Grounds have to have direct contact with bare metal and be of a sufficient gauge for the job. Use the voltage drop calculator link below (under Resources)to determine if the wire gauge is sufficient.

If the problem remains even after the remote Ford relay/solenoid is installed, a starter heat shield may help. Another option is a "permanent magnet" starter. They're less inclined to suffer from heat soak. Then there's the heavy duty "high torque" mini starters- also permanent magnet type starters.

[edit] Ignition timing

If the timing is advanced far enough, the engine will turn over slowly when it's hot regardless of what is done. This often happens when there is a big cam that needs a lot of initial advance or needs the timing locked at full advance.

The solution for this is to use a remote ignition interrupter switch. Generally the switch is a momentary on-type switch that is installed in positive wire to the ignition coil. This will allow the engine to be turned over by the starter, without the ignition being armed. Once the engine is spinning over, the switch is released to energize the ignition, and the engine fires up.

[edit] Battery terminals

Clamp-on replacement battery terminals are to be avoided!
Do not be tempted to save money by using one (or more) of the cheap, clamp-on style battery cable terminals. Even if they are tight and look fine, all too often they will cause high resistance and can make it seem like the battery is bad, etc. If the terminal fails on a cable there are better terminals that may be used to replace them, or the entire cable can be replaced with a cable having a larger diameter and quality terminals already installed.

[edit] Resources

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