Buddo95

I just did a drivetrain swap from a 97 to my 89. Everything went smoothly during the removal and installation but now i have everything hooked back up it wont start. The fuel pump relay rapidly clicks when the key is the on posistion and while the engine cranks. I could really use some help here, Im totally lost on this one and i only have a few days before i ship out. Thanks in advance!

RTorrez1

Try swapping it out with the one for your electric fan and see if it starts.

Buddo95

Swapped relays around and all click the same. I jumped the relay and the pump runs.

DFlintstone

When you say "jumped", did you use power from somewhere else, or just jump across the relay?

Buddo95

Across the relay. I experimented and the relays ground isn't working. Doesn't the ecm control the ground for this relay?

DFlintstone

Here is my best guess. 30 should be hot. 86 gets juice from the Ign, 87 goes to the pump, and 85 is grounded by the ECU (to run it). I don't have time now to double check that in the diagrams. (you can find some in a link in my signature)

DFlintstone

Yea, pretty sure that would be 85. (PCM 91 on, ECU 87-90)

Buddo95

You would be correct and I have the right voltages at 30 and 86. Its just the ground to the ecm. I took a piece of wire and with the relay in touched 85 to the - post and the clicking stopped.

DFlintstone

Huh. (I'm assuming you mean battery negative post). Well, Cruiser has mentioned pulling the plugs off of the ECU and tweaking the pins to tighten them a bit. It's pretty rare for an ECU to fail. (you could also try grounding it at the ECU, and/or checking that wire for continuity.

The ECU needs a good signal from the CPS to tell the pump to keep running. Also grounds on these things are known to be an issue. I suggest you take a look at Cruisers stuff found in the first link in my signature. (the ecu itself grounds at the dipstick stud, you should check/clean that).

EDIT! If it's buzzing with just "ign. on" it's no way the cps.

Checking a Renix CPS really only takes three minutes. I'd check that trickster if my radio didn't work! >>

Anyone with an 87 to 90 Renix Jeep has two or three minutes to see that their
CPS is putting out at least .35 AC volts. I don't mean to come off rude, but if you can't connect a meter and turn the key, we may not be much help.
AC voltage CPS test; Small clips on the meter leads really helps. (you need contact with those little pins while it cranks)

Unplug the connector at the back of the manifold with wires going down to the bell-housing/cps.
Probe the two wires to the cps with the meter set on AC volts. (a 200 scale on mine).
Crank the engine and note the voltage. (jump it if your battery is low)

Mine somehow works with only .3 my wiring/grounds must be pretty spiffy. .5 seems to be a good mark. One article talked of .5 to .8. **Update, it soon failed. I installed Napa #css980 in my 90. That part will work back to 87.

Buddo95

Well I just replaced the cps but I'll definently check it. Could you post a pic of the grounds at the dipstick stud I have a feeling something is missing there. Thanks for the help

DFlintstone

I edited that. If it's acting up with just the ign on, it can't be related to the CPS.

Seriously, check out Cruisers stuff. It might be in his post #5. If you look at your dipstick mount you will see a bunch or grounds there. The ECU, TCU and more. Cleaning, or at least checking the grounds really should be first....I got distracted over that relay buzzing.

Here, > https://www.cherokeeforum.com/f51/cru...x-tips-153657/

Buddo95

I got it fixed. I felt like I missed something when I put the new engine in. I missed the ecm grounds at the dipstick they got tucked behind the head so I put them in their place and no buzzing now. Thanks for your help

DFlintstone

Hey Cool ! And the crowd cheers! Well done. (and it's an ECU!)

Buddo95

I've heard it called both Lol. So another quick question, I've got pressure at the rail and I've got spark but it still won't fire. If I've got spark that rules out cps doesn't it? What else am I missing?

DFlintstone

Electronic Control Unit, 87-90. 91 on, Power-train Control Module. I guess the only reason not to just say "puter'", is if you didn't state your year.

One reason for a no start with spark, compression and fuel is, it's flooded.

One reason it could be flooded:


There is a brittle little tube. It comes out of a rubber plug, low on the engine side of the throttle body. It runs back to the firewall, then up to your MAP sensor sort of high up there near center on the firewall. CRUCIAL!! The MAP will tell the ECU to dump fuel with less/no vacuum. Any crack or blockage and it will be pig-rich. (black smoke, fouled black plugs).

Has crisp, blue spark? I can check my CPS in about the time it takes to put on my shoes,, Btw...