The Automotive Oscilloscope

Look in the back of the auto shop, you might see an old oscilliscope

When I started working on cars in the 1980’s there was always an Oscilloscope in the shop. These were usually the big Sun type machines that would reside in the back corner of the shop, often gathering dust. I remember when I started in the business asking some of the mechanics how to use the machine in our shop. Most of the guys I asked would just tell me “don’t worry about that machine, you will never use it”. 

Plug in Diagnostics

In the 1980’s Porsche was using the 9268 LH/EZK tester which was soon replaced with the 9288 Bosch Hammer for diagnostics. The 9268 LH/EZK tester was only used on the 928 models and gave very limited information. The 9288 Bosch hammer was a step up from the 9268, but by today’s standards was still a very simple and limited tool.

the Automotive Oscilloscope, 9268 LH/EZK tester
9268 LH/EZK fault code reader used on the Porsche 928 Models

What it signaled though was the death of a lot of the older tools that were used in routine maintenance and diagnostics. Mechanics began relying on these newer tools to tell them where the problem is, and what to replace.

The automotive oscilloscope, Bosch Hammer Porsche tool number 9288
Bosch Hammer 9288 Used on 911, 928, 944, and 968 Models from 1989 on

The old ways

In some ways I was lucky, in that I was able to work with a couple of mechanics that knew the older tooling and how to use it. They were also not shy about showing me how to use it. The automotive oscilloscope is a powerful tool that will give you so much data on a car. However, in older car repairs today it is so underutilized. Most shops that I speak to today don’t have an oscilloscope and often the ones that do don’t know how to use it.

Modern oscilloscopes today have shrunken to hand held tablets and most of the ones that we find in an automotive repair shop are the Snap On or Bosch branded units. Unfortunately, these units are marketed towards todays modern computer controlled vehicles and not used on the older cars. These units are also very expensive, often costing north of $5000.00 depending on the model and the options.

Extra Information

These types of oscilloscopes are still good tools, but the cost makes them unreachable for most mechanics and shops. One of the selling points of these units is that they give you all of this extra information built in to them. Most of these units will have pin locations and connection points, they will also show you what the wave form should look like. Some will even have technical service bulletins built in.

The problem is for the old Porsche models this does not do us any good. Most of these units only go back to 1981 model vehicles, and usually only Domestic models at that. So, this means for us to buy one of these testers we have to still pay for all the add on’s, but if your shop doesn’t work on the covered vehicles it is wasted money spent.

What I Use

I have three different types of oscilloscopes in my shop. I have two bench top units and one portable unit. My bench top units are both Siglent branded units and are an SDS 1052DL and a SDS1102X models. These are both Labscopes, which means that they are designed to be used in a laboratory environment. Now both of these two scopes are two channel units, which means that you can watch two different signals at any given time just like the Snap On units. The big difference is the price, the SDS 1052DL sells for about $260.00 and the SDS 1102X for about $484.00.

SDS 1052DL
Siglent SDS 1052 DL Labscope
Click the Picture to view on Amazon

So, what’s the catch? You have to know how to connect and adjust the scope for it to work. The Snap On version has already locked out most of the adjustments of the scope itself. Instead they will just tell you to connect line “A” to pin ”x” and line “B” to pin “y”. learning how to use these tools is really quite simple and if you are willing to put in the time, they will give you a wealth of insight into the vehicle.

Siglent SDS 1102X Labscope
Siglent SDS 1102X Labscope
Click the picture to view on Amazon

On vehicle Oscilloscope

The other scope that I use is the Hantek DSO1062B. It is a battery powered combination two channel scope with a built in multimeter as well. I also have two accessory units with this unit, the Hantek CC-65 Current Probe and the HT-25 Ignition system adapter. This compact portable unit allows me to run a complete system check on the car. I can use the multimeter to measure basic resistance readings and voltages, and the oscilloscope to look at secondary ignition and primary level voltages. This scope cost me $356.00 and the current sense and ignition monitor cost another $75.00, bring the grand total around $430.00.

Hantek DSO1062B hand held scope
Hand Held Hantek Digital Oscilloscope, DSO1062B
Click picture to view on Amazon

How I Use My Oscilloscopes

I really like my hand held unit and use it all the time. The only way for us to see electricity is by using a scope. Most people think that the scope is only going to be used on the secondary ignition.  I use my oscilloscope to monitor all different kinds of circuits. When using a scope, I can see if the charging circuit is smooth, this indicates that the alternator regulator is working correctly or not.

Technical Service Bulletin CDI Tachometer(Opens in a new browser tab)

The automotive oscilloscope
Coil firing event showing a Double Coil Pulse. This was due to an air gap that forced the energy to jump the gap before passing through the coil

I also use it on trigger circuits and component power circuits to check for interruptions or anomalies. The screen shot above, shows a coil circuit primary issue, where there was a gap in the circuit. The screen shot below shows a CDI power circuit that was dipping from 12.5 volts to 11 volts every 65 microseconds. Turns out someone wired a fuel pump into the circuit and every time the fuel pump pulsed; it took power away from the CDI box. Without an oscilloscope finding these types of issues would have been impossible.

Power supply being interrupted
The yellow line shows the +15 line to the CDI being pulled down due to an aftermarket fuel pump being installed and tapping the CDI power supply

Learning to Use an oscilloscope

Learning to use these types of oscilloscopes is really quite easy. Once you have mastered the settings, which can seem a little daunting at times, these units will give you a wealth of information. Especially today with the cost of these units being so affordable it just doesn’t make sense to be without one.

SIGLENT SDS1204X-E Super Phosphor Oscilloscope 4-Channel 200 MHz 1 GSa/s 14 MB
Hantek DSO5102P USB Storage Oscilloscope Oszilloskope 2 Channels 100MHz 1GSa/s
SIGLENT SDS1204X-E Super Phosphor Oscilloscope 4-Channel 200 MHz 1 GSa/s 14 MB
Hantek DSO5102P USB Storage Oscilloscope Oszilloskope 2 Channels 100MHz 1GSa/s
SIGLENT SDS1204X-E Super Phosphor Oscilloscope 4-Channel 200 MHz 1 GSa/s 14 MB
SIGLENT SDS1204X-E Super Phosphor Oscilloscope 4-Channel 200 MHz 1 GSa/s 14 MB
Hantek DSO5102P USB Storage Oscilloscope Oszilloskope 2 Channels 100MHz 1GSa/s
Hantek DSO5102P USB Storage Oscilloscope Oszilloskope 2 Channels 100MHz 1GSa/s

If you or your shop already has an oscilloscope pull it out and use it. Hook it up to the battery and watch the charging system work. Look at what happens when a relay triggers. Knowing what the electrical signature looks like when everything is working will help you identify when something is not right. So keep practicing.

Understanding Ignition Timing(Opens in a new browser tab)


12 thoughts on “The Automotive Oscilloscope”

  1. I really appreciate the examples of using a scope to evaluate the CDI system. With the original CDI systems being as much as 50 years old, the ability to properly analyze them with an oscilloscope has never been more important. Ideally, replacement of failed CDI components would only be done after such analysis is completed. This would help to prevent premature failure and/or underperformance going forward.

  2. Thompson Griffin

    Thanks for the great information for us.
    Best regards,
    Thompson Griffin

  3. Mead Henneberg

    Thank you so much for doing the job here, everybody will
    surely like your post.
    Best regards,
    Mead Henneberg

  4. That is interesting. I’ve just ordered a ‘scope to fault find a Mercedes that’s consistently burning out it’s EZLs (ignition control modules). At 1200€ each, this cannot continue.
    My feeling is that the burn-out is taking place on engine start as I recall hearing the thing fire just once before death. I may be wrong.
    I shall start with examining the 12v feed during start-up and take it from there.
    Thanks again.

    1. Ray,
      A couple of things for you to keep in mind. look for poor ground connections as these will increase the current demand. Also make sure that the secondary shielding on the ignition wires is correct. Often bad secondary systems can cause damage to control units. So keep an open mind and let me know if we can be of help

      1. Thanks Kurt.
        Yes, all those things are considered and instructions to the garage on Thursday when they install a new alternator and starter motor will be, “Please check, clean and reinstall any and all earth connections you can find”.
        I have done that for all those that I could access from above.
        I installed new spark plugs, wires and coil in preparation for this latest phase of work. We also removed, cleaned and reinstalled all the earths I could see. All readings from negative battery to engine, diagnostic socket, areas close to the EZL all now read well below 1 ohm – closer to 0.2 in most cases.
        I’m at a bit of a loss after successfully maintaining the ignition system in this car for over 20 years. Then 3 costly failures in the last year.
        Life can be a challenge.
        I’ll keep you posted.
        Thanks again.

        1. Ray,
          One of the things you might want to add to your testing is doing voltage drop tests on top of the resistance test. A voltage drop test is a way of testing the circuit while it is under load and in use. This allows you to comfirm the circuit while it is actually being used. Voltage drop accross the circuit should be 0.10 volts or less.
          thanks and good luck

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