Electrical failure that can make an old diesel vehicle stop?

Factual Questions
1

I have an older diesel truck (1998 Mercedes 814) and I want to install a GPS tracker. This device has also a remotely controlled relay that can be used to stop the vehicle’s engine.

Normally you put the relay in series with the fuel pump power wire. When the remote stop function is engaged, the relay cuts the power to the fuel pump and the engine stops running.

The problem is that this is an older truck and has a mechanical fuel pump. Is there any other device on that truck that cutting electrical power would make the engine stop?

2

Well, there must be, or you wouldn’t be able to shut it off with the key…

Are the fuel injectors electrically triggered? They are on my Powerstroke.

3

Most trucks have a fuel shutoff solenoid and fuel solenoid relay. A 98 model may have electronic injectors as well.

4

this. my old '84 F-250 had an electric fuel shut off solenoid in the mechanical pump.

5

I’m not familiar with the injection system on your truck.
Newer diesels have electrically fired injectors and open circuiting the power from the key would shut the system down.
Older diesels had a fuel cut off solenoid on the pump. Cut the power to the solenoid, fuel is no longer supplied to the pump, engine stops.
Really old diesels had a mechanical cut off with a big knob on the dash you pushed in to cut off the fuel on those cars you could take the key out and the engine still ran.
If you can locate a PDF of the wiring I can probably tell you where to install it.

6

I seem to recall that on my old early 80’s 240D, the shutoff was actually vacuum powered. Hopefully by '98 Mercedes figured out that as the word’s largest producer of diesel vehicles, they probably shouldn’t have every single thing run off vacuum.

7

The engine turns off with the exhaust brake, which is a button on the floor, operated with the left heel. It won’t let you turn the key if the engine is running, but I suspect this is merely so the steering wheel won’t lock while the truck is moving.

Rick, I have once replaced the fuel pump myself, there’s nothing electrical on it, just a pulley and the fuel lines.

I have though of instaling some electrically operated valve that when engaged will bleed the air from the air tanks. This will cause the parking brake to lock, but it’s kinda dangerous.

8

Your Volvo was electric. No vacuum on a diesel it had to have a vacuum pump for the power brakes and AC controls.

9

Does the throttle have an electric idle control that holds it open?

10

Automotive electronics professional (specializing in alarms and remote starters) checking in here.

To accomplish what you want, the relay can simply be used to interrupt the main ignition wire coming from the ignition key switch. As beowulff said, this would have pretty much the same effect as just turning the key off.

(You’d probably be interrupting only one ignition circuit with your relay , and the truck might have two or more ignition wires, as well as some accessory wires. Therefore, when the GPS unit energizes your relay, the engine would stop, but things like the radio and climate controls would likely remain operational until the ignition key is turned off completely.)

However, most people would recommend you interrupt the starter wire, not the ignition wire. Why?

If the ignition were to turn off with the vehicle in motion, the driver would lose engine power (of course), but also power steering and in short order, the power brakes too.

An astute, alert driver should theoretically be able to react and carefully coast to the side of the road, but it’s also possible to end up coming to a stop in the middle of a highway, or worse yet, losing control going around a curve.

This could happen not only to the car thief, but also to the legitimate owner in the event one of the wire connections comes loose, someone triggers the GPS unit by mistake, or (very rarely) the relay fails.

Therefore, you might want to consider a starter kill.

—If someone steals the truck, they’ll still be able to have safe control to their destination, until they shut it off. Then, they won’t be able to start again, and at this point the GPS unit can be used to locate the stolen truck.

—If a malfunction occurs while the legitimate truck owner is driving, the truck will still make it to its destination, with no roadside breakdowns. It just won’t start again until it’s fixed, that’s all.

—It’s much safer and easier to deal with a parked vehicle that won’t start, rather than a moving vehicle that shuts down.

Useful links for relay wiring:

http://www.the12volt.com/relays/relays.asp

http://www.the12volt.com/relays/page2.asp

11

No, there’s a manual hand screw at the base of the steering wheel for idle control (BTW, there’s no throttle on a Diesel engine)

12

Any diesel I’ve ever seen has had a throttle plate.

13

If I’m reading this correctly the “ignition key” does nothing but engage the starter and turn on accessories and everything else is mechanical including the fuel pump, injector distributor and engine exhaust brake. You would have to add something inline that cuts off the fuel.

14

AFAIK diesels have no throttle plate, hence no engine vacuum.

15

Don’t have time to do a lot of searching but this is the first one I googled.

16

You haven’t looked at many diesels then.

17

It’s a diesel, so no ignition. The starter kill is probably not a bad idea…

18

That’s not a “throttle” in the sense of “a device that controls engine speed by metering the intake air.” Those devices are there to improve EGR aspiration in certain situations. A “throttle” directly controls the engine power output.

19

Naw, a Benz 240D. A completely different slow overbuilt euro sedan! The old W123’s had vacuum everything, including the windows and locks. Even the kickdown on the automatics was vacuum controlled, oddly enough.

20

Sorry, I should have been more specific.

True, a diesel engine won’t have a typical “ignition system” consisting of coil, distributor, spark plug wires, and spark plugs.

However, just about every car around is going to have electrical wiring behind the key-operated ignition switch.

There will be various wires there: A source of constant power, and then output(s) to supply the engine’s electrical devices, the climate controls, radio, and starter.

If the OP finds the wire that shows power in both the Run AND Start positions, interrupts this with the relay, the vehicle would cease to run when the GPs energizes the relay.

It would be intellectually interesting to find out what circuit(s) in the vehicle are powered by the individual wire(s) coming from the ignition switch, but for the purposes of making the car stop running, we don’t need to concern ourselves with that. The automaker has already designed the vehicle so that the engine will shut off when power is removed from the wire coming from the ignition switch.