Mysterious silver box...

[mejulihn]

On the later cars there is a small silver box about 2.5" x 2.5" that plugs into the FI harness and is located under the battery tray. I read somewhere this is a type of rev limiter. Anybody have more specific info on this little gadget? What does it really do and why?

[Dave at Pelican Parts]

The L-jet cars have the "dual relay" and the injector resistor pack in that area. I don't know if there is a version with the rev-limiter or not.

The CA-spec 75-76 2.0s (with catalytic converter) have a rev-limiter in that area. I haven't seen one myself, but I have an FI wiring harness that has a provision for it, and I have heard it described to me as a silver box.

It works by cutting off the fuel. The signals from the FI trigger points go through it. When those signals are close enough together (i.e., the RPMs are high enough), the box stops passing them through to the ECU. The ECU thinks the motor is running at 0 RPM, so it stops firing the injectors. When the pulses (which are still occurring, just not going through to the ECU) get far enough apart, the limiter starts passing them through to the ECU again.

That is how the functioning has been described to me.

I believe this was done to avoid dumping unburned fuel into the exhaust, which is what happens if you limit the revs by shorting out the spark. Raw fuel in the exhaust would burn when it hit the HOT catalytic converter, which could result in it exploding or overheating and melting down.

--DD

[scotty914]

well to add a little more to daves post


the relay pack has 2 relays in it one is the power to the second relay which is on when the key is on the other comes on when either the starter is running or the air flow sensor flap is open

the idea is one comes on with the key so the fuel pump cant run with out the key on

the second directly goes to the reg board

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scott thacher
75 1.8 running at last
75 under rust
(fine german rust)

[mejulihn]

Dave,
Thanks. Your explanation makes a lot of sense, although my '76 does not have a catalytic converter as it is a true 49-state car. It is a 2.0 so the L-jet dual relay idea is not applicable. By the way, if I remove the device, the car starts and runs normally -- which you would expect.

[bowlsby]

To add more detail:

In 1976, both USA and CA 2.0 914 cars have the speed limiter.

In 1975, there were a few 2.0 cars without the speed limiter.

On the 1975 2.0 car I have seen (I don't know if it was a CA or USA car) was an early serial number, so I can't determine if the factory just added them in the mid-to-late model year, or if there were other conditions.

[scotty914]

on a 1.8 if you remove the relay pack it will not run at all even if you jump out the relay on the reg board

ask me how i know

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scott thacher
75 1.8 running at last
75 under rust
(fine german rust)