Fuel Pump Relay Troubleshooting (Page 1/1)
Will APR 21, 05:04 PM
So here's a weird one...

Mostly stock 1988 Formula wouldn't start one evening to head back from my dad's house to mine.
I did some super basic checks. In keying on, I heard a click from the relay cluster by the air filter, but no fuel pump priming... thus leading me to think it was the fuel pump. I had to get on the road, so I took a different car.
Shop replaced the fuel pump (and alignment and tires since it was due), but car still wouldn't start.
Replaced relay, drove car home. The click I was hearing had been the A/C relay instead, since it ALSO clicks at key-on if the A/C is on.
I also replaced the dead oil pressure sender.
When I started it the next morning, and after sitting for several hours every time thereafter, the fuel pump does not prime and I experience extended crank times. I checked the new relay and the old relay and they're both ok.

I do some electrical troubleshooting and find that when I probe the relay control signal from the ECM, it shows ~700 ohms to ground with the key off, and open circuit with the key on. That's not right. The 700 ohms is expected if the output driver is tri-stated, but I should see very low resistance for 2 seconds, followed by open circuit until I crank, if the ECM were trying to prime the pump.

Has anyone ever experienced their stock ECM deciding not to prime the pump anymore?

If the driver had straight up died, I wouldn't have expected that it would transition when the key was operated.

[This message has been edited by Will (edited 04-21-2019).]

Spoon APR 21, 10:33 PM
I never experienced anything like that but if I did I'd pop in the spare ECM I keep on the shelf and rerun my testing. Also I guarantee someone will jump in and tell you to check all your grounds.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

pmbrunelle APR 22, 12:48 AM
Ummm, are you aware that the ECM does not provided a switched ground to the relay coil?

It is in fact a switched positive. Therefore, a "resistance" measurement would be more meaningful if measured between the relay control wire and a hot-in-run wire.

But rather than putz around with an ohmmeter, I'd just use a test lamp.
theogre APR 22, 10:14 AM
AC Clutch Relay Does Not click or turn on when the engine starts or Key on and engine Not running. 1 or 2 other relays upfront switches on and maybe off as you turn the key.

Relays work or not but have different failure mode like...
Relay Coil is dead.
Relay can click but contact have problems and won't connect.

Switching AC Clutch and FP relays was your first thing to do.
Still no relay and have long crank time is OP sender getting enough pressure to turn on the pump. So you replace the pump likely for nothing and worse took to a shop that very likely use wrong type of hose in the tank that won't last long.

ECM can "die" and cause many things. Any solder crack on 30+ year old unit can kill the whole thing or just 1 function and often never set a "code."
Yes. ECM sends Power to the FP relay so use a light or volt meter not Ω between grn/wht and blk/wht wires on relay plug. Do Not use Ω to measure anything to/from ECM because that itself can blow parts in the ECM.
Save the PROM to switch to a replacement unit. See my Cave, ECM Heat

Measuring Relay Coil Ω may not help most DIY. FP and other relays have a Diode and "Shorts out" the coil one way and not the other. This is so to stop power problems when you shut off a coil.

You can, likely should, fix/clean all grounds but likely not the main problem here. You want to fix grounds at minimum because can cause low volts to FP etc and that can burn Relays etc.
See my Cave, Electric Motors and Wire Service

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

edfiero APR 22, 03:37 PM
On my 84 Grand National, I just recently had the problem of NO PRIME at key on and extended cranking time.
Replacing the ECU resolved the problem.
Will APR 27, 09:23 AM

quote
Originally posted by pmbrunelle:

Ummm, are you aware that the ECM does not provided a switched ground to the relay coil?

It is in fact a switched positive. Therefore, a "resistance" measurement would be more meaningful if measured between the relay control wire and a hot-in-run wire.

But rather than putz around with an ohmmeter, I'd just use a test lamp.



Yah know... I had it my head that they always switched ground... didn't even look at the diagram for that when I was looking up wire colors... D-oh.
Will APR 27, 05:15 PM
I checked the socket and have +12V for 2 seconds when the key is turned on.
The relay works on the bench... Not sure if I had an intermittent somewhere or what. I'll see if it starts quickly tomorrow morning. It's unlikely that the fuel rail will bleed down enough between now and when I step out for the evening to adequately check.