Ignition fuse blows during cranking

Ignition coil fuse No.40 blows during cranking of the engine. 2004 Suzuki grand vitara it cranks but does not start. also check engine does not come on when igntion is turned on, but I get service engine light. reference volt is good 5.3 volt Ecm is alive and does turn fuel pump relay on for 3 seconds and shuts off as should be. My only problem Ignition fuse No.40 blows upon cranking and shuts the whole power circuit off to ECM,ignition coil, alternator etc. One more thing meter fuse and ignition fuse both show shorted and both share power side. Bulb gets illuminated when intalled instead of the fuse in either ignition fuse ( 40) or meter fuse ( is connrcted ti=o body module and AC control. Dear readet Can you see the attachment? lease let me know. if not I can fix it sorry I am new and getting familiar with the web site. My question what steps Ishould take to eliminate the source of short or component pulling toomuch current that is leading to blowing of ignition fuse 40.

Please refer to the attachment. Ignition fuse No. 40 blows also the fuse no. 43, 41, 19, and 40 all share power side being supplied by black /blue wire coming from Ignition switch. I removes the fuses from all the above fuse terminals and checked not the power side but the other terminal or leg of the above fuses and all show ground when tested with light test connected to battery positive. Does this mean all the above fuses terminals are shorted? Is it possible four fuses are shorted to ground at the same time. Also to make matter more complicated the ignition switch wires please see attachment Yellow /BLK and Blu/red wires also show ground when tested with the light test connected to battery positive. I removed the ignition switch to eliminate it as a suspect, bur the wires before the ignition switch also show ground. Does this mean ignition switch wires are also shorted to ground? These are too many shorts to be real or consequence. Last question if a fuse is only hot at on position ( when ignition switch is on position) does this mean this fuse show ground when tested if not energized i.e. when ignition switch is on off position and circuit not energized. The car does crank but do not start. I am looking forward to some ideas and guidance how to tackle this short thanks fred975.

Welcome to the forum Fred. I moved your topic to the repair questions section where you will get more help.
Please be sure to include the year, make and model of whatever you are working on.

On a power side switched circuit, one side of the intended load is connected to constant ground and the other side is waiting for voltage to be applied. You will measure ground on both sides of the load until the circuit is completed.
So while burning that fuse does indicate a short to ground or a component pulling too much current, the ground potential you are measuring on the component side of the other fuses sounds like a normal condition, at least at first take.

So the fuse holds with the key in run then blows when the engine is cranked? The most obvious difference between KOEO and crank is the coil will be switched in crank. So I'd try unplugging it first. Injectors would be next suspect if they're also fed by F40.

Hi Matt, All the coils have been disconnected and the injectors are on a different circuit. But still when I crank the engine stillthe fuse 40 blows. My suspicion is ignition switch circuit which feeds fuse 40. Can you see the attachment I have attached a copy of the power circuit to this thread.

That link is junk. No diagram

Hi Matt/Noah,

I have unplugged every thing ( igintion plugs, alternator etc) except the pcm and I have narrowed down the short to fuse no. 40 circuit. It is fuse no.40 feeds the followings
all igntion coils/sparkplugs
PCM
Alternatorr
noise suppressor

But as I said i have unplugged every thing except the PCM. But what is interesting the ground wire for coils and pcm all grounded together in one spot and with KEyOEO it illuminates a test bulb that has been inserted instead of the fuse 40 holder to alert me of the short. When I disconnect the ground wire of the Ignition coil circuit or the PCM ground wire the test bulb turn off. What is your understanding how can a ground wire causes a short. By removing the ground wire to the ignition circuit the bulb light turns off indicating the short is diconnected. one more observation with ground wire connected i measure 9.5 volts on the power wire of the ignition circuit and with ignition wire disconnected I measure 3.8 volt. There is hige volttage drop of the power wire when the ground wire is connected what is your understanding. Pls help. Please help giveme your ideas.

Is there any kind of remote start or alarm system spliced into the ignition circuits that you can see?

No but I am suspecting the ignition switch to be shorted beacuse the S wire has voltage with just the kery on engine off and there should not be voltage unless it is in start position ( cranking position). do you agree? is it possible the ignition switch to be the source of short?

Any other tips why I see the short in fuse 40 is seen when I place a test bulb instead of fuse 40. What else should i Look at?

Fred975 wrote:

Any other tips why I see the short in fuse 40 is seen when I place a test bulb instead of fuse 40. What else should i Look at?

Possibly because there is a path to ground through the control side of the oxygen sensor heater relay.
You can remove that relay and see if the bulb doesn't quit.
If there was a true short to ground in that circuit, the fuse would open with the just the key on. It wouldn't matter if the engine was cranking.

Thank you Noah for good tips.I do really appreciate you took the time and pulled the diaggram on line. As you see in the diagram that Fuse 40 feeds a lot of components.
PCM
O2 sensor relay
Ignition coil
Alternator
Fuel pump relay
I agree O2 sensor has solid ground but I did remove the O2 relay and I even cut the Blaclk and white wire ( power wire from F40) to isolate the short. Also I have cut the black /White wire feeds the ignition coils to isolate the short so Right now I have only power going to fuel pump relay and Pcm left connected, but still the bulb illuminates. I tested the wire to PCM and it is not grounded but still the bulb stays lit. Talking to you helps me think out of the box.Please keep suggesting other tips and hopefully with your guidance I isolate the short. Thanks agian.

Interesting...
And with this wire cut, does the fuse also open while cranking the engine?
I'm having trouble finding a detailed power distribution diagram. The color one from Mitchell just has an arrow that says "engine controls, starting/charging".

Hi Noah,

The short is still evident anb the light bulb is still lit. The answer is yes. How did you paste the diagram?I have the power distribution diagram I use Alldata they hve all diagrams for 2004 Suzuki grand vitara 2 wheel drive ( rear wheel drive). May goal to is shut off the that bulb light becaues it show the short still active. Do you agree?

Just to be perfectly clear, the answer is "yes, the fuse continues to blow"?
The goal should be to get the car running, I honestly don't care much about what the bulb thinks if the fuse doesn't blow...

You will have to save the diagram as a .png .bmp format, then click the tab that says attachment, find the file you want to upload, then click insert.

Hi Noah,

I have already lost one PCM and I am on my econd computer because of this short issue and fuse blowing. That is why my priority is to take care of the short and get the light bulb to turn off first otherwise I will burn the second PCM. As you may know for Suzuki since nolonger is made in USA it is hard to find parts. But I do appreciate you are providing tips to guide me through to find a resolution for the short. do you have the ignition circuit what else could be causing the short? thank you so much. Do you need tme o send you the power distribution.

I understand your apprehension. I am not convinced by the results of the bulb test in this case however. Correct me if I've misinterpreted your posts. You stated that with the key on, with a bulb in place of the fuse, the bulb lights. But with the key on, and the fuse in place, the fuse doesn't blow. Only in the cranking position does the fuse blow, so only in the cranking position is the cisuit over loaded. So we can't exactly say the bulb is lit with the key on for the same reason that the fuse is blowing with the key in the cranking position.

I agree that is the dilemma. with Bulb in place and Key on, the bulb is lit but not blowing any fuse. Also as you stated when the fuse in place is not blowing with key on position, only in cranking position. If the circuit is overlaoded is it possible the ignition switch causing the overload and messed up causing the overlaod. I forgot to mention the S wire at the starter shows voltage of 8 volt with key on only. The S wire is not supposed o have voltage with only key on, it issuppose to have voltage only in cranking position.

I had a truck blowing a PCM fuse yesterday because of a short to ground and it made me think of the problem you're having.
I found it with an ohm meter in the fuse box on the leg of the fuse that feeds the PCM

You can see here it's a dead short to ground.
Not sure what a good circuit would look like, I did the same test on the TCM fuse that did not have a problem.


I then reset the min max values and began wiggling the harness.

You can see the max value changed when I was close to the short.

This test may or may not be helpful. While I agree the bulb should not be lit during your testing, I'm still very confused as to why the fuse only blows while cranking...

Thank you Noah for thinking of my issue. Can I ask you this test indicates the wire that is feeding the PCMis shorted or touching the metal part some where? So let us suppose that would be the case in my Suzuki too once I do this test. if I by pass that the wire that feeds the PCM then ,the short should go away and the bulb should turn off do yuou agree? Regards. Fred

While I do agree in theory, I'm trying to get you away from the bulb test to try and quantify the short condition with the key on engine off that for some reason doesn't blow the fuse.