Help with PCM fuse that keeps blowing Electrical Issue

Terminator Dave

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OK. Here's the skinny. F-21 is the actual B+ fuse, 5A for the PCM. On pin 55, you should see 12V. You must have ground on pins 3, 25 & 55. The PCM will definitely not like an open end to a 30A fuse. Fuse 26, a 30A fuse, supplies voltage, and passes allot of current to other items that are inputs to the PCM. If fuse 26 is blowing, pull all the lines from the output side of the PCM relay. Something there is shorted.

Now, I can tell you for a fact that those straps on the back of one or more heads, NEED to be connected. If you don't have those lines connected, or the grounds I mentioned above, the PCM will find its on ground, any you may not like the results. That tends to burn up PC foil on the PCM, unless the ground line, the ones above, are connected. The company I worked for added ground fuses for just this reason. It is much better to burn open a ground fuse, rather than replace burnt components, or fix burnt foil.

I hope I didn't put anyone to sleep. If you need more help, I can help you in the car, and we can go over it together. Used to do that as well.
 

hotcobra03

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something says to stay in the ccrm area of harmess..

I'm expecting a short somewhere under air cleaner..

my thought..f26 splices(s108) at tearoff to c1232.

here 1 of the other wires in same harness is a signal from ignition,


the last short I chased..not my car,but I had to open harness..
sadly I never found the actual point..but once I opened harness short went away..I taped the line than rewrapped harness..all is well ..pos,they still use it 6yrs later..
 

Terminator Dave

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Does F-2 or F-8 blow? I'm guessing not. F-26 feeds the PCM relay. See if removing the PCM relay stops F-26 from blowing. That will tell you allot.
 

*Jay*

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F-26 feeds the PCM relay. See if removing the PCM relay stops F-26 from blowing. That will tell you allot.

Agreed, also when you remove it look for any signs of damage or excessive heat. Also give it a good shake and listen for any noise, it should be relatively silent.

I would also remove the battery cables, pull F26, pull the PCM Power relay and check if pins 8 or 10 are grounded to the body or negative battery terminal. Also follow the BLK/ORG wires from the CCRM to the battery junction box and check for any damage.
 

03' White Snake

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I had the same issue, it was the noise suppressor on the back of the head. The harness is live with key on. I'd follow it back in the harness and make sure it's not pinched or grounding out.
 

03' White Snake

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I had the same issue, it was the noise suppressor on the back of the head. The harness is live with key on. I'd follow it back in the harness and make sure it's not pinched or grounding out.
 

Toolsplusmore

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I have both noise supressors unplugged and have verified they are not grounded out. I will check pin 55 of the PCM for voltage as well as checking ground on pins 3 & 25. I was under the impression that the PCM relay is in the CCRM when that is removed the fuse no longer blows and the power probe switches from short to ground to open circuit. The same thing happens when you leave the CCRM in and remove the actual PCM. I will check pins 8 and 10 on the CCRM and see if they are grounded to the battery negative. I will report back my findings this afternoon when I get home from work.

Dave, what are you suggesting I try when you said "If fuse 26 is blowing, pull all the lines from the output side of the PCM relay." Thanks for all the help so far I still am puzzled but hopefully will get it resolved.
 

Terminator Dave

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That's OK. If you pull the PCM relay, that effectively unloads the output of the relay. The voltage pins from the relay, not the coil and ground, can't be shorted to ground. Try pulling the relay first, and let us know if the fuse still blows.

On another note, and I almost hate to bring it up, but those lines from of the back of the head, the ones bolted to the head, can't be hot. The block is ground. They are there for RFI, radio frequency interference, and again, are usually braided line. I would measure the lines, but my car is up in the air.
 

hotcobra03

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Screenshot_2018-09-10-12-53-12.png


look again..f26 feeds 8/10.

relay gets switched and 24 gets powered..

s169..see red circuit 361..
power goes straight to injectors and black box on head.
than feeds fuse 2/8 also red circuit 361..

if you pull fuse 2/8 install f26.

and try key again..
it looks like this will help to path of short.

unhooking all components that are on circuit 361 will help on if a component is cause of short
 

*Jay*

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Looks like you are correct about the relays being internal, take the CCRM out and do some bench testing. Pins 8/10 should be shorted together, 8/10 to 24 should be open when completely removed. If that checks good energize the relay on the bench. If you are able to apply 12 volts across 13/18 (13+, 1:cool: and see if it operates the relay i.e. closes 8/10 to 24.
 

*Jay*

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*Cant edit my post, not sure why the smiley showed up*

To bench test the relay apply 12v to pin 13 and ground to pin 18.
 

Toolsplusmore

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I will check all of the above when I get home, Dave are you referring to the noise supressors or a braided ground line? The things I unplugged looked like this.
 

BlckBox04

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#26 30a fuse is the one that will blow when you turn the key if you have a connection issue with a noise suppressor on the back of one of the cylinder heads.

I don't want to hijack his thread but is it possible that this would also cut the power and make the gauges sweep during a run? I'm also having an electrical issue, although it's not blowing fuses, in my 2-3 or 3-4 shifts where the car cuts out for a split second then comes back on.

I've blown fuses mid run but they were for the fuel pump
 

BlckBox04

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#26 30a fuse is the one that will blow when you turn the key if you have a connection issue with a noise suppressor on the back of one of the cylinder heads.

I don't want to hijack his thread but is it possible that this would also cut the power and make the gauges sweep during a run? I'm also having an electrical issue, although it's not blowing fuses, in my 2-3 or 3-4 shifts where the car cuts out for a split second then comes back on.

I've blown fuses mid run but they were for the fuel pump
 

*Jay*

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I would go over all of your battery power and ground lines/straps (batt, chassis, engine).
 

Toolsplusmore

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Jay took the CCRM out and 8/10 are shorted together out of the car and are open to pin 24 once 12v is applied to 13 and ground to 18 the relay clicks and 8/10 then show closed to 24.
 

Toolsplusmore

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With the CCRM out and a new fuse installed the fuse does not blow if the key is turned to run and the circuit tester shows that it is an open circuit at that point.
 

Toolsplusmore

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Hotcobra with fuse 2 and 8 out the fuse still blows and the circuit tester still shows “short to ground”. It does not matter if both are out or just one it still shows short to ground and blows the fuse.
 

hotcobra03

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c133..

engine harness..big black conector with 10mm bolt.

than just below that close to engine is another main connector.
c145?

try these 1 at a time..

trying to isolate the area .

I don't have pc today to say what connector circuit 361 goes thru.

also after second look,it was coils and black box's that s169 leads to(pg1 of 3) on small manual.
 

Toolsplusmore

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Hotcobra, I will check those two connectors when I get back home. The only part I am not sure of is that when you unplug those connectors aren’t you breaking the circuit at that point so the fuse can’t blow? At that point it would always show open correct? Regardless I will unplug both when I get home and will post he results.
 

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