Diesel Engines: How Do You Know If Your Glow Plug Relay Is Bad?


– Preheating system: definition

– Warm-up relay: what is it?

– Warm-up relay: in case of failure

– Warm-up relays: purchase and prices

Diesel engines use a fuel with low detonation power, which requires a preheating system to allow the fuel to ignite. The preheating relay is one element of this system. This post will explain how it works.

Pre-heating system: definition

Unlike petrol, diesel is impossible to ignite instantly. It has to reach a temperature of 90° C before it starts to burn. In order to produce the spark required to start a diesel engine, the temperature inside the combustion chambers must reach 900° C.

Diesel engines therefore have glow plugs that heat the combustion chambers, an electrical relay and an electronic control unit, also known as an E.C.U.

Glow relay: What is it?

Installed in the engine compartment, the glow relay is a module that controls and manages the glow caused by the glow plugs. When it is energized, its secondary contacts close, allowing the battery current to flow through the glow plugs. This mechanism heats the air in the combustion chambers.

It also controls the operation of the glow indicator light installed in the dashboard, as well as the duration of glowing (and post-glowing on equipped vehicles) according to the engine temperature, determined by the electronic control unit.

Glow relay: in case of failure

Here are some of the most common cases where the preheat relay may be involved:

– If the Warm-up light on the dashboard does not come on, the relay may be the cause of the failure: check the circuit fuse.

– In the event of a glow relay failure, the dashboard glow light may also be flashing or constantly on.

– In case the power does not reach the glow plugs or afterglow plugs, check the power supply to the glow relay with a multi meter.

Good to know: When changing a glow relay, as well as any other electrical equipment on your vehicle, remember to disconnect the battery.

Warm-Up Relay: Purchase and Pricing

The prices of the preheating relays vary according to:

– the diversity of car models and the engines they are equipped with;

– the car market segment (popular, utility or high-end);

– its origin: manufacturer’s spare parts or adaptable parts.

Given the huge offer on the market, it is difficult to establish a realistic price range.

You will find your vehicle’s serial number in a section of its registration document. This will help you find out the serial number of your car or utility vehicle from the dealership of the make of your car or utility vehicle, in the spare parts stores, or on the Internet, on specialized sales sites. You will be able to compare the different offers. You can also buy a second-hand heater relay on the Internet or in second-hand car parts stores in your region. The best advice however would be to take your car to a professional motor vehicle electrician where they would be able to determine the exact cause of the problem with the relays. At times, it can get extremely difficult into determining the issues which could be coming from a faulty BSI unit instead.

Our next publication will be on How to Connect an Automotive Relay? Stay posted, and remember to share and comment on this post with your friends. Should you also wish to read on something of particular interest to cars and motors, please do not hesitate to write your request below. We’ll get back to you asap!

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