A service engineer from one of our sister concerns finds himself in a funny situation one evening. He was returning from a customer’s location deep inside the desert. He started at 5 pm and was hoping to reach the highway in four hours.

It was a new light truck, a hit among the deserts, with genes of regular winners at the Dakar rally on those days. 4WD, diesel engine with intercooled turbo charger,…

Nothing wrong in his plan to maintain a speed of 120 km.

He  had checked the lights, brakes, horns…and everything was fine when he started. An hour later he switched on the headlamps only to find them inoperative.

He applied the brakes and was trying to park the vehicle by the roadside when the lights came back to life. But the relief was short lived. The lights went off again.

It did not took much time for him to realize that the headlamps have become speed sensitive. The lights went off exactly at 60km/hr speed and came back again once the speed dropped below 60 km/hr.

This is when he called me. I had no idea what was happening. I could not think of a connection between the headlamps and the vehicle speed. The headlamp circuit was a simple one, a resistive multiplex type multifunction switch and headlamp relays controlled by the body control module [BCM] . I refered the manuals, but could not find any links between vehicle speed and headlamp operation. I decided that BCM had gone mad. It was in the CAN communication network and vehicle speed reading was available to it!

I advised him to maintain his speed below 60 km/hr. His immediate response to my advice can’t be included here. But it was a classic one! It is years now, still I remember it in full…

He reached home safely 12 hours later.

We replaced the BCM without checking anything and soon realized that there are things more complex than we realized in that vehicle.

This vehicle was to frequent the PDO [Petroleum Development Oman] areas and had undergone some modifications to meet the PDO standards. Roll cages, 80/120 speed limiter,…

There was another requirement …

The headlamps have to go OFF when the ignition is switched OFF, though the headlamp switch remained ON.

In some vehicles the headlamps will go OFF when ignition is switched OFF. But in this model, original circuitry is such that the headlamps will not go OFF when the ignition is turned OFF if the headlamp switch is in ON position.

 Need to modify headlamp circuit to meet this requirement…

We connected the scan tool.

There was no change in the input switch status…

And there was no change in the BCM output also…but the lamps were going OFF once the speed reached 60 km/hr.

 BCM was controlling the headlamp relays correctly, but the relays were going OFF when the speed reached 60 km/hr.

There are different agencies who do the PDO modifications and each has its own methods in modifying the headlamp circuit. It was one of those agencies who did the modification in this vehicle. The modification was working fine. Headlamps were going OFF when ignition switch was turned OFF with the headlamp switch turned ON.

We checked their modification. It was a simple circuit. They inserted a normally open relay in the headlamp relay control circuit and controlled that relay with a ignition switch output voltage.

When the ignition switch is turned ON, the relay they fixed will turn ON and close the head lamp relay control circuit. Now BCM can control the headlamps based on the input from the headlamp switch.

When the ignition is switched OFF, the relay they fixed will turn OFF and open the headlamp control circuit. Now BCM cannot turn ON the headlamp relay and the headlamps remain OFF though the headlamp switch is ON.

Everything fine upto this point!

Who expects the ignition voltage to disappear when the speed reached 60 km/hr?

That was what happening. The voltage to the control side of the relay they fixed turns to zero when the speed reached 60 km/hr.

A little about expert electricians at shops that do such modifications…

They attend a lot of vehicles, of different makes and models, and do only specific jobs like this. They are very experienced in what they do and have reached a stage where workshop manuls do not mean much. When they need a point to tap ignition voltage, they just take a test lamp, go under the dashboard, try different locations at random with ignition switch turned ON and OFF and decide.

The same happened in this also. We found 12V at the point where he tapped, but the voltage was going to nearly 0 V when the speed reached 60 km/hr.

We found that the point they tapped was not directly connected to the ignition switch though the voltage became 12 V when the ignition switch was turned ON.

We changed the connection to another ignition switch output line and the headlamp started functioning properly.

I could not find why the voltage was disappearing in that line when the vehicle speed reached 60 km/hr. The speed sensor circuit had no connection to that point.

To start from a system wiring diagram and reach the pins in the vehicle is easy. But the opposite, starting from a pin and reaching the system wiring digram, was not easy with the wiring digram manual of this vehicle.

That Friday, I took the help of my assisstant, a diagnostic technician, and we spent some time on a similar vehicle.

That day I came to know that my knowledge about the operation of turbo intercoolers were not complete.

There was an intercooler fan that was controlled by a control module located under the dashboard. And… the intercooler fan stopped working when the vehicle speed reached 60 km/hr. The control module was cutting off the voltage in the fan control line.

Hope you now understand what went wrong with the modification … from where the electrician tapped 12 V…

Why the intercooler fan should stop when vehicle speed reaches 60 km/hr?

I searched the web… but the information I got belongs a level higher than that of an automobile engineer whose knowledge in fluid mechanics and thermodynamics is meagre…


Not related to this post…

I don’t believe in Charles Darwin’s theory of evolution. But each week something  happens that ridicules my (dis)belief. Last week was no exception!