LJ78 Glow plug wilson switch guide

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Aug 30, 2010
LJ78 Glow plug override switch guide

This is how I improved upon the stock Glow Plug System in my 70 Series Landcruiser.
I was finding the stock glow plug system wasn't warming the glow plugs long enough during cold weather.
I wanted to put all of the information into one thread since the information is quite scattered across posts.
I selected this method of modification so I could leave the Superglow plug system intact and working for future truck owners. This method is the safest and least intrusive.

First I'll quote Doug from the Diesel tech forum for an informative summary of diesel systems.

Found this on a generator forum and thought it was an informative read.

Also check this report and test of diesel fuel additives, it's a few old, but should still apply. http://www.thedieselgarage.com/forums/attachment.php?attachmentid=12976&d=1195365354


All about Diesel Fuel. The cold, smoke, Injectors, Sulfur....

Diesel Fuel

What is Diesel Fuel?

Diesel fuel is principally a blend of petroleum-derived compounds called middle distillates (heavier than gasoline but lighter than lube oil) and may or may not contain additional additives. Other middle distillates include kerosene and No. 2 Heating Oil. Diesel fuel is designed to operate in a diesel engine where it is injected into the compressed, high-temperature air in the combustion chamber and ignites spontaneously. This differs from gasoline, which is ignited in a gasoline engine by the spark plugs.
Why are diesel engines so popular?

Diesels engines deliver 20% to 40% better fuel economy than gasoline engines. Diesel fuel does not require as much refining. The engines are simple and easy to maintain. There is no spark plug, ignition cables, carburetors and complicated fuel control devices. Diesel fuel has less harmful emissions from diesel fuel than gasoline. Now with new ultra low sulphur diesel fuel the benefits to the environment are huge.
Winter and Summer Diesel

In order to keep diesel fuel low temperature flow characteristics, fuel suppliers adjust the fuel properties through the year according to historical temperature data and where the fuel is sold. Generator owners may have fuel sitting in their tanks or in storage for long periods of time. In cold weather it would be advisable to add a fuel anti-gel additive. Anti-Gel treatment may be conveniently poured into the fuel tank before filling. It is inexpensive and worth spending the time to do it. More on this later.
Diesel Smoke

When a cold diesel engine is started, the walls of the combustion chamber are still cold. The low temperature in the combustion chamber results in incomplete combustion, and the white smoke.
Even after the engine has started, the temperatures in the combustion chamber may still be too low to induce complete combustion of the injected fuel. The resulting unburned and partially burned fuel is exhausted as a mist of small droplets that is seen as white smoke (cold smoke). This situation normally lasts for less than a minute, but the exhaust is irritating to the eyes. A fuel with a higher cetane number will shorten the time during which unburned fuel is emitted to the atmosphere.

Cetane is to diesel engines as octane is to gasoline engines. The Cetane Number is a measure of how readily the fuel starts to burn (auto-ignites) under diesel engine conditions. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.
What makes A Diesel Engine Noisy?
The noise produced by a diesel engine is a combination of combustion noise and mechanical noise. Fuel properties can affect only combustion noise. Some combustion noise is reduced in generators by using direct injection. A good muffler system can reduce a great deal of noise. Increasing the cetane number of the fuel can decrease the amount of knock also.
Low-Sulfur Diesel Fuel

Ultra-low sulfur diesel (ULSD) began replacing conventional diesel fuel in 2006. The new fuel will contain 97% less sulfur than conventional diesel—sulfur will be reduced from 500 parts per million (ppm) to 15 ppm.
There are many ways of removing the sulfur content. The cheapest of these involves hydrotreating, a process that removes sulphur by treating it with hydrogen. Unfortunately, hydrogen is highly reactive and also reduces the lubricity, or lubrication properties, of the end-product diesel.
Since the removal of sulfur from diesel fuel, diesel shops have been flooded with work caused by lack of lubrication in the new fuels. Cars, Trucks and Generators can all see the effects of this. Fuel Injector and Fuel pump failure can be common since they are not lubricated like they where before. Some fuel companies have their own additives in the fuel to bring back the lubricaton properties however it may not be enough.
Leading diesel manufactures now recommend using a fuel additive such as Stantadyne with every fill up to restore the lost properties.
Additives are cheap and easy to use. Check your gas station or automotive parts dealers. They are as common as windshield washer fluid where diesel products are sold.
In your Diesel Generator the only lubrication your fuel valve, fuel pump and fuel injector receives is from the fuel itself. Since failure of these components are not covered by any warranty it is important to take care of them. Taking care of them is easy if you use a fuel additive that restores or improves lubrication and antioxidant properties to your fuel.

  • Stanadyne White Paper on Diesel Fuel
  • How Stuff Works The difference between gasoline, kerosene, diesel fuel, etc.
  • Vegetable Oil is Used as Fuel
  • FAQ Exxon Diesel Fuel
Fuel Injectors

Most diesel engine problems are related to the fuel system. The fuel injectors and fuel pumps must be lubricated and kept deposit free. Since Sulfur has been removed from most fuels now, a fuel system additive that helps clean and lubricate the fuel system is required. You should use the additive with every fill up. Failure to use it will result in unusual wear and tear that will not be covered under warranty.
How the fuel is delivered is the most common reason for engine failure. Often looking at the color of the smoke will tell you a lot about what is going on.
Black Smoke

Excessive diesel smoke is due to incomplete combustion, a rich air-fuel mixture. This may be the result of problems with the fuel injector pump or its timing. It may be a clue that the air cleaner is choked. Worn or damaged fuel injectors and adulterated diesel fuel can also cause this.
Sometimes when a fuel injector fails it can leak fuel and drip. The excess fuel does not burn off and is pushed out into the exhaust where it can collect, burn and overheat the system. Often deposits left there will also glow cherry red and burn off. Black smoke is an indication that fuel is not burning properly.
White Smoke

White smoke occurs mainly during cold starts, when the fuel tends to condense into liquid and does not burn due to cold engine parts. The most common reason for white smoke are in-operative glow plugs, low engine compression, a bad injector spray pattern, late injection timing or injection pump problems.
Blue Smoke

Excessive blue smoke indicates problems from low engine compression and/or worn piston rings, scored cylinder walls or leaking valve stem seals The blue smoke is caused by crankcase oil entering the combustion chamber and being emitted after partial combustion through the exhaust
Diesel Fuel in cold weather.

Below 15 degrees F, wax crystals begin to form in diesel fuel. These will clog the fuel filter and stop the engine as the temperature drops toward 0 F. Any good “winter fuel conditioner” for diesel fuel will keep the fuel moving to at least -20 degrees. Follow the instructions on the bottle!
Here is an example of oil waxing in cold weather. On the right is oil treated with an anti gelling additive.
Deposits and Corrosion

Residual carbon deposits from combustion can build up in nozzles and around the orifices and can obstruct the atomization of the fuel into the air preventing complete combustion. Deposit modifiers and detergents soften the hard deposits allowing for their removal. The detergent will clean the soft deposits and prevent additional deposit build-up. Also, they incorporate an anti-oxidant, which helps fuel maintain its potency and a corrosion inhibitor to help protect fuel injection parts from rust and​

First I dismantled a Battery Voltage Readout LCD I bought from Princess Auto (Schumacher Brand) and it replaced my now extinct adjustable suspension button:


Now I can see in real time, all the time what my battery is doing, and I can tell what the glow plugs are doing based on the voltage readout.
Some gents have recommended a light that is in parallel with the glow plugs.
This is also a good way of knowing if your glow plugs are working properly but a voltage readout offers more information on your entire system.

After that I ran a #16 AWG sized wire from No.1 Glow plug relay to one side of my momentary push button ( I actually pushed 5 spare wires through the firewall for future mods)
I use wire loom for all my cabling, it is cheap and offers extra protection against abrasion from vibration:

Cut into the glow plug light and momentary pushbutton:







By wiring it this way, the current Toyota "Superglow" system is intact and it is safer to operate the momentary push button because all the glow plug current doesn't have to go through the switch.
Also, using the existing No.1 relay, the switch can only be operated with the key in the "On" or "Start" position.
You don't want children or curious friends able to energize the glow plugs without the key in the ignition. Kids love to push buttons!

If you will be using this method to override your glow plugs, it's important that the rating of your glow plugs is above 10VDC operation.
You can prematurely burn out your glow plugs if excessively glowing the OEM 6VDC rated glow plugs.

Test for correct operation:


When you first turn your key to "ON", the Toyota Glow timer will energize No.1 Glow Relay which sends 12VDC across the glow plugs.
This provides maximum Wattage (Heat). This will typically be on for about 3 to 4 seconds before the timer disable No.1 Glow Relay.

After No.1 Glow Relay de-energizes, No.2 Glow Relay will energize. No.2 Glow Relay will provide 6VDC across the glow plugs for a much longer amount of time.
6VDC across the glow plugs will generate 1/4 of the Watts (ohms law).
Eventually the timer de-energizes No.2 glow relay and the engine will be running nice and warm.


During initial start, use the momentary pushbutton you installed to place 12VDC across the (11VDC rated) glow plugs. ( longer than the Toyota Timer).
Use the momentary switch to superglow for around 10 to 12 seconds initially. Your truck should start every time.
It is recommended to superglow for a duration of 10 seconds initial on, then cycle 5 seconds on-off until warm. Pushbutton for 5 seconds, then off, again for another 5 seconds and attempt starting engine.
Be conservative with your pushbutton. 15 seconds should be the maximum time under the coldest conditions that you should be pushing it in.
You may drain your battery or prematurely wear out your glow plugs if you push the button longer than 15 seconds.

Here is a diagram I put together based on my own troubleshooting experience:


Sequence of operation review:

  1. Water Temperature Sensor (thermistor) tells Pre-Heating Timer how long to run No.1 and No.2 glow plugs.
  2. Key to "ON" : No.1 Relay sends 12VDC to glow plugs giving it initial "Superglow" for a short period. (Around 3-5 seconds)
  3. Pre-Heating Timer shuts off No.1 Relay and energizes No.2 Relay.
  4. No.2 relay runs glow plugs at 6VDC to aid in the start up and continue warming up the engine. (Runs for a while depending on Water Temp. Sensor).
  5. After everything is satisfied the Timer shuts off all Voltage to the glow plugs.
  6. When you push the momentary switch, you are basically running the "Superglow" No.1 Relay longer than what the Pre-Heating Timer determined.

Looking at the graphs I would say that over the course of 20+ years, 6VDC plugs accidently get replaced with 11VDC plugs, the Resistor, Current sensor, Pre-Heating Timer, or Water Temp. Sensor lose their accuracy in timing correctly.
The graphs show what the timer aims for in timing depending on temperatures. It seems Super glowing (12VDC) a few more seconds than the OEM timer improves start up of the vehicle.

So in theory 12VDC on a 11VDC rated glow plug will produce approx. 500 Watts.
When the timer drops to 6VDC (after only 3 seconds) only 1/4 the Wattage will be produced (ohms law).
So say that two glow plugs are dead, you will get only 250 Watts for the first three seconds (Superglow 12VDC), and then a mere 62.5 Watts for the remainder (Afterglow 6VDC).

So I guess a combination of over rated plugs, shortened glowing period and 1 or 2 dead plugs can lead to a pretty cold engine start!

Last edited:
nice write up.
me, i run 12V glowplugs and the momentary switch
yours is an excellent recommendation. very profesional, very clean install.
congrats and thanks.
very good explanation on the glow plug system. what i have done different to tie in the superglow system, i disconnected the timer and connected some jumpers to the plug to manually control the #1 relay & the #2relay with a 2-way momentary switch. the superglow system is still intact except for the old timer. my voltage gauge also shows what's happening at the glowing cycles. thumbs up on the write-up!
Hey Holks I am a novice and confused. I have the 12h-t with the two heat screens (not glow plugs). How can I accomplish the same thing in my system. My preheat timer is not functioning. How do I wire in a manual pre-glow using the existing preheat screens? Thanks Randy randynshoremn@me.com
Hi mark great write up you have included all my proplem is there any way you can help me or send those pictures of the wilson switch you installed iam unable to see the pictures tx
good explanation, 500w sounds wonderfully generous! I would never have gone to 15 secs, but good to know that is a limit to look at. Thank you.

I installed a wilson switch from directions on a few of the links on mud for my hj75's 2h. Lost marbles comes to mind. Wilson switch was one of the first things I did after change all fluids. Much prefer it, pulled out that little pre timer computer. Count longer 10 during winter and shorter 6-8 during summer. Trial and start or no start, you get a feel. As I use to do on my old bj40. Feels more interactive and a car thief would have to know a little about older diesels to start it too.

Just on a side note, I have been playing around with an el cheapo pop tester for injectors. I leave a spare injector installed so the pop tester tubes keep wet with diesel and pressurized. When the temp warms with the day I watch the pressure rise on the dial of the tester. At least 2x maybe 3x (39celsius in the shade the other day) the cold pressure with fuel expansion. Makes me think that the warmth also helps the diesels start with more diesel pressure in the tubes.
Why one should always buy fuel at the coolest times of day, you get more fuel.
My dad use to warm up his 1956 massey tractor with a blow heater below the block and a blanky swaddled on the top and sides before starting during winter. God he loved it.

I can't see the pics either.

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