Blown TCM fuse

Tiny
PSYCHO3328
  • MEMBER
  • 2000 PLYMOUTH BREEZE
  • 2.4L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 130,000 MILES
The TCM fuse blows every time I try to start the car. The car will not start. I have not sprayed starting fluid or brake clean in the intake yet. Also, I do not hear the fuel pump come on when I turn the key. Any suggestions would be appreciated.
Thank you
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Monday, September 25th, 2017 AT 6:28 PM

14 Replies

Tiny
CARADIODOC
  • EXPERT
A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a twelve volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it is not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

The problem you will run into is the automatic shutdown, (ASD) relay, fuel pump relay, when a separate one is used, and the transmission relay all only turn on for one second when the ignition switch is turned on, then not again until the engine is rotating, (cranking or running). It is not practical to be cranking the engine so you can take a series of test measurements. Instead, remove the relay for the affected circuit, then jump terminals 30 and 87 together in the socket. That will keep the circuit live, even with the ignition switch off.
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Monday, September 25th, 2017 AT 8:18 PM
Tiny
PSYCHO3328
  • MEMBER
Thank you. The info you gave was very helpful. I know I'm not that bright as the fuse that keeps blowing is the PCM, ASD.
Should I do the same procedure for this diagnosis
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Saturday, September 30th, 2017 AT 12:37 PM
Tiny
CARADIODOC
  • EXPERT
Yup. I just posted a reply to your other question to that effect. Most blowing ASD fuses revolve around shorted oxygen sensor heaters or the wiring going to the oxygen sensors. Look for a wiring harness that fell down onto hot exhaust parts. If everything looks okay, unplug the O2 sensors, then if the short appears to be gone, reinstall the ASD relay and fuse then try to start the engine. If an oxygen sensor is shorted and then unplugged, the engine will run, but a number of other fault codes will set relating to those sensors. Just erase those new codes after the repairs are done so they don't confuse anyone in the near future if another problem develops.
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Saturday, September 30th, 2017 AT 8:44 PM
Tiny
PSYCHO3328
  • MEMBER
Also the test light bulb did not change. It continued to stay bright and never got dim. Nothing changed when I turn the ignition key to crank the engin. There were no wires on the exhaust manifold that I could see. I did take your advise and did put the fuse in place and jumped across #30 and #87 with a test light. Nothing changed. Test bulb did not go dim under cranking/turning the engin over. When I jumped across the relay the fuse did not blow. Any other suggestions will be much appreciated.
By the way, thanks for your time on this. Good learning experience.
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Thursday, October 12th, 2017 AT 4:03 PM
Tiny
PSYCHO3328
  • MEMBER
Dang it. The first part did not post. Okay I jumper with the test light on the fuse for the PCM/ASD. Nothing changed when I crank the car. I jumped between Spade 30 + 87 and did the same test. Nothing changed. I then placed the fuse back into the PCM ASD and put test light between 30 and 87 on the relay. The fuse did not blow but the light did not change under cranking condition.
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Thursday, October 12th, 2017 AT 4:13 PM
Tiny
PSYCHO3328
  • MEMBER
I know this looks confusing if you need more info please ask for it. I'm between a rock and a hard place and really need your help. Thank you
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Thursday, October 12th, 2017 AT 4:15 PM
Tiny
CARADIODOC
  • EXPERT
The bright light in place of the fuse or the relay is showing it is getting the full 12 volts. That means it's getting 12 volts from the battery, AND a good ground, which is what we don't want. That's just a different way of saying there's a short to ground on that circuit. The stuff in that circuit has some electrical resistance. That is necessary to limit current flow to a safe, normal level. The short is bypassing that resistance. With no resistance to limit current flow, that current will be much too high, and the fuse will blow. Your test light or my test bulb has resistance. By sticking that in the circuit where current has to flow through it, the bulb limits current flow to a safe level. That's why the bulb is bright, and the fuse no longer blows. A brake light bulb will only allow about one amp of current to flow through it. Most bulbs in test lights pass even less current; typically 1/4 amp or less.

Regardless of which bulb you add to the circuit or where you plug it in, when you locate what is shorted and remove that short, current will have to go through the test bulb yet, and the normal circuit resistance. Think of the test bulb's resistance as one person standing on a garden hose. Water flow will be reduced because of your foot's resistance. The normal circuit resistance is like a second person standing on that garden hose. Water flow will be reduced even more. The short is like removing the second person, and their resistance, from the hose. As long as you still have your foot on the hose, (the test bulb in the fuse or relay socket), water flow will be restricted and limited to a safe level.

Even if that wondrous explanation is hard to follow, you found the important clue. The test bulb is bright, indicating there is a short in that circuit. The goal now is to not try to make the short go away, except when you are doing something that you can control and follow. I've had people bring me a car that blows a fuse once a month, and the circuit works fine the rest of the time. After I put in my test bulb, and see that it's bright, the owner pounds on the dash, or slams the hood, then says, "see, the short goes away when I do that". At that point I tell them they just removed the only clue I had, and they should bring the car back in a month when the fuse is blowing again. What do they expect I'm going to do when the short is gone? The fuse isn't blowing because there is no defect, and nothing to find. Grrr.

You have a permanent short which is a real lot easier to find because you will know exactly when you do something that makes it go away. The bulb will go out or get dim. Start by unplugging items in that circuit. I suspect the most likely cause would be an oxygen sensor heater grounded internally or to the sensor's ground wire. Unplug those sensors. The ignition coil and injectors are in this circuit. They are turned off by the Engine Computer removing their ground circuits, so even if one of those parts were shorted, the fuse wouldn't blow until you started cranking the engine. That eliminates the coil and injectors as suspects, but not the wires going to them. You can unplug each of those parts while watching the brightness of the test bulb. If the bulb dims, the better suspect is you disturbed a wire that was grounded when you moved that plug around.

The two small wires on the back of the alternator are in this circuit. There isn't much inside the alternator that can short, so if removing those wires makes the short go away, look closer at those wires.

I can't get into the online service manual web site for that last few days, so to tell you what else is on this circuit, I have to come up with suggestions from memory. Look for an EGR valve, a charcoal canister purge valve, or anything else that might get 12 volts only when the engine is running. If you do not have a separate fuel pump relay, the pump is powered through the ASD relay, then that becomes another potential suspect. Fuel pumps don't short very often. A better suspect would the wire going to it is pinched or rubbed through.

Look for a huge bundle of wires laying on the inner fender where they can slide back and forth as the engine rocks. I've seen those have multiple wires rubbed through the insulation and the paint rubbed off the inner fender. You might get that to act up by tugging on the car while it's in "park", to rock the engine.
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Thursday, October 12th, 2017 AT 7:39 PM
Tiny
PSYCHO3328
  • MEMBER
I'm an electrician by trade so I understand current and what it can and can't do. All I was stateing was that the test you suggested did not show a fault by disconnecting the o2 sensors.
Where on line can I find a good schematic of the circuit so I can find all components that apply to this circuit?
Thank you for all the help you've given me.
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Thursday, October 12th, 2017 AT 7:50 PM
Tiny
CARADIODOC
  • EXPERT
I listed the main ones. For online manuals, we use Mitchell OnDemand, and Alldata. Independent shops spend over a thousand dollars per year for unlimited access. You can buy a one-year or five-year subscription for just one car starting at around 26 bucks. I haven't been able to get into Mitchell for few days. When I can, I'll look for anything in that circuit I missed.
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Thursday, October 12th, 2017 AT 9:08 PM
Tiny
PSYCHO3328
  • MEMBER
Cool. Thank you for checking. The car is in a parking lot 15 miles from home so it's a little challenging to get time to work on it.
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Friday, October 13th, 2017 AT 5:56 AM
Tiny
KEN
  • ADMIN
Sorry about the access thing I hope we got it figured out in the mean time here are the wiring diagrams (below) for the transmission and TCM with a guide for anyone who needs it.

https://www.2carpros.com/articles/how-to-check-wiring

Please let us know what happens so it will help others.

Cheers, Ken
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Friday, October 13th, 2017 AT 1:08 PM
Tiny
CARADIODOC
  • EXPERT
Yippee! I got my nose in the diagrams. Thanks, brother Ken. First of all, there is a 20-amp fuse, fuse # 5, in the under-hood fuse box, that feeds the ASD relay and some other circuits. After the relay there's a 10-amp fuse # 1, also under the hood. Which fuse is blowing?

The 20-amp fuse feeds the coil side and the contact side of the ASD relay, and it's the feed circuit for the Engine Computer that's attached to the under-hood fuse box. I'd be surprised if the computer is shorted, but there is a diode in it between that circuit and ground. It's in there backward so it doesn't do anything until someone connects the battery backward, then the diode turns on and looks like a dead short to force the fuse to blow, thereby saving the computer. It's not common, but those protection diodes have been known to short. If it is shorted, your test bulb will go dim when you unplug the computer. That short will be there all the time. You don't have to turn on the ignition switch or crank the engine for the fuse to blow. It will blow as soon as you plug it in.

If that 20-amp fuse only blows when you turn on the ignition switch, the short is after the ASD relay. That relay feeds the large number of circuits, and one of them is protected by the 10-amp fuse. Just to double-check, remove the 10-amp fuse, then see if the test light stays dim. I suspect it will still be bright because if the short was on that 10-amp line, that's the fuse that would be blowing. That 10-amp fuse only feeds the heater circuit for the downstream oxygen sensor after the catalytic converter.

The other circuits coming off the ASD relay go to the alternator, the ignition coil pack, the front oxygen sensor's heater, the Engine Computer, (to verify the ASD relay turned on when it was commanded to), and the four injectors. All of those wires are dark green / orange.

The only other thing I wasn't aware of is a noise suppression capacitor. That is also fed with a dark green / orange wire, but the diagram doesn't say where it's mounted. The only clue I can find as to its location is the splice is listed as "engine wiring harness, in breakout to ignition coil". It says "TO" the coil; not "near" the coil. That splice is fed from a main splice listed as, "engine harness, near cam sensor breakout". To check the capacitor, if there is no connector to unplug, just unbolt it and hold it away from the engine.
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Friday, October 13th, 2017 AT 6:12 PM
Tiny
PSYCHO3328
  • MEMBER
Hi. It's been awhile. I jumpered the relay as you said then placed a headlight between the fuse and cranked the motor over. The bulb got brighter as I cranked but it did not start. If doing this test, shouldn't I hear the fuel pump come on? Or did I not understand the explanation you gave.
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Tuesday, October 31st, 2017 AT 6:04 PM
Tiny
CARADIODOC
  • EXPERT
It's unlikely the engine will start with the bulb in place of the ASD relay or the fuse. The bulb is bright during cranking indicating there's a short after it. That means everything in that circuit including the ignition coil pack and injectors will be getting fed with 0 volts instead of 12 volts. A different way of looking at this is you only have 12 volts to split up between the bulb and the rest of the system. The brightness of the bulb shows that most or all of the 12 volts is being dropped across it. That leaves 0 volts, or close to it, for the coils and injectors. The fuel pump is on a different circuit that is run by a different relay with its own fuse. That circuit should be getting its full battery voltage during cranking. Typically the noise from the starter makes it impossible to hear the fuel pump.
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Tuesday, October 31st, 2017 AT 6:39 PM

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