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Electrical

1930’s and 1940’s Chevrolet Truck Typical Wiring System

Tuesday, January 16th, 2018

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Additional Points of Interest on Early GM Wiring

1. Six volt systems MUST have 2 woven wire cables as a ground to allow plenty of current flow.
a. One from the battery to the frame rail. See drawing.
b. One from the ear of the starter attaching bolt to the frame rail.

2. The insulated cable from the battery to the starter switch, see drawing, MUST be a heavy one gauge thickness. NEVER use a small diameter 12 volt cable. It cannot carry the extra current flow required
by the starter. A small cable will cause the starter to turn slow!

3. As much as 75% of all electrical troubles are traceable to poor connections in the circuits.

4. An old timer way of tracing down an electric drain in your truck:
a. Touch a removed battery cable end against its battery post. If you have a short, you will see a tiny spark due to current flow. Sometimes dim outside light is necessary.
b. Disconnect suspected areas where a short may exist. When you no longer have the tiny spark, you have found the electric drain.

1934-38 Horn Wire Metal Loom and Connection

Friday, February 10th, 2017

Bet you didn’t know!

The two electric wires extending from the main harness run vertically beside the intake or exhaust manifold to the horn, depending on the year.

Here is the way Chevrolet did it on trucks and cars: From 1938 and older trucks and cars the two horn wires run vertically up to the horn between the exhaust manifold and the intake manifold. When looking at the rear of the intake manifold mounted horn (on the early six cylinder) the two wire attaching posts are on a 4 o’clock position. This results in the two wires being close to the heat of the exhaust manifold. To better protect these wires from heat damage, the factory harness includes a 14 inch corrugated metal loom as part of their complete harness assembly. This metal loom was on all early Chevrolet inline six cylinder vehicles as well as on factory replacement harnesses.

Even with this loom protection, there was still occasional heat damage to the two wires! To solve the problem beginning in 1939, the vertical horn connecting wires changed position. Now, the wire was found on the exterior side of the intake manifold. The two connecting posts were rotated to be at the 8 o’clock position. This made it possible to keep the wires away from the manifold beyond heat. Thus, the protective metal loom was no longer necessary.

Surprise: The mechanical part of the 1934 to 1953 horn can be rotated by the hobbyist. Remove the six securing fasteners, on the perimeter rotate it to the new position, and retighten the securing nuts. Therefore, the horn is easily changed from the early to the later years design. This replacement loom is now available from Jim Carter Truck Parts and other full stocking dealers.

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1934-36 horn. Connecting points at the 4 O’clock position

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1937-38 horn with wires in metal vertical loom (beside exhaust)

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1939 and newer on left. 1934-36 on right (connecting post on opposite sides)

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1939 and newer. (Mounted on intake) 8 O’clock position. No metal loom required

6 and 12 Volt Alternator – Warning

Monday, October 10th, 2016

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Alternator Warning

As some owners now replace their original electric generators with a modern alternator, here is an important warning that must be considered.

This is in regards to the in-dash original amp gauge used on most all vehicles. This gauge was made for a lower amperage flow provided by the early factory generator, usually a max of about 35 amps (sometimes 45 amp if factory air conditioning) on most 1950’s vehicles.

When a modern alternator is added, sometimes they have the ability to create a current as much as 75 amps.  Sometimes this is not good! The original dash amp gauges were not made to carry this high charging level and they could be permanently ruined if one thing happens.

If your alternator equipped older vehicle has a totally drained battery (lights left on, small electrical short, etc.) there may be trouble.

The alternator charging the dead battery starts operating to its full capacity when the engine begins running.  Remember during this charging period, if it’s a 60 to 75 amp alternator, it may ruin the original amp gauge with the “catch-up” to reach full battery charge.  If the battery is almost at full charge, no damage will occur.  The older amp gauge is not made to withstand this high current flow.

The older gauge can be identified by the two posts on the back side (positive and negative post). All current created from the alternator passes through the amp gauge. If this is a concern, running the current through an add on volt meter below the dash will be the option or use an alternator with not over 50 amp charging capacity.  A 50 amp alternator will provide the service most require on older vehicles.

Dents on Original Horns

Wednesday, July 20th, 2016

After 35 years in business, a walk-in customer told us why so many car and truck horns have miscellaneous dents. They are on the surface sheet metal in no particular place.

To our customer, it was easy to understand. He told us: “If the horn fails to operate the vehicle owner hit it with a wrench or hammer to start it working again”.

We went a step further. If the first few hits does not get the horn to make noise, then you hit it harder!

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Applying 12 Volts to a 6 Volt Starter

Wednesday, March 30th, 2016

There comes a time with some 6 volt vehicles that an emergency jump start is necessary. Maybe the battery has been drained due to a light or ignition switch being left on. Or maybe it was stored over a winter without the battery trickle charger being attached. Whatever, the reason, you feel helpless without another readily available 6 volt battery to use as a jumper (and you need to move the vehicle that day!)

Buying a replacement 6 volt battery at a local auto parts store seems a last resort, particularity at the near $100.00 price. The option is to use your readily available 12 volt battery and jumper cables to get it started.

This can be an acceptable idea in an emergency but with some important limitations. If your vehicle was running when parked and the 6 volt battery did not freeze over the winter, it should start quickly with a 12 volt battery. Connect positive to positive and negative to negative when adding the 12 volt cables from the 6 volt in the vehicle. No need to unhook the depleted 6 volt battery.

If the engine still remains free (will turn over) a 12 volt jolt will get it spinning at twice the RPM as did the 6 volt original starter. It if drove and was parked under its own power, then it should start quickly! If at the higher RPM and it will not start, you probably have another problem.

Pouring a “small” amount of gasoline in your down-draft carb. Using the 12 volt cables requires less time to have it connected before starting. Less time pumping gasoline into a dry carb requires less time connected to a 12 volt system.

DANGER: Jumping 6 volt vehicle with a 12 volt battery can be done at a “maximum” of 15 Seconds! After that the solder in the commutator begins to melt, some wiring insulation is turning black, and the starter switch may be turning blue. Connect the 12 volt cables “ONLY” when you are ready to start the engine. Have a person ready to immediately to remove one of the 12 volt cables the second it becomes running. Remember: Not over 15 seconds or parts of your 6 volt system are cooked.

SIX VOLT TRUCK AND CAR OWNERS. A MUST READ!

Monday, June 8th, 2015

One of the more common reasons for slow engine turnover using an original six volt system is under size battery cables. Most of today’s auto parts stores only stock the smaller diameter 12 volt design. Unknowing owners mistakenly replace their original worn cables with shinny new ones that are as much as half the diameter as needed. In Addition the owner does not know he needs two. Ground cables: One from the battery to the frame and one from a starter mounting bolt to the frame.

Six volt starters require twice the electrical flow to operate properly. Don’t blame your six volt system for slow unacceptable starter motor speed! Many restorers go to the expense of changing their 6 volt system to 12 volts. They feel their original system was inferior and believe what they did was necessary. The problem could have been corrected with just adding the three proper cables.

Remember: Millions of cars and trucks were made with 6 volt electrical systems each year. If they had not operated correctly they would have come with 12 volt systems!

Battery Cables 6 Volt

Buy Parts for 1934 to 1946 Trucks

The 20 Year Chevrolet Horn

Wednesday, April 16th, 2014


This new horn design was introduced in 1934. It is attached direct to the 207 six cylinder engine and was so successful there was almost no changes through 1952.
As shown in these photos this 1934-36 horn was attached to a flat foot that secured it to the center of the intake manifold. A long nose directs the sound to the area very close to the radiator cooling fan.
A slight change to the exterior appearance occurred with the new 1937 216 cubic inch engine. Possibly to keep it away from the high temperatures of the exhaust manifold, the horn was relocated. It now was attached to the forward leg of the intake away from engine heat.
The “bell” part of the horn was shortened to keep it the same distance from the fan. There, it remained on cars and trucks with the 216 engine through late 1952.
It was so well designed it rarely required attention. A single screw secures the rear half circle cover. When removed the inner workings are exposed for an occasional tone adjustment.
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1934 – 36
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1937 – 52

1934-1946 GMC Tail Lights

Thursday, February 11th, 2010

Early GMC Tail Light

Though items were shared between GMC and Chevrolet trucks between 1936 and 1946, General Motors made sure many parts remained very different during the early years the GMC preferred very few things to be similar to Chevrolet.  Their customers needed to see an almost stand-alone truck with the higher price of the GMC.

One very obvious difference is the change in tail lights.  There is no comparison to Chevrolet.  The massive GMC stamped one piece steel bracket combined with a redesigned 5-inch tail light makes the pair a “one-of-a-kind”.  They do not interchange with Chevrolet during those year.

It was not until the new body style in mid 1947 that the two brands shared tail lights. When the larger GMC’s 5-inch light was discontinued on trucks in 1947, Chevrolet introduced it on their 1949 through 1952 station wagons and early GMC buses. It was placed in the center of the gate and was the only factory light on the vehicle.

Even though 1936-1946 taillight was used for so many years, it is becoming very difficult to find. Most GMC pickup restorers use the reproduced Chevrolet rectangular design and only a few GMC perfectionists are aware that there is a difference.

A shop in the US is attempting to remake this bracket; however, if this happens the tail light will be almost as big of a project to find.  It is not being reproduced.

Hint: This tail light also was used on Chevrolet, Buick Oldsmobile Station Wagon tail gates from about 1949 through 1952.  Therefore you will see more lights than GMC brackets at swap meets.  See Photos

1934-1946 GMC Tail Lights 1934-1946 GMC Tail Lights 1950 chevy taillight 1951 old taillight

Early Ignition Wire Protection

Thursday, February 11th, 2010

During the 1930s and 1940s many auto and truck manufacturers protected the long ignition wire between the dash switch to the coil beside the engine. The technology of wire insulation wrap during these early years was a woven cotton covering and it was more susceptible to damage from oil and antifreeze in the engine compartment. For protection, a metal wrap was placed around this electrical wire. See attached photos from a 1941-46 GM truck. A metal cap even covers the connection on the top of the coil for protection. It unsnaps to disconnect the ignition wire from the coil.

The cable system was very good 60 years ago, however, today it can create much frustration. The cotton insulation on the inside wire has deteriorated and pieces can drop away. In time the hidden wire gets exposed and may touch the outer metal cable causing a dead short. The engine stops with no notice. It may occur only on a rough road or during a fast turn. The engine may run well at idle or not run at all. One can imagine how difficult locating the problem can be. Opening the two ends of the cable and replacing the original wire may be good insurance. A modern vinyl covered wire will never be seen inside the original metal wrap.

Early Ignition Wire Protection 1

Early Ignition Wire Protection

1941-1946 Horn Parts

Thursday, February 11th, 2010

One of the more asked questions on the 1941-46 GM trucks is regarding horn contact parts. Most have been damaged over the years and new owners are unsure how they were originally assembled.

Below, is a diagram from a 1940’s GM Master Parts Catalog and gives an excellent view of the parts used in the assembly.

Most items are currently reproduced, including the upper bearing, rubber bumper, cap assembly, steering wheel, mast jacket, 3 finger horn cap retainer, and internal cap spring.

Note: Added are the available part numbers from the Jim Carter Parts Catalog.

Horn Contact Parts

Speed Up 6 Volt Starting

Thursday, February 11th, 2010

One of the more common reasons for slow engine turnover using an original six volt system is under size battery cables. Most of today’s auto parts stores only stock the smaller diameter 12 volt design. Unknowing owners mistakenly replace their original worn cables with shinny new ones that are as much as half the diameter as needed. Two ground cables are also required: One from the battery to the frame and one from a starter mounting bolt to the frame.

Six volt starters require twice the electrical flow to operate properly. Don’t blame your six volt system for slow unacceptable starter motor speed! Many restorers go to the expense of changing their 6 volt system to 12 volts. They feel their original system was inferior and believe what they did was necessary. The problem could have been corrected with just adding the three proper cables.

Remember: Millions of cars and trucks were made with 6 volt electrical systems each year. If they had not operated correctly they would have been built with 12 volt systems!

Battery Cables 6 Volt

Buy Parts for 1934 to 1946 Trucks