No fuel/spark fixed with a technical tap on the RPM relay

[jangell]

I took my car for a real drive for the first time in a year and a half. Everything worked great with the new exhaust and cams. It does feel like it has more power now, although it's hard to quantify, especially given how long it's been since I last drove it.

Anyway, I pulled into a gas station and the car stalled (automatic transmission). I filled the tank up, and couldn't start it. After about 10 minutes of diagnose, I determined that I had no spark and no fuel, but that I could jump the RPM relay and run the fuel pump just fine. Realizing the relay might be a problem, I gave it a technical tap (i.e.: I rapped it against the stainless a few times), plugged it back in, and the car started right up.

My current theory is that when the relay got warm, it started to stick. This may also explain weird intermittent issues I've had with the car stalling and being hard to start, being stranded for two hours until the car magically decided to start again, and random sudden and total yet brief (less than a second) loss of engine power as though you had taken your foot off the gas.

Just wanted to pass this information on for anyone else who might have weird problems like this. Guess I'll be contacting bitsyncmaster for one of his solid-state upgrades. :)

-- Joe

[WelmoedJ]

Perhaps it's the time to consider replacing the original RPM relay for one of Dave's (Bitsyncmaster).
It's all electronics inside, no mechanical parts.
Many of us already are using it without problems.

[jangell]

Thanks -- I've already sent him a PM.

-- Joe

[Elvis]

No Spark ? then it's not the RPM Relay.


But resoldering the relay's contacts can help a lot with fuel problems.

All you need is some soldering equipment that one usually has at home anyways.

It takes 5 minutes and costs nothing.

[jangell]

The engine would crank, but there was no spark and no fuel. The coil had ~1.5v on it with the key in the "run" position, which seemed pretty odd -- I was expecting 12v. The lack of spark was confirmed by pulling a plug and watching it while cranking.

I honestly don't know exactly what the RPM relay does beyond running the fuel pump and frequency valve. The "RPM" name made me think it might have to do with spark, but now that I think about it I thought I heard of people limping home with the fuel pump jumped, so I guess that doesn't make sense.

I failed to find information in the wiring diagrams about the ignition system (beyond the one that explains what fuses the key turns on). Is there a diagram for how the coil gets power?

Thanks!

-- Joe

[ccurzio]

I honestly don't know exactly what the RPM relay does beyond running the fuel pump and frequency valve. The "RPM" name made me think it might have to do with spark, but now that I think about it I thought I heard of people limping home with the fuel pump jumped, so I guess that doesn't make sense.


The name makes sense when you understand the function. It's pretty simple, actually. If the relay detects RPMs, it supplies power to the fuel pump. That's its primary function anyway. It also supplies power to the Lambda relay and the heater for the WUR.

I can't recommend Dave's solid-state version enough, even if your original has nothing wrong with it. In addition to the nice key-on/key-off pump priming, you have significantly lower power requirements and the option of a one-second pump prime once an hour while the car is off.

[jangell]

Right, I meant the WUR heater, not the frequency valve -- I remembered having to jump it for something when I was diagnosing my high idle issues. Thanks!

-- Joe

[Bitsyncmaster]

Your battery may be low to have read only 1.5 volts on the coil. I'm pretty sure you can run with that voltage but during a start, one resistor is bypassed and you will get more power to the coil during cranking. Then after your alternator is running, you get more voltage anyway.

99% of the RPM relay failures are due to broken solder joints. That is why rapping it (not on the stainless) or just letting it cool can get it working again.

[DMCMW Dave]

If you lose ignition (i.e. bad connections at the firewall ballast resistors) the RPM relay won't see "RPM" and won't turn the fuel pump on.

[opethmike]

If you lose ignition (i.e. bad connections at the firewall ballast resistors) the RPM relay won't see "RPM" and won't turn the fuel pump on.

+1

My car "broke down" like that a few days ago. Turned out to be a loose connector on the ballast resistor. Give that a look, clean the connections up, and tighten them.

[jangell]

If you lose ignition (i.e. bad connections at the firewall ballast resistors) the RPM relay won't see "RPM" and won't turn the fuel pump on.

Ah, that explains why both failed. I did fiddle with both the ballast resister wires and the bulkhead connectors, but I can't recall if I tried to start the car before I tapped the RPM relay. I'll check those more carefully if it happens again. Thanks!

-- Joe