Residual Fuel Pump Relay Voltage

Topic: Residual Fuel Pump Relay Voltage (Read 3743 times) previous topic - next topic

Residual Fuel Pump Relay Voltage

August 07, 2007, 11:01:29 AM

Many moons ago the Cougar ped out on me while on a 2k mile road trip. It also had a partially fried EEC (not related to the breakdown), which required me to ground the FP output on the self test connector in order to run the fuel pump. Once I towed the car back via U-haul, I replaced the fuel pump, fuel pump relay, and the EEC. When I cranked the car all seemed well, until I turned it off. The fuel pump kept running. In face, the fuel pump would run any time the battery was hooked up.

I left the car in the garage and forgot about it for a while, until last Saturday. I crossed my fingers and plugged in the original Motorcraft FP relay. The relay came on for two seconds with KOEO, ran while the engine was on, and stopped when the engine was turned off.

I probed the relay points (from the EEC) and noticed it went to 12V immediately when turned to KOEO, and settled to a constant 7V after about two seconds. This doesn't seem normal. Any explanations?

Residual Fuel Pump Relay Voltage

Reply #1 – August 07, 2007, 04:06:14 PM

Probing which wire: R, Y , PK/BK H, T/LG D

Residual Fuel Pump Relay Voltage

Reply #2 – August 07, 2007, 04:26:05 PM

I am probing 85/86. The relay is actually a SPST, so there is no 87a.

Residual Fuel Pump Relay Voltage

Reply #3 – August 07, 2007, 10:18:34 PM

85/87 ? Are these wire numbers? Is this the FP relay?

Residual Fuel Pump Relay Voltage

Reply #4 – August 07, 2007, 10:46:25 PM

Ack, snagged by the hotlink!

Residual Fuel Pump Relay Voltage

Reply #5 – August 07, 2007, 11:41:02 PM

That does seem like too much voltage drop when it is supposed to be off. You would think most of the voltage would be dropped across EEC pin 22 (wire 97) and ground.

If we get a cool day here sometime soon I'll check it on my car.

Residual Fuel Pump Relay Voltage

Reply #6 – August 08, 2007, 09:14:37 AM

Doesn't seem right to me. If you're probing the two wires that go to the relay coil (86 and 85) your voltage across the circuit should be 12 volts. Since you're taking your measurement directly at the EEC you can rule out a bad harness, etc. It would seem the EEC's FP relay driver is wonky...
*EDIT* It occurs to me that the voltage should be "0" after a few seconds of KOEO, as the EEC-IV should not be trying to run the pump without the engine running. You should be getting no voltage reading at all, which would still lead me to believe the EEC's FP relay driver is wonky... just a different kind of wonky.

Residual Fuel Pump Relay Voltage

Reply #7 – August 08, 2007, 10:40:20 PM

Check the feed coming out of the relay to battery ground and then the ground side of the relay to battery positive. Maybe we can see where the voltage drop is coming from, unless it is across the relay terminals. Also if you ground the self test connector does the voltage change any? Maybe, it's possible that there is residual voltage after the eec opens the ground? Softtouch may be able to test his vehicle for the voltage reading. A schematic would be extremely helpful, as I can't remember the circut, if the feed is vpower or dedicated and the ground route into the eec. Lastly, does eec power or ground affect any of the fuel pump relay?

Mike

Residual Fuel Pump Relay Voltage

Reply #8 – August 09, 2007, 11:45:42 AM

you lost me

you cant read it that way from what i see unless there **IS** a problem so you are definatly on to something if your reading from eec pins 1 to 22.

The fuel pump relay will get coil input power from the EEC relay contacts when the key is turned on. The eec relay coil is in line with the ign sw waiting for you to give it power.

you cant read the coil of the fuel pump from the eec with both meter leads unless you have a faulty ign switch making partial contact or the eec relay contacts are not opening up when de-energized.

page 57/58 of the 1987 evtm.

If there is a faulty ign sw or eec relay contacts,, the eec relay will have power coming out of the wire color black yellow. This black yellow is spliced to the coil input of the fuel pump relay.

if infact there is a problem,, and,,,,,,, the only way you would be able to read across the eec (both meter leads) to find it would be from pins
1 to 22 on the eec.

i read your post and you said "from the eec" so either way from the coil or eec, there cant be power unless something prior to it is defective like i menitoned.

to troubleshoot,, and make the problem go away,,,,,,,,,
tap on the eec relay and see if the 7v goes away. If it does,, your spring in the eec relay is getting weak.

if not..........

bang on the ign switch and see if you can make the 7v go away. If it does,, somethings up in the column.

Residual Fuel Pump Relay Voltage

Reply #9 – August 09, 2007, 12:46:22 PM

My term "from the EEC" was a general term meant to describe that I was probing the low current side, not the high current side. Very vague...

I probed the connectors directly, the relay was unplugged.
I tested the R (361) and T/LG (97) connectors.

A little more info:

With the replacement STANDARD MOTOR PRODUCTS Part # RY46T installed, it would run whenever there was power at the battery. But sometimes it would wait until KOEO to engage (most common when I had left the battery disconnected for a week or more), but run until the battery was disconnected. At no point did the FP ever stop once it began. To add to the confusion, after sitting for 3 months - the relay started with KOEO, and stopped while KOER about 2 minutes later. I yelled out "Those Chinese sonsofbitches are going down!" and removed the RY46T relay. I noticed it was hot. This is when I probed the connectors. I then slapped the old Motorcraft relay in and everything worked just fine. 2-second burst with initial KOEO....ran during KOER...and turned back like normal.

I agree with Thunder Chicken, it sounds like a wonky EEC FP driver (pin 21). That was the problem with the previous tranplanted EEC...so I hope there isn't some short in the wiring responsible.

jcassity, thanks for the EVTM pictures.

I've got plans for this weekend, but I'll take a looksie next week to see if I can pinpoint the problem.

Residual Fuel Pump Relay Voltage

Reply #10 – August 09, 2007, 02:05:46 PM

It cooled down to 95 today so I took some voltage readings on my '84 3.8 fuel pump relay.

Across the relay coil:
K-OFF 0.00
KOEO 11.15--2 sec--0.01
K-OFF 0.01---5 sec--0.00

EEC side of coil to ground:
K-OFF 0.00
KOEO 0.76--2 sec--11.96
K-OFF 12.04--5 sec--0.00

Two seconds is too quick for my meter to settle down on an exact voltage. But you get the idea.
The five seconds is the delayed dropout of the EEC power relay.

With your OHM meter check the two relays to make sure the connectors match. Same pair goes to the coil. Same pair goes to the contacts.

I probed with the relay installed..All bets are off on the way you probed it.

Residual Fuel Pump Relay Voltage

Reply #11 – August 09, 2007, 02:49:45 PM

Quote from: softtouch;167463

EEC side of coil to ground:
K-OFF 0.00
KOEO 0.76--2 sec--11.96
K-OFF 12.04--5 sec--0.00

This is exactly what I'd expect to see, as silicon transistors when used as switches have .7v voltage drop across them... Sounds as if the EEC has a defective driver transistor... Of course there is the possibility there is a resistor in the output and is correct, for that particular EEC... But I'd think it would use a relay different than softtouch's example...

Residual Fuel Pump Relay Voltage

Reply #12 – August 09, 2007, 11:17:45 PM

Circut 361 appears to be vpower. There should be close to battery voltage at that terminal of the relay with the key on, dvom to good ground. Softouch's tests show the expected voltage drop across the relay. If an electrical malfunction has caused issues with both pcm's, I'd look at everthing real hard before buying another pcm. There may be a short to ground on a vpower item or circut. Reading the voltage at relay 361 to a good ground should reverify the ground problem into the eec driver. Please let us know what you find.

Jcassity, great schematic pics!

Residual Fuel Pump Relay Voltage

Reply #13 – August 10, 2007, 01:09:54 AM

Quote from: t.birdsc;167620

Circut 361 appears to be vpower. There should be close to battery voltage at that terminal of the relay with the key on, dvom to good ground. Softouch's tests show the expected voltage drop across the relay. If an electrical malfunction has caused issues with both pcm's, I'd look at everthing real hard before buying another pcm. There may be a short to ground on a vpower item or circut. Reading the voltage at relay 361 to a good ground should reverify the ground problem into the eec driver. Please let us know what you find.

Jcassity, great schematic pics!

thanks

my thoughts go directly back to the eec relay,, if there is power coming out of the wire i mentioned at the eec relay with the KOEO,, then there lies the problem. The eec isnt associated with this problem from what i see. The eec relay could care less about the eec, its just sitting there waiting for the key to be turned on. It just so happens to have wire branching off prior to the eec relay contact leading to pin 1 on the eec. you could disconnect the eec KAP lead pin 1 and the eec relay would still deliver power to the input side of the fuel pump relay coil. just ground out the tan / lg wire and it will run.

so,,,,,,,,to verify with another test i just thought of,,
disconnect the black orange wire coming off the eec relay preventing power delivery to the eec.
ground out your tan/light green wire at the fuel pump relay or at the eec self test conn
turn on your ign sw
eec relay should energize and deliver power to the fuel pump relay (it should run full time in this setup)
turn key off and the fuel pump should stop.
no go back and measure the voltage drop conditions across the fuel pump relay
if the voltages are the same, the the problem is not eec related.

i think again, you need to go back one step and start with the eec relay. with what you described, the power finally stabalizes at 7 volts after fluxuating, it tells me there is a contact somewhere barely touching for some reason.

Residual Fuel Pump Relay Voltage

Reply #14 – August 10, 2007, 06:15:06 PM

Measured the voltage on my FP relay socket with the relay removed.
The relay coil wires red and tan.

KOEO 11.90 2 sec then 10.60 It my drift around for awaile, down to 9.99 but settles at 10.60.

So I don't think you can draw any conclusions from this. Looking at it with the relay plugged in is the way to go.

If I am understanding this correctly, it works right with the OEM relay and acts crazy with the new relay.
Sounds like the new relay is not the correct replacement for the OEM relay.