Fuel pump relay trouble

99grandcheroki · 11-19-2009, 01:00 AM

1999 Grand Cherokee

I need a little help sorting this problem.

The car cranks, but does not start.

The fuel pump does not prime when ignition is turned on.

The fuel rail has no pressure after an attempted start.

The fuel pump relay tests good.

The fuel pump fuse is good.

The fuel pump works when jumped at the relay connections and pressurizes the fuel rail.

The relay positive main fuel pump hot is "hot".

The relay is NOT activated when I try to start the car.

The positive (hot) relay switch leg is activated when the ignition is turned on.

BUT, the relay switch ground connection is not grounded.

That is what I tested thus far. Any ideas on what to check next?

Thanks in advance for any help.

ZeeJay1997 · 11-19-2009, 04:38 AM

If all you say is is good is indeed good, it's time to suspect the PCM or the low side wire coming from the PCM.

jeep210 · 11-19-2009, 06:44 AM

Do the same test on the ASD relay. The control ground on both relays is connected to the PCM. The PCM grounds the circuit to turn both of them on. If you have a ground problem on one, you may have a problem on both. If one works and the other does not, it may be a wiring problem. If both are dead, the problem is probably the PCM.

Check to see if you have a spark. If the ASD is not activating, it shuts down the coil, injectors and alternator.

99grandcheroki · 11-19-2009, 11:25 PM

Thanks for the feedback.

Here is what I did now:

I tested the ASD relay and found it to be good.

I tested the control ground on the ASD relay connection and it is NOT grounded when the ignition is turned on. By this, I mean the ground that would trigger the relay. The switch hot for this relay does turn on when the ignition is turned on, and stays on.

Based on your feedback, I am assuming that this points to the PCM telling the ASD and Fuel pump to stay OFF.

I purchased an OBD-II scan tool and tried to scan the car. I only read a "no link"...meaning that it cannot connect to the car. I tried the tool on my neighbor's honda and it worked fine...reading "0 codes".

Is this something that I can continue to diagnose myself?

Thank you

rooster51 · 11-19-2009, 11:32 PM

There are codes for PCM failure/malfunction but you wont get them if the PCM is unaware of its own malfunctions. Your friends scan on his vehicle confirms ZeeJays diagnoses.

99grandcheroki · 11-20-2009, 12:07 AM

Just another thought.

Could this be related to the security system? As I understand, the security system can shut things down. How would I know this?

rooster51 · 11-20-2009, 12:28 AM

The scanner read "No Link" on his Jeep but read "No Codes" on his friends vehicle. A symptom of a bad PCM, interface malfunction.

99grandcheroki · 11-20-2009, 12:45 AM

From what I gather, it sounds like the consensus is that my PCM is shot.

Looks like my choices are....

$100 for a dealer diagnosis, plus towing

or, $200 for a rebuilt PCM...

1999 - 2006 Jeep Grand Cherokee COMPUTER ECM PCM ECU:eBay Motors (item 110424135511 end time Dec-11-09 15:06:14 PST)

With the possibility that this was not the problem.

rooster51 · 11-20-2009, 12:53 AM

Seems legit, except its MANDATORY to disconnect the negative wire from the battery before you remove the old and install the new PCM. Then start it and let it run (dont drive it) for 15-20 minutes so the new PCM can "learn" your Jeep and adjust all that needs adjusting. Fuel/air ratio, spark, timing, etc...

ZeeJay1997 · 11-20-2009, 05:25 AM

Before you go buying a PCM, confirm B+, ignition, and ground to the PCM. The grounds are black/tan and connect to the block. Ignition is orange, and B+ is red/yw.

99grandcheroki · 11-20-2009, 10:13 AM

Thanks ZeeJay...I will check these things.

Any leads on a wiring diagram for this?

ZeeJay1997 · 11-20-2009, 03:43 PM

The best i can do for you is a 97 diagram. The colors shouldn't change from year to year, but stranger things have happened.

I think the best way to go at without a diagram is to pull the connectors off, take the back shell off so you can see the wire colors, flip it over and check voltage/ground at the pins. There is more than one ground.

99grandcheroki · 11-20-2009, 04:34 PM

97 wiring diagram is better than non.....thank you.

Is this something you can e-mail or post?

Sextiva · 11-20-2009, 05:25 PM

How do you know the relay is good?

99grandcheroki · 11-20-2009, 05:30 PM

yes...both ASD and Fuel Pump relays tested good.