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Discussion Starter #1
I have searched for a diagnosis to test the drive(fuel pump) relay on our spiders with L jet injection and have not found one. The tachy signal is difficult/impossible to simulate when testing the relay. When I "hot wire" the fuel pumps the engine starts and runs great, if I disconnect the hot wire jumper while the engine is running the engine dies. The relay does "click" and there is continuity between the input terminals, but I sense something else internal to the relay is faulty.

thanks all
 

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Keeping in mind that I've had MG's since I got my driver's license and my understanding of things electrical is based on the Smoke Theory (when the smoke escapes from the wires things stop working), the best advice I can offer is replacement with a known-good drive relay.

If you can figure out how to provide a tachymetric signal, it might be possible to test the drive relay. Peruse this thread: Fuel Pump Relay Test and especially reply # 31.
 

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Discussion Starter #3
I was hoping somebody had a test that could be completed with a VOM? Perhaps a schematic of the drive relay exists?

Amazon has a Bosch relay for $178.00, I am resisting.
 

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But Mad North-Northwest
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Basic question, but have you verified you're getting +12V to terminal 30 of that relay's socket? It's a straight run from the battery so you should have voltage there unless there's a wiring problem or the 8A fuse is blown.
 

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My electronic test equipment includes a pulse generator and I would be happy to help with testing these relays. My ownership of a Bosch Spider was very brief and I have an incomplete understanding of the relay so I would have to learn a little about them.
 

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But Mad North-Northwest
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I don't think terminal 31b actually needs a pulsed tach signal. It's just a relay...I can't imagine there's anything in there that would require a square wave. 31b is "switched negative" per Bosch terminal designations, which I take to be just grounded during operation. I could be wrong, though.

From looking at the wiring diagram and Bosch terminal designations:

30 is the +12 input (direct from battery via 8A)
87 is the +12 output (to the fuel pumps)
15 is switched +12 (from switched side of fusebox)
31 is ground (in this case via the L-Jet ECU)
31b is switched ground (via the coil / Ignition ECU)
50 is +12 when the starter is running

So I *think* the way to test the relay out of the car is ground 31 and 31b, apply +12V to 15 and 50, and then see if you have continuity from 30 to 87. You should then be able to disconnect 12V from terminal 50 and you should still have continuity from 30 to 87.

I COULD BE WRONG HERE, HOWEVER, so I'd double-check this yourself. I don't *think* this could fry the relay, but I'd hate to potentially blow up a $180 relay so proceed with caution.
 

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Discussion Starter #7
Basic question, but have you verified you're getting +12V to terminal 30 of that relay's socket? It's a straight run from the battery so you should have voltage there unless there's a wiring problem or the 8A fuse is blown.
Check, 12v at terminal 30.

What I have heard/read is the fuel pumps should activate when the ignition is turned to the "on" position for a second or two. This does not occur with our specimen. The relay we are trying is used. I have cut another one apart and seen the pc board and circuitry, so it is possible the "pulsed signal" is required for #87 to be powered.

This is a minor setback in the big picture, the car involved here is the "sandy spider" salvaged from the flood waters of hurricane sandy. It has been a long road (over a year) but she is running good and will be roadable soon.

all the input from this board has been invaluable, thanks to all
 

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What I have heard/read is the fuel pumps should activate when the ignition is turned to the "on" position for a second or two.
My theory (see disclaimer in reply #2 ) is that the drive relay sees the initial pulse (when the ignition is switched on) as the start of the expected tachymetric signal. When it does not continue (since the engine is not running) the drive relay cuts the power to the fuel pumps.
 

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Discussion Starter #9
The disclaimer would include "the smoke theory?"

I would bet 5 you are correct in saying the power to #87 is interrupted if the tachy signal is not received. With our example the fuel pumps do not run(at all) with the ignition in the on position

All that said, do we have a real test for the drive relay?
 

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The disclaimer would include "the smoke theory?"
Yes. And also Tom Magliozzi's "unencumbered by the thought process"...

do we have a real test for the drive relay?
Well, what is the tachymetric input? Is it just 12V on & off in time with the coil discharge? I don't know for sure but that'd be my guess.

The engine idles at ~ 900 rpm. That's 15 RPS (Revolutions per Second). Times 4 cylinders that each fire every other revolution would mean the coil fires 30 times per second at idle.

Can you replicate that as the input to the drive relay?
 

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But Mad North-Northwest
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The disclaimer would include "the smoke theory?"
Yes, there's always a risk of letting the smoke out. That being said, I based the above on the details of the wiring diagram, as well as a passing knowledge of how the Bosch double relay works (this is how the Milano handles the same drive relay functionality). The double relay uses the AFM flap switch rather than the coil ground as the "hold open" for the relay, but beyond that the idea is pretty much the same.

Double Relay Demystified

I've no doubt the drive relay has a PCB inside, but likely it's a similar resistors & diodes setup as the double relay. I'd *really* be amazed if it actually needed a pulsed tach signal instead of just a ground, but like I said I've been wrong before.

Beyond that, no, I've never seen a test diagnostic for these parts. I assume most mechanics would just swap in a known good one.
 

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I'm pretty sure that Papajam understands how it works. Maybe he can enlighten us?

My guess is that it needs a pulse and the PCB is a timer circuit.
 

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Discussion Starter #13
Yes. And also Tom Magliozzi's "unencumbered by the thought process"...



Well, what is the tachymetric input? Is it just 12V on & off in time with the coil discharge? I don't know for sure but that'd be my guess.

The engine idles at ~ 900 rpm. That's 15 RPS (Revolutions per Second). Times 4 cylinders that each fire every other revolution would mean the coil fires 30 times per second at idle.

Can you replicate that as the input to the drive relay?
That is the question, "how to bench test with a simulation of the tachy signal" and what exactly is the tachy signal? does anybody know?
 

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That is the question, "how to bench test with a simulation of the tachy signal" and what exactly is the tachy signal? does anybody know?
I expect that it is a signal similar to the one that is transmitted from the coil to the tachometer. If that is the case it is easy to simulate with a decent function generator. I posses a signal generator but not a relay to test. If someone has a good working relay to lend me I will determine what signal is needed to test it. It may then be possible to build an inexpensive pulse generator specifically to test the relay. I will be happy to do the work and build a prototype test generator but I do not have a Bosch Spider and I have no plans to own one so I have no financial interest in getting a relay for the testing.
 

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Discussion Starter #15
Thanks for the replies all, I suspected a bench test for the drive relay was out of reach for the average backyard mechanic. I will look for a used relay.
 
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