Quote:
Originally Posted by mysocal911
1. The issue of less reliability with the high component count verses the original relay
hasn't been discussed, i.e. the reliability decreases as the component count of any
system increases.
2. A solid state fuel pump relay is less reliable and prone to failure when the fuel
pump stalls or may generate excessive start-up currents than a mechanical relay.
3. Most all automotive OEMs still use mechanical (contact type) fuel pump relays.
4. The inductive discharge voltage when the fuel pump turns off can over-stress
solid state switching devices.
5. Solid state relays are more prone to failure during an intermittent over-charging
alternator. |
All good points! Let me know if this addresses all the concerns or if I need to include more details. I welcome all the input I can get. Keep in mind this SSR is installed in my Porsche too, if there is an issue you can be sure I will want to fix it!
1. As a rule of thumb reliability does decrease with complexity. Device failures are the summation of the probability of failures for all individual components plus assembly defects. Mechanical relays use largely stamped metal parts in their construction along with complex assembly processes such as winding the coil, stamp welding, and riveting. Each of these processes inherently will have radically higher defect rates than equivalent electronic manufacturing processes used to create the SSR. Only high-quality name-brand electronic parts are used. The odds of any of these parts being defective from the manufacturer is similar to winning the lottery. In terms of assembly the SSR just needs parts placed somewhat accurately on solder paste which is then heated in an oven. The surface tension of melted solder will pull the part into the correct position on its own. There will also be version without the Pump-Prime feature which reduces the part count by more than half. Officially, I will warrantee it for double the period of highest warrantee of any replacement DME relay.
2. The transistors inside the SSR are rated radically above the 15 amp fuse that protects the fuel pump. The fuel pump could be in full stall until the battery runs dry and the transistors that drive it would never feel more than warm to the touch. I am not sure what the issue would be with startup current. The windings in the fuel pump (an inductor) will dampen the current flow (flywheel effect) when power is applied far more efficiently than switching it on slow. Dave, could you clarify the concern? I just want to make sure I’m not missing anything.
3. OEMs use mechanical (contact type) relays because they are cheaper, and not because of reliability factors. Car makers don’t hesitate to save a buck event if that means some people will get hurt or worse because of it. It has only been in the last several years that the semiconductor industry has come out with transistors that can handle very high current without huge heat sinks. At some point in the near future the cost of a mechanical relay is going to be the same as an equivalent transistor. When that happens OEMs will start switching over.
4. The “kick back” voltage generated by inductive devices when the electric field collapses is easily handled any number of ways. In the SSRs case, the transistor is a high-power P-Channel MOSFET configured as a high-side switch (switching Battery Positive Voltage). All MOSFETS have a “Body-Diode” that connects the input and output of the transistor (i.e. Drain and Source). In this configuration, when the fuel pump generates a high voltage spike it forward biases this body diode and is simply absorbed into the vehicles power system. Operating the power windows ,let alone the starter, generates far bigger voltage spikes.
5. All electronic devices attached to a vehicles power system must be protected against voltage spikes and high alternator output. The SSR has the same power protection circuit found in your Porsche’s DME (actually it’s a good measure better). If the SSR fails due to alternator issues, you will likely need to replace the DME too. At least the SSR will be covered under warrantee for like 4 or 5 years.