If you landed here, you are likely having problems with your pre-heat glow plug timer (PHGPT), or possibly the older mechanical relays in the BJ42. I have as well, and this page will give you some insight into what I have done to correct this failure. This page also embodies other electrical upgrades as once I started tracing the wiring harness, and in general started to dissect the electrical system, other thoughts and upgrades seemed to merit my time. 

Regarding the Pre-Heat Glow Plug Timer or Toyota SuperGlow System

There  are several options to correcting this including a simple push button approach, but I opted to re-engineer the pre-heat timer and the Toyota Superglow glow plug heating system using a Basic Stamp II.  I also wanted a backup so have also designed a push button into the circuit in case anything something goes wrong with the BSII or any of the related components.

I went with the BasicStamp as I wanted to learn more about electronics and micro controllers as they provide incredible flexibility to control and monitor nearly anything. I also wanted to move to a solid state relay for years of reliability, as while the OEM mechanical relays have performed without fault, they are getting older, so while I was doing surgery, it made sense to update the entire system. Control of a solid state relay was also more straight forward for the BasicStamp and reduced control circuit complexities and reduced the amount of wire in the engine compartment.

The Basic Stamp II also allowed me to integrate with various circuits in the truck to both monitor and control what is going on with the glow plugs, engine water temperature, and glow plug resistance; allowing me to replace the original PHGPT. And while getting relatively deep into the electrical system and wiring harness, I also decided to:

  1. Replaced the mechanical glow plug relays with a large solid state, 225 amp relay.

  2. Fabricated and installed various digital displays to provide dynamic readout and health status of the glow plugs, their operation, engine coolant temperature, and in the future, many other functions, including a dual battery voltage monitor.

  3. Replace the 12v neutral headlight relay for better lighting and eliminating the battery imbalance issue that would be created if a single headlight were to burn out. I also replaced the headlights with H4s, so I can carry an extra bulb, and simple replace it on the road when it blows. I also installed 10 awg wiring to the headlights along with a set of relays that are activated with the original headlight wires, virtually eliminating any wire induced voltage drop and corresponding light output loss.

  4. Installed a Solar switch that converts 24v to 12v, so I can now run laptops, inverters, the BasicStamp, radio, etc, that all require a 12v supply. There are other ways to achieve the same, but this seemed like the best set of trade-offs.

  5. Installed two auxiliary fuse boxes: one to provide an isolated, direct, and short connection to the headlights, another to serve the other 12v circuits that I have now installed in the truck.

  6. Augmented interior lighting with LED lighting.

The BSII and associated circuitry replaced the functionality of the original Toyota PHGPT, monitoring the engine temperature and glow plug resistance. Based on these readings, I have coded the BSII to fire the solid state relay according to the heat timing curve in the Toyota factory manual and factory glow plug specifications. I also performed additional empirical bench testing to derive the relationship of heat to resistance on both the glow plugs and water temperature sensor.

The original Toyota design called for two relays, one for a fast heat at 24v, and a second that runs through a large resistor to drop the voltage to roughly 14v as a maintenance heating cycle. Since this is not possible with one relay, I have coded the BSII to run what I am calling a macro pulse width modulation at roughly 1.6:1 second cadence to achieve the maintenance voltage/heating after the initial 24v period brings the plug to approximately 900 degrees F. This also reduced to amount of wiring, should that be of interest. Frank at PerfectSwitch was also kind enough to alter this relay to better suit the BSII, changing the control circuit to an isolated ground, utilizing the extra pins on the existing port to drive a remote LED for the dash, and tweaked the on and off voltages.

I have written design documentation, notes, schematic diagrams, parts lists, etc while designing and building this, and further have drawn CAD/CAM DXF files used to cut the stainless steel gauges assemblies on my CNC table.  I will likely post some of this additional information here in the future, but let me know if you have interest in this before that happens.

And finally, this project is not yet 100% complete: it has been installed as a prototype in the truck and works well. I now need to build the final circuit board that will reside back in the original Toyota Preheat timer circuit board box. It may take a bit of time, as I am now restoring a friend’s Mustang. For some of the project photos available to date, see below: