Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Pulled Reverse Voltage Diode - Now Car Won't Start it

Hope someone can help me out here. I accidentally pulled the reverse voltage diode from the under steering wheel fuse box while car was running. (I know, shouldn’t have done rhat. It was dark and I thought it was the fuse puller). Car immediately shut off of course, but now it won’t restart once I put it back in. Cranks powerfully and no problems before, so I definitely caused this.

So I thought I might have created an OBD code that kept it from starting, so I shorted out the POS/neg cables for about 2 hours to try and clear memory, same problem. Checked fuel cutoff switch and it’s not popped. Tested the diode and it seemed fine. is there a chance that there’s still a code on it? Or maybe the diode blew? Is there something else I’m not thinking of? Help please. Losing my mind.

Edit: both reverse voltage diodes test out fine, and they can’t be installed backwards. Also, when trying to connect a diagnostic to the OBD2 port it just says “error” after it reads. But the port is getting power because my diagnostic isn’t self-powered. Finally, I noticed today that the check engine light does not come on when first turning key, and it did before. Double checked ECM fuse, but not sure where to go from there.


I’m happy that you mentioned pulling that out with the car running. This is good information for the next person who removes electrical stuff with engine running. Thank you for helping. My advice would be to do the thing you’re going to try next anyway. I hope that part is available new and you don’t have to find one at a salvage yard.

1 Like

Can the diode be physically installed backwards? If it can, try it the other way. The pictures I see online show that it can be installed backwards. If that doesn’t work, replace it.

1 Like

Thanks for the idea Mustangman. I should have mentioned but there’s a dummy-proof shape. Can’t go the other way. Car shops don’t have the diode anyway, I guess it’s pretty rare. Also, diodes in both dashboard and engine compartment test out correctly and exactly the same.

So you tested resistance forward and backward so you know it is OK? Or swapped out another to see if the diode is blown. I hope that is it… if a stray spark killed something else, focus on the diode you pulled and see where that lies in the circuit diagram.

BTW, I’ve never seen a diode in the form of an ATO fuse before, or really any diode outside of an alternator. Interesting!

Yes, swapped the diodes and no difference. They are right there in the two fuse boxes as if they are another fuse, but have the diode symbol on them. I’m afraid that when I pulled the diode, it caused the ECM/ECU to fry. But I don’t know how to test that.

You need a scanner to access the OBD2 info. Key-ON and it will try and connect to the ECU. A no-connect may identify a fried ECU.

I like ForScan for my android phone. It is very Ford-centric. Doesn’t cost much for the expanded functionality. You’d need a Bluetooth OBD2 plug in and your android phone or tablet to connect.

Also available for Apple phones and tablets.

That further confirms it…I have a scanner and it just says “error” when it tries to read. So ECU is probably fried?

Sure sounds like it is fried.

Check power supply to ECU. Maybe the fuse supplying it is open…

I would also check to see if the car will crank with the diode removed. Just to make sure it is installed properly and making contact. If it won’t crank with it removed, you can assume it is making connection with it installed and the car again cranks.

Thank you, good idea.
Fuse to ECM is good and double checked it had continuity. Diode in the engine compartment must be in place to crank, and it doesn’t matter which diode I use. There is power but car will not crank unless either diode is installed. Opposite under the dashboard. Doesn’t matter whether the diode is there or not, it cranks.

OK, now check to see if ECM is powered using your DVM. You may be able to probe the OBD port for the same power that feeds the ECM.

Ok, can you elaborate on how to do that? I need to know working diagram I assume. Might be getting over my head.

To see if the ECU is getting power just check the DC voltage on the power lead going to the ECU while the ignition is ON.

I find it strange that removing a blocking diode caused this problem. I have to wonder if the diode in question is really a zener diode. These special type of diodes are used to regulate voltage and operate in the reversed biased mode compared to normal diodes. They are rated to regulate a various voltages but each has its’ own voltage spec, like 12 volts or 5 volts, as examples.

It would be nice to know if the diode is working correctly while in the circuit. To check the operation turn on the ignition and using chassis ground as your reference, check the voltage on each side of the diode, one at a time. Let us know what the voltage readings are on each side of the diode while the ignition is on. If it is a zener diode then you will most likely see around 12 volts on both sides of the diode.

If the diode is designed like a fuse body see if there are some metal tabs on the top of each side of it, just like fuses are designed. If they are there then you can test the voltage on each side of the diode by placing your meter probe on each tab, one at a time, using ground as your reference point. I tend to think that the ECU is really okay but you may have damaged it possibly. We need to check all the power sources to the ECU before saying it is bad. It would be real helpful to have a factory wiring diagram to refer to for this issue.

Make sure you have battery power on pin 16 with the key on.

1 Like

OBD2 is definitely getting power. I can’t tell if power is also going to ECM.

The diode doesn’t have metal pins on top or side, so I can’t check while it is plugged in. However, when testing diode with multimeter outside of the power module it works correctly. I get a power drop going one way, and nothing going the other way. Furthermore, both diodes test exactly the same and I’ve interchanged them to verify results.

Interesting note about the diodes…the diode under the steering wheel that I accidentally pulled while the engine was running and caused this problem has absolutely no bearing on whether the car cranks. I can crank with or without it installed. However, the other diode, which I never touched, must be installed for the car to crank.

Another update, I have removed the ECM and opened it up. There is no obvious burn or damage on the inner circuit board. Now, I know this doesn’t mean it’s not fried, but figured I would see something. The software could be jarbled, but my mechanic friend hooked up his full diagnostic set and said that he should see if the software was bad. He just gets an error at OB2 and can’t read anything.

After looking into these diodes after I made my original post about it I see that the diodes seem to be just normal diodes. Hopefully the problem is just due to a lack of power getting to the ECU. It seems the data bus at least isn’t working for some reason and you need to find out why that isn’t working first. It would be good to know what we are working here and I don’t see any mention of what it is in this thread. If you haven’t made sure all the fuses under the hood and in the dash have power getting through them then I suggest you do that next. You should also get a wiring manual for the car so you can refer to it to see how things are wired together. Knowing the make and model may allow us to find some info on the net possibly.

Edit: I now see that the vehicle is a Ford Contour. What year?

Sorry if I missed that. It’s a 1998 and I tested all relays, fuses, and both diodes. I was hoping it was something simple that tripped and wouldn’t allow the car to fire.

It seems as though I’m heading to a wiring diagram to fix, in which case I’ll be calling a car electrician. It would be fun, but I don’t have the time to shoot wires.

Have you made sure that VOLTAGE is getting to all the fuses while the ignition is on? If you haven’t already made sure all the connections going to the ECU are connected I suggest you do that also.