Engine problems after touching the battery terminal with the ECM of my 1994-1995 BMW 325i
We wrote this article because customers ordered a performance chip for their 1993, 1994, or 1995 BMW 325i/525i and, after installing the chip, accidentally touched the car battery’s positive terminal to the metal chassis of the DME, which is grounded. After that, the engine started having different problems, like a very low idle, turning off by itself, low power, and so on. Such problems persisted even after replacing the DME (ECM or ECU).
Avoiding the problem in the first place
The key here is to ALWAYS disconnect the car battery before doing any electrical work. Especially when working with the DME, because the battery is in the DME’s way. Whether you’re inserting or removing the DME from its bay, you risk touching the car battery terminals with the DME’s metal chassis. So please at least disconnect the negative terminal before doing any work.
Symptoms you will notice
If this has already happened to you and your car’s engine is having problems like the need to keep it accelerated to prevent the engine from shutting off, or if it is very hard to start the engine when cold, or if the engine is consuming much more gas than normal, then do not replace the DME.
The culprit
Replacing the DME will not fix the problem. Even when you touched the battery terminal to the DME’s chassis and sparks flew, another part got fried. It was the AFM (Air Flow Meter). On very rare occasions, the DME could have sustained some damage because of the short circuit that occurred, but most of the time, and I would say 99% of them, the problem will be a damaged AFM.
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A test you can do
To do a simple test, just disconnect the AFM connector and start your car. If the engine problems seem to go away, or at least aren’t as bad as before, then you can be sure you need to replace the AFM, but not the DME or any other part.
What is the AFM
The AFM is a device used for measuring how much air is entering the engine, in this case, by volume. The DME needs this information to calculate exactly how much fuel to inject based on incoming air under different driving and environmental conditions, and to maintain a proper air/fuel ratio (AFR).
Location of the AFM
The AFM is located in the incoming air pathway between the air filter and the intake manifold, and it has a rounded electrical connector. If you stand in front of the engine, you will find it on your right, near the intake manifold. Check the picture below as a reference.
Below is a picture showing a couple of AFMs used in the 1993-1995 BMW 325i and 525i. To disconnect the flowmeter’s 4-wire terminal, twist its cover counter-clockwise (like opening a water bottle) and pull it out.
Below is an example of the 1993-1995 BMW 325i / 525i AFM module, so you can have an idea of what it looks like.
Replacement
You can find a replacement AFM at an affordable price by searching eBay or a recycled/refurbished parts shop. A new AFM will be expensive and will do the same job as a good, used one.
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