Fuel Relay #103 -- Is it really necessary?

[jose456891]

Recently my fuel pump relay went bad. The first instinct was to jam a piece of thin cardboard between the poles on the point of the coil to keep it closed. This is essentially the same as putting a jumper.

After replacing the capacitors 100µF, 22µF I pugged it back in the car. To my surprise it kept cutting the coil off and on erratically.

So I took it back to the lab and started to decode the function of the PCB to figure out what exactly this little bugger does, and is it really necessary.

On my Volvo 855, this relay receives power from the ignition switch position "I" to the PCB board. The contacts it closes receive battery power from the middle "J" relay, which is also powered by ignition swtch position "I".

The PCB seems to be a simple monostable vibrator circuit. Early 93 Volvos have a different design then the 94-97. They both use the same monostable chip, but in a different package. See the following image.

93vs94relay103

NXP Clone

As you can see int he first picture above, both PCB's main chip's number is: 14538B, Manufacturer: Motorola
The second picture is: 74HCT123, Manufacturer: NXP; this is a "clone" on the 94+ PCB.

As per manufacturer's description:

The MC14538B is a dual, retriggerable, resettable monostable
multivibrator. It may be triggered from either edge of an input pulse,
and produces an accurate output pulse over a wide range of widths, the
duration and accuracy of which are determined by the external timing
components, CX and RX. Output Pulse Width T = RX CX (secs)

This chip is designed to trigger from 100µs to 1s depending on the capacitor and resistor values.

VisualDescription

Screen Shot 2015-10-06 at 6.26.32 PM.png (81.77 KiB) Viewed 5951 times

The typical application schematic for this chip (relatively equal to the PCB) is the following:

typicalapplication

Screen Shot 2015-10-06 at 6.27.14 PM.png (71.61 KiB) Viewed 5951 times

The chip and additional circuitry triggers the fuel pump on a timer. The PCB is blind in the sense that it has no inputs. It simply triggers the coil on/off based on a timer. The following is the typical waveform produced by the chip:

Waveform

Screen Shot 2015-10-06 at 6.26.21 PM.png (56.55 KiB) Viewed 5951 times

Perhaps the reason behind putting a monostable vibrator chip behind the fuel pump is to modulate the duty cycle, thus perhaps reducing stress on the fuel regulator, and perhaps extending the life of the fuel pump. Perhaps the PCB monitors the resistance of the fuel pump via its power feed and cycles it to cool it off. (some one please chime in)

My Volvo was assembled on November of 1995, rendering this component 20 years old, its 21st birthday is in a month!. The capacitor replacement did not fix the relay, upon further inspection the coil's resistance fluctuated upon flicking the sides. It fluctuated from 4 Ohms (good) to infinite (bad). It was a broken/melted core wire on the coil.

The only situation where I see this relay providing benefit to the fuel system integrity is when the operator leaves the can on, but the engine off. No need to overwork the fuel pump when the engine is off.

I conclude that replacing this relay long-term, or until next paycheck, with a jumper wire is OK, and will not hurt the fuel pump. While a jumper is installed, I recommend not leaving the car on without the engine running. I noticed that my Volvo was a lot peppier than before. I gained a couple of ponies with a jumper.

Don't worry about the fuel pump running when you remove the keys, the middle "J" relay will cut the power off.


Here is a link to replace the capacitors: viewtopic.php?t=9681

[RussB]

Cool post, but way over my head

[Ozark Lee]

Most of the timing on the circuit is related to pre-start priming. It also cuts off the fuel pump if the engine dies which is a desirable trait after you run into a tree and break a fuel line.

I guess the upshot is that you really want to avoid running into a tree while you have the jumper wire in place but I usually try to avoid that anyway.

There is no fuel pressure sensor so the relay doesn't have any way to act as a pressure regulator.

...Lee

[cn90]

I personally think this is Volvo's over-engineering thingy.

In my BMWs, the fuel relay is a simple relay, the typical Bosch 85-86-30-87 type that has been around for 30 years.

[Ozark Lee]

It likely is over-engineered. My Porsche 914 had the simple On/Off relay but most of them burned to the ground as a result. The mid engine Porsche had an ill conceived drain for the middle engine hatch that drained directly onto the battery. The battery acid washed off and its first stop was over the fuel injection lines and they would, without warning, spring a high pressure fuel leak. The bad part about the leaks were that they were small and they atomized the fuel as it sprayed which made it much more volatile.

...Lee

[abscate]

Motorola didn't design this chip for Volvo, it was designed for multiple applications outside automotive.

You are reading way too much into this, the design of parts like these is driven by supply chain considerations much more than EE.

Over-riding a fuel pump relay long term with either mechanical or electrical means is a poor choice from both a maintenance and safety stand point.

[mecheng]

In addition to what Lee posted which is pretty important. Can you not over heat the fuel pump if you leave your cars in the ignition w/o it running. Say if your listening to music, waiting in the car for someone. Who hasn't done this?

Nonetheless, interesting info.

[rspi]

Most of the timing on the circuit is related to pre-start priming. It also cuts off the fuel pump if the engine dies which is a desirable trait after you run into a tree and break a fuel line.

This.

[jose456891]

Most of the timing on the circuit is related to pre-start priming. It also cuts off the fuel pump if the engine dies which is a desirable trait after you run into a tree and break a fuel line.

This.

Not with a "vibrator" circuit. The chip suggests that it actuates the relay on/off either based on voltage feedback or a timer. The 100µF capacitor and the resistor in the circuit generate the following waveform:

Cx/Rx:

Waveforms

Screen Shot 2015-10-07 at 2.14.07 PM.png (45.19 KiB) Viewed 5893 times

Depending on configuration, the chip will trigger the relay coil either on falling or rising edge of Cx/Rx. There are no "intelligent" inputs from the ECU to the relay. Most likely the ECU powers the board, effectively priming the fuel line upon startup, and turning it off on an accident. The relay has little to do with that.

Still, this relay is a vibrator....

A dumb relay could just as well replace it; while keeping the safety and pre-priming functions intact.

I still don't understand the need for the monostable vibrator circuit. What was Volvo thinking when they made this relay?

[cn90]

Volvo 240 didn't have this fuel relay. Modern BMW does not have this relay.

It is Volvo over-engineer genius that makes your life fun.