Replacing factory Fuel Pump Relay with Bosch Relays

Anybody have a reason why this would be a bad idea? The LH2.4 fuel pump relay has two relay coils. As we all know, they're unreliable, and replacements are fair quality at best.

It's a pretty simple circuit that could easily be replicated with two Bosch 4-pin relays and an adapter harness...unless I'm missing something.

Not missing anything that I can tell.

I had the same idea a month ago, when one of the coils in my relay went bad. Scribbled up a drawing and everything, but a brand-new relay "at cost" through the shop sounded nicer.

Other than the two "standard" relays (as long as they can take the current), you'd only need to add an appropriate diode to your new wiring. Steal it from the old relay? It's hiding under one of the coils.

Could it be so simple? Diodes are cheap. I wonder if we can find the value of that diode.

I think the ECU converts the coil signal to a ground so that ground can then be used to trigger the relay. Is that correct?
Dave B

It's not that simple. The relay has a 'pulse sensor' built in to convert the signal from the ICU/coil (-) to a constant on signal.

hiperfauto said:

The relay has a 'pulse sensor' built in to convert the signal from the ICU/coil (-) to a constant on signal.

Click to expand...

1981 VW Rabbit, with CIS, had this....I don't think this is relevant to LH 2.4

Yes, engine RPM had to be detected before fuel pump would energize.

dbarton said:

diode.

Click to expand...

Diode is forward-biased, so when relay is activated by pin 21 to fire up injectors/IAC/AMM, LH is aware of this voltage via feedback at LH pin 9.

Just an estimation via 1993-240 diagram...need to evaluate electrically.

1. Ignition switch feeds pin 35 on EZK-116; it triggers LH to turn on
2. Then LH 2.4 grounds out pin 21
3. With pin 21 grounded, then injectors/etc and LH pin 9 have power
4 Injectors/etc power is fed back into relay's pin 6, which trips fuel pump relay when LH pin 20 grounds.

Two Grounds
LH Pin 20 powers fuel pump
LH Pin 21 powers injectors, IAC, and AMM


LH Pin 9 may be a feedback loop to make pin 20 work or tell LH there is power to injectors
If there is no power at injectors, there is no reason to fire up fuel pump. This is a safety.

LH Pin 20, most likely, grounds out when
a. engine shows RPM,
and
b. key switch is turned on, then some 5 seconds of grounding
===============

MasterBlaster said:

only need to add an appropriate diode

Click to expand...

I would have to review a diagram for a Bosch Relay, but diode is there for "Volvo" relay's circuit.

Diode would not be relevant...not needed...important point is not to bypass fuel pump safety, where fuel pump does not fire up unless pin 20 "says so."

It would be nice to know how much current is flowing thru relay's pins 4 in one test, and then pin 2....and compared to what a Bosch relay needs

hiperfauto said:

It's not that simple. The relay has a 'pulse sensor' built in to convert the signal from the ICU/coil (-) to a constant on signal.

Click to expand...

If i read the text correct it is saying the pulse sensor is triggering the relay from the ignition signal, to ensure the fuel pump is running whenever the ignition is, and shut it down when not needed. The relay array could just as easily be triggered by the ignition switch, or aftermarket ecu, or any place I am not thinking of that produces voltage when the engine is running/cranking.

84B23F said:

1981 VW Rabbit, with CIS, had this....I don't think this is relevant to LH 2.4

Yes, engine RPM had to be detected before fuel pump would energize.

Click to expand...

You're correct. LH2.4 uses a different fuel pump relay. Disregard my earlier post.

This design makes great sense from a safety perspective. A friend of mine's daughter is wheel chair bound and had the lower half of her body 2nd and 3rd degree burned because while "distracted driving" she flipped her car which poured gas all over her. The mother said her medical bills have passed $3M now, and she is still wheel chair bound almost a year after the crash. Her vehicle released gas into the cabin where she was trapped.

Safety when dealing with gasoline, is absolutely essential.

NotSoFresh said:

If i read the text correct

Click to expand...

Negative...engine RPM was needed....I bought a 1981 VW Rabbit with CIS that had a problem; it would run fine, then shut off, so PO sold it cheap since mechanics couldn't fix it.

I hot wired fuel pump, and engine never died, so I went to salvage yard, and picked up another relay; problem solved.

In LH 2.4, we need to know how much current is passing thru LH's two grounds, and then compare to what it takes to fire up a Bosch Relay. Otherwise, LH's ground circuit might quit if too much current is required to trip a Bosch Relay.

When engine dies, that fuel pump relay must shut down...if a fuel line was busted (accident), then fuel pressure would drop at injectors, and kill engine. It's about a safety

GM - 1971 Vega used oil pressure sender to run fuel pump
GM - 1989 2.8 used oil pressure sender to run fuel pump

1971 used idiot light on dash, and 1989 had oil pressure gauge on dash, so both were wired differently.

On B230F block, if more than one oil port exist for installing an oil pressure sender, then fuel pump could be energized this way.

I'll try to talk to an experienced electronic tech about that diode. If that's all in Volvo's relay, then it could be pin 20 oscillates between positive and negative, and Bosch did not want the positive feeding those other circuits (injectors/AMM/IAC)...only logical reason I can think of.
.
Diagram I used
.

Note - When oil pressure drops very low, and when oil pressure sender controls fuel pump, then engine shuts down, due to decreased oil pressure. There would be a way to energize pump(s) at starting via using a diode on power from starter's solenoid wire.

Instead of the Kustom relay mods, I'd really recommend getting a 2nd junkyard relay and swap it in now. If it works, leave it in place and put the original in the trunk as a spare. In the future, if it fails, you have a known good spare ready to go. You could even get 2 spares if you're worried.

If you kustomize it, no one other than you will be able to diagnose it. I can just imagine the posts here in a few years with "WTF did the PO do to the electrical?", complete with ugly pictures. Followed by an expensive repair shop bill for replacing all the kustom wiring with original wiring and an original relay.

That being said, and this being turbobricks, you can use any black colored diode you want. If you want to get picky, a 1N4004 should be fine (or anything in the 1N4001 to 1N4007 range). Relay coils take ~100 to 200mA, so a 1A diode is plenty.

My best guess for that diode is that it prevents back-powering the injectors/iac/maf through the ECU. It may also reduce power drain when the key is off. I'd guess that the ECU has split power inside -- a few critical circuits are powered off the battery at all times (pin 4), with main power coming in from the relay (pin 9).

FWIW - Bosch was using a double relay on VW Buses, 75-79. and two diodes were used. Not related at all to LH 2.4, but diodes were used to prevent reverse current flow.

Fuel pump's power when engine was running was controlled by Air Flow Meter (AFM): if no air flow was detected, then pump's power would be cut off. "This mechanism also serves as a failsafe system: if the engine dies for whatever reason, the AFM flap will open the switch and cut power to the fuel pump."

I've been lead to believe that the reliability is significantly improved if you dramatically drop the current the OEM set up has to deal with. Seems a lot easier to simply add another relay that is triggered by the output of the OEM unit. Then the OEM unit only has to deal with the negligible current needed to close the coil in the new relay and you preserve all the safety aspects of the OEM set up. Thankfully, with the new motor, I'm completely out of the OEM relay business.

^^^This is what you do.

For reference the early kjet system 74-77 used an airflow sensor switch to cut off the fuel pump if there was no airflow detected. It also used two relays for this function. Then the failsafe was changed in 78 to the rpm sensing relay.

bobxyz said:

Instead of the Kustom relay mods, I'd really recommend getting a 2nd junkyard relay and swap it in now. ...

Click to expand...

This is my thought as well, despite this being Turbobricks. If it is your own car and no one else ever drives it, and you'll never sell it, you still might have repercussions when the mods to this important part of fire safety in a crash are questioned by some adversarial attorney.

Also, I agree with the OP's premise; that these circuit board relays were inherently less reliable originally, then say a Bosch cube relay, but I've seen the high current side of the internal wiring improved by the manufacturers, using solid metal between the contacts and the connector pins instead of passing it through wave-soldered joints.

bobxyz said:

My best guess for that diode is that it prevents back-powering the injectors/iac/maf through the ECU. It may also reduce power drain when the key is off. I'd guess that the ECU has split power inside -- a few critical circuits are powered off the battery at all times (pin 4), with main power coming in from the relay (pin 9).

Click to expand...

The power is indeed split, and the circuitry to pick the system relay of the duo is minimal and only there for the need to burn off the AMM. The diode is in the path from the transistor on the hybrid that selects the pump relay, and despite making my living mostly reverse engineering, I can't explain its presence with any certainty.

All I do know for certain is that transistor on the hybrid used in the old production (colored label) 5xx ECUs fails at that point, and not based on time on the ECU or mileage on the car. I suspect it is the deterioration of the grounds at the fuel rail deciding when to open that transistor, but Bosch quietly replaced those hybrids with a redesign in that area.

And as popular as the idea is here and elsewhere, I think the mods replacing the k-jet fuel relay, or the LH2.4 FI relay with cube relays work because (1) in doing so the modder is replacing the wiring and crimps and (2) starting a new life cycle with all new parts which will never be tested or criticized 30 years hence.

Just resolder yours, get a newer one with the improvements, or if you're worried about the coil winding breaking, be careful not to nick it when you have it apart. Keep another in the glove box, because "we all know" any spare you carry will never need using.

dl242gt said:

For reference the early kjet system 74-77 used an airflow sensor switch to cut off the fuel pump if there was no airflow detected.

Click to expand...

Apparently, this was an unsafe method if vehicle rolled over in the wrong position so AFM's flap remained opened, and fuel pump would kept on running, I assume.

RPM and oil pressure were two means used by Bosch and GM, respectively. GM's oil pressure sender was less complicated setup than Bosch's relay, but GM may have had a patent back then, so Bosch had to do it a different way. 1971 Vega was first vehicle I recall with an intank fuel pump

I'm not going to do this modification, but here's how GM's 1971 Vega did it with an intank fuel pump-shown in photo below. Discussed in this forum's thread. Having that "primer" button was not GM's idea.

Another poster there posted another diagram, which is incorrect way of doing it. His diagram reverses how relay is tripped on terminals 85/86. Using ignition voltage to trip relay is better since if its wire shorts out to ground, a fuse will blow (as shown below). Using a ground to trip relay is not good since if its wire from oil pressure sender to relay got grounded, it would trip relay.

As far back as the Chevy Vega GM used an oil pressure switch to ground the in-tank fuel pump relay. Could it be so simple as the relay needing an oil pressure switch? Just realized the the post above mine pointed this out. The mentioned link shows that relay terminals 85 and 86 can be reversed if needed.