8 replies to this topic

[Steve Scheifler]

I’m working on another NB with an intermittent “miss”. This one is a ‘99 Champ car that started acting up last December. Since then they have replaced nearly everything relevant including the engine wiring harness, except the ECU and injectors. Today I just warmed it up, wiggled wires and did some baseline runs on the dyno. It never flinched, but tomorrow I’ll beat on it some with all available OBD channels logging.

However, I wanted to share something I found today. Since the main relay is a common failure point I tapped around on it a bit and noticed it was pretty warm. So I took some thermal images of the fuse & relay box. All the other relays are essentially at the air temp while the main is a great deal hotter. And this is with the hood up and fuse box cover off. It was a new one that they installed recently so I swapped in the spare and it acted exactly the same. So this probably isn’t the cause of their issue but it may explain why they often fail if this is normal? Seems warmer than I would design it to run anyway.

https://drive.google...ew?usp=drivesdk

https://drive.google...ew?usp=drivesdk

[Martinracing98]

Does the relay fit snug. That seems like there is high resistance at the connection.

[Steve Scheifler]

Yes, although a little difficult to be sure because of the pressure required to push over the retaining tab. All four contacts clean and none show signs of heat or arcing, sprayed with Deoxit.

[Steve Scheifler]

If I can reproduce the miss on the dyno I’ll hit the base of the relay with some freeze spray.

[Dave D.]

You would think with how many cars out on the road in various states of repair(some rat traps, some low mileage lovelies), that if there was an issue with the Main Relay running hot, a common issue would have shown itself by now? Or does the conditions we operate on track push the issue over the edge?

[tylerbrown]

You would think with how many cars out on the road in various states of repair(some rat traps, some low mileage lovelies), that if there was an issue with the Main Relay running hot, a common issue would have shown itself by now? Or does the conditions we operate on track push the issue over the edge?

 

If you make some google searches on Main Engine relay failures and the miatas, it seems to be a pretty common issue. Especially when someone runs and aftermarket fuel pump, it sounds like it hurts the main engine relays real bad. 

 

One solution people have found is to add another relay into the system for just the fuel pump. It is my understanding that this would not be legal in SM, but it is also not a performance advantage, so maybe letters could be penned to the SMAC to get something like this approved?

 

Here is a link to a forum that has discussed this topic: https://www.miatatur...-failure-91002/

 

The fix they have is quoted here: 

 

"The cause for the master relay failure is a combination of two main issues. First, with time and mileage, combined with exposure to heat and elements under the hood, the main relay contacts become dirty causing a voltage drop and therefore higher current. Over time this becomes a self-energizing effect, overheating the connections and eventually damaging the contacts with relay failure.

 

However, the main culprit is the current demands of the fuel pump. As the various connectors and contacts in the harness corrode with time, the resulting cumulative voltage drops steadily drive the current draw up. In a normal low current circuit, this would not lead to a relay failure. But, since the electronic fuel pump has a high current demand, which increases as the the device gets older, the stress on the relay becomes too much and failure is imminent. So replacing a dead relay is not the cure.

 

The best solution is to remove the high current draw from the main relay circuit, the fuel pump. This is pretty easy to do. You will need a standard universal 30A or 40A 4 prong relay, 12 ga wire, 25A inline fuse, some crimp connectors and some zip ties.

 

Disconnect the battery. Remove the bulkhead cover behind the seats to access the fuel pump wire (red-blue). Remove the right trunk panel to expose the battery. Cut the fuel pump power wire (red-blue) several inches below the tank and, with butt connectors, crimp enough 12 ga wire to the two separate wires to run back to the battery. Connect the wire wire from harness to the 86 pin of the universal relay and the wire to the pump to the 87 pin. Next, connect the 85 pin with a crimped eyelet/stud connetor under the factory ground at the inner fender pinch seam. Finally, connect the 30 pin to the positive battery cable clamp bolt (use an eyelet/stud crimp connector) using the inline fuse. Zip tie the wires securely, as well as the relay.

 

If everything is wired correctly, you should be able to reconnct the battery, reinstall the panels and drive it like you stole it."

[Steve Scheifler]

It is a fairly common failure point on both street and race cars. I need to check what all draws power through it but I believe the coils and fuel pump at least. The coil on this car is new but perhaps the fuel pump is drawing more current than it should.

[FTodaro]

When will we be at a point that we contact a vendor to manufacture a wiring harness to replace the OEM harness and have SCCA sanction the move.

 

Wiring issues are going to keep getting worse until these 20+ year old cars will be unraceable.

 

Ralph, this right in your wheel house.

[Steve Scheifler]

I think Bill Agha is already making engine harnesses with the blessing of Mazda. But this car already had that replaced.