• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Latch Relay/Circuit?

R

Rob Dimel

NAXJA Forum User
Location
Texas
Some background- 89 Laredo with an A/T has crank/no start. The fuel pump doesn’t prime and there ‘s no spark. It’s killing me because it’s my kid’s daily and we haven’t been able to get it started since January. I think it started when he was trying to trace some wiring for the A/C. Not sure if he shorted something or what. In any case, i’ve refreshed all the grounds per Cruiser’s tips, checked and/or replaced the crank position sensor, MAP, TPS, coil and ICM. Nothing.

I was triple checking all the wiring to the relays. The B Latch relay has 12v to tge red wire, but nothing at either of the pink wires. I checked them at the relay and at the plug out from the PCM. Nothing at either end. Checked with key in run and while cranking. Does that mean the PCM isn’t powering them, and could that be our issue?
 
Not sure how you have power on the red wire, but not the pink since the pink is the trigger and red feeds out. No spark would make me suspicious of the crank sensor. Don't assume a new one is good, btw.
 
I swapped PCM and now I get spark. The relay cycles as it should on power up. The fuel pump relay clicks but the fuel pump isn’t priming. If I run direct power to the pump, it kicks on, but not through the relay. I’m only getting 7V at the pump, and that’s after bypassing the ballast resistor. I’m wondering if that 7V isn’t enough to kick it on.
 
Can't post FSMs here, but on the diagnostic connector, the smaller one is D1. The two on the outboard side, front of vehicle are D1-5 (red) and D1-6 (orange). D1-5 should have 12v from the battery. D1-6 is direct to the fuel pump. If you jump the pins, the fuel pump should engage.

You can try swapping around the relays.

Q8pzVnj.jpg
 
I have an FSM. That thing has been invaluable. I’ll take a look at those ports, and I’ll try another relay. I should have tried a different relay. I should have tried a different relay first off.
 
My D1 pins are a little different than the pic. I have connections in 4, 5 and 6. My diagram says 4 is the anti-lock module, 5 is battery and 6 is blank on the diagram. I jumped 5 and 6, didn’t do anything. If I pull the orange wire off the relay and run it direct to battery, fuel pump runs (without the key on). Tried three relays, nothing got the pump to prime or run unless I direct connected it. We direct connected the pump and cranked it. Didn’t start. Checked that I’m still getting spark, and I am. I feel like we’re close, but this is killing me.
 
So the above was from my '88 FSM in the Fuel Injection System - Multipoint section. I don't have ABS so the diagram matched my posted picture of my own XJ.

Sounds like you have some wiring issues. That Pin D1-6 is spliced to the same orange. Have a look at the wiring diagrams.

I had an intermittent misfire on #3. Traced it to a bad splice inside the main bundle along the valve cover. Literally took months to finally isolate the issue and about 15 minutes to fix it.
 
We’e redone every splice we could find. I have the entire harness pulled out of the split loom right now so I can check everything. It’s killing me because it’s my kid’s daily driver and it’s been stuck in the garage not running for three months. Going to have to just keep probing everything I guess.
 
Not sure what else to suggest at the moment. Keep posting updates, even with no progress to keep your thread on the 1st page.

Perhaps someone will come along.

What PCM did you get and from where?
 
It’s a MasterPro reman computer. The one that came out of it was also a reman. Looks like it was a MasterPro too, based on the labeling.

So, we traced wires, tried cranking it several times the other night. I tried direct wiring the fuel pump to see if we could get it to start. It didn’t. Think it might have flooded. Tried a little starter fluid. It sputtered, backfired, and grenaded the muffler.

I’m on hitch at work for the next week out of state, so I’ll have to wait to dig at it any more myself. In the meantime, my son is continuing to trace wires via the wiring diagram.
 
My son found a splice that was corroded, and cut it out. He soldered it and put heat shrink on it. That solved the fuel pump issue. Now it primes as it should. So, we have spark and the fuel pump is working. He tried cranking it, but still didn’t start. Not sure how long he cranked it, I’m thinking it will take a bit after sitting a couple months.
 
Nice progress!!!

Where was the splice located?
 
Still not starting. Have spark (from coil and checked each plug wire), have fuel (40 PSI and each of the injectors looks to be firing with a noid light). Any thoughts?
 
Frustrating!!

Let's go back to basics. You need fuel/air, spark and compression.

The each fuel injector gets 12v from the ECU when it's time for it to inject fuel. All of the injectors share a common ground. Check the ground at each injector. The ground also should connect to Pin D1-3 on the diagnostic connector.

You can try spraying some starter fluid down the TB while cranking. If it lights up (maybe not actually run), then the issue is in the fuel injector circuit.

The latch relay not having voltage at the pink is concerning. It should have power with the key on. The latch relay, Pin 5 (pink) gets power from the ECU and then grounds back through the ECU. The Pink wire on pin 4 gets power once the relay closes. You should also see voltage the the Diagnostic connector D2 (large one) Pin 5 (also pink). The FSM shows a splice joining the ECU, Latch Relay and Diag Conn pink wires. The latch relay remains energized for a few seconds after key turn to allow the ECU to reposition the IAC for next startup.

Have you tried swapping some of the relays?
 
Last edited:
The latch relay and all of the relays are working. The pink wires show 12v since we swapped ECU. I tried starting fluid and it backfired at the exhaust and literally blew the muffler apart. Split it like a beer can. If he cranks it even without starter fluid, it sputters and backfires. It’s a whole lot closer to starting than we’ve been in months, just won’t actually start.
 
You haven't messed with the distributor, have you? If removed and incorrectly indexed for reinstall, could screw up timing royally.

Also, since this is a Renix, it might be worth checking the shear pin holding the drive gear onto the distributor - it caused member EcoMike quite a bit of trouble trying to track down a persistent misfire (and later high-idle backfiring) a few years back - LINK
 
Good call. Also check the plug wires to make sure they're in the right firing order.
 
Rob Mayercik said:
You haven't messed with the distributor, have you? If removed and incorrectly indexed for reinstall, could screw up timing royally.

Also, since this is a Renix, it might be worth checking the shear pin holding the drive gear onto the distributor - it caused member EcoMike quite a bit of trouble trying to track down a persistent misfire (and later high-idle backfiring) a few years back - LINK
Click to expand...


Yeah, we actually did remove the distributor. We indexed it based on Cruiser’s info, but I’m not guaranteeing it was done correctly. The sheer pin is a good call, I’ll have to take a look. Thanks for the continued information. I appreciate y’all.
 
You must log in or register to reply here.

Similar threads

P
Fuel pump will not prime
2
Replies
35
Views
226
Patrick dimel
P
D
1996 4.0 No Crank, Starter Works When Jumped
2
Replies
20
Views
291
DirtySouthXJ
D
I
2000 Cherokee XJ Misfire Mystery
Replies
12
Views
190
Tim_MN
Tim_MN
Z
1990 renix electrical issues
Replies
3
Views
89
cruiser54
C
R
No spark
Replies
1
Views
1K
Ecomike
Ecomike
Back
Top