No start and fuse 18 and 27 blown

M

Mercury

TJ Enthusiast
Joined
Jul 18, 2018
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235
Location
Louisville, KY, USA
Hello!

1999 TJ 4.0 manual. At 2am today I was driving home, stopped at a light, and when I pressed the gas to move the engine stalled and would not restart. It cranks hard, battery is fine, but wouldn't start.

Today I checked my fuses and it looks like #18 under the hood, labeled as "fuel injector", was blown. I replaced it, tried to start the Jeep, the fuse blew again. Discovered that fuse #27, leak detection pump and O2 sensor, was also blown. Replaced both and the engine fired right up.

I checked the fuel injector wiring and I don't see anything exposed or rubbing.

Short of going over the entire harness what would be the next place I should check?
 
Both of those fuses are powered by the Automatic Shutdown Relay (not necessarily relevant to the issue, just pointing it out). Fuse 18 powers all six fuel injectors (the switching of the injectors by the PCM is done on the ground wires for those injectors) and the coil. Fuse 27 powers both oxygen sensors and the leak detection pump. Here are the relevant pages from the Factory Service Manual (found in the TJ Resources section here):

1719263102195.png


Since you're blowing two fuses, I'd look for chafing or other damage to the harness in areas where the wiring for both circuits is in the same section of harness. That seems the mostly likely cause of two fuses blowing. Unfortunately, the only way the manual is helpful for figuring out which circuits are near each other is by looking at the connector and splice locations, which I circled in green above. Go to Section 8W-90 in the manual to look up the appropriate figure for each splice/connector, and then look at the figures. From there, you may be able to discern common sections of the harness for the two circuits.

And I don't think the fuel pump can blow those fuses. It's on a different circuit fed by Fuse 19 in the PDC.
 
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macleanflood said:
Fuel pressure?

Got a clamping ammeter? I'd check the amperage that fuel pump is pulling.

-Mac
Click to expand...

Wish I had a clamp, but I don't. I have a DMM and an o-scope.

sab said:
Both of those fuses are powered by the Automatic Shutdown Relay (not necessarily relevant to the issue, just pointing it out). Fuse 18 powers all six fuel injectors (the switching of the injectors by the PCM is done on the ground wires for those injectors) and the coil. Fuse 27 powers both oxygen sensors and the leak detection pump. Here are the relevant pages from the Factory Service Manual (found in the TJ Resources section here):

Since you're blowing two fuses, I'd look for chafing or other damage to the harness in areas where the wiring for both circuits is in the same section of harness. That seems the mostly likely cause of two fuses blowing. Unfortunately, the only way the manual is helpful for figuring out which circuits are near each other is by looking at the connector and splice locations, which I circled in green above. Go to Section 8W-90 in the manual to look up the appropriate figure for each splice/connector, and then look at the figures. From there, you may be able to discern common sections of the harness for the two circuits.

And I don't think the fuel pump can blow those fuses. It's on a different circuit fed by Fuse 19 in the PDC.
Click to expand...

That's incredibly helpful, thank you. I'm not very good with this FSM, but I'll do some digging and try to figure it out.

Would you explain what the red arrow is referring to?
 
Update: tried to start it, blew 18 again.

A buddy suggested testing if it was a failed injector by unplugging the harnesses to each fuel injector, then one at a time plugging injectors in and trying to start it. I unplugged all of the injectors, tried to start, fuse blew.

27 is fine.
 
Mercury said:
Update: tried to start it, blew 18 again.

A buddy suggested testing if it was a failed injector by unplugging the harnesses to each fuel injector, then one at a time plugging injectors in and trying to start it. I unplugged all of the injectors, tried to start, fuse blew.

27 is fine.
Click to expand...

Inspect the wiring harness to the injectors.
 
Mercury said:
Would you explain what the red arrow is referring to?
Click to expand...
There are two pages there. Page 8W-10-11 on the left and page 8W-10-12 on the right. The red arrow that I drew is how you continue from one page to the next. The triangle with the B in it shows a connection, and the note tells where. For instance, the B on page 8W-10-11 says "TO S126 (8W-10-12)", so you go to 8W-10-12, find the B and follow it to S126.

Mercury said:
Is there any reason why having #27 blown would cause #18 to blow when I started it, but when it was replaced not blow #18? I feel that's important but I'm not sure how.
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That is why I suggested looking for a section of harness with both circuits in it. That's how it's important. Chafed harnesses behave exactly that way - intermittent problems that shouldn't be related, but are. That's only one possibility, though.
 
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sab said:
There are two pages there. Page 8W-10-11 on the left and page 8W-10-12 on the right. The red arrow that I drew is how you continue from one page to the next. The triangle with the B in it shows a connection, and the note tells where. For instance, the B on page 8W-10-11 says "TO S126 (8W-10-12)", so you go to 8W-10-12, find the B and follow it to S126.


That is why I suggested looking for a section of harness with both circuits in it. That's how it's important. Chafed harnesses behave exactly that way - intermittent problems that shouldn't be related, but are. That's only one possibility, though.
Click to expand...

Ahhh, okay. Thank you for that. If the rain isn't too bad tomorrow I'll start crawling. After I decipher this FSM a bit.

Here's something I thought of: I drove 5 hours yesterday and the Jeep died at the end, but before I left I tried changing out my battery. The battery I put in turned out to be a dud so I put my old battery back in. I wonder if I knocked anything around there.
 
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