what can an oscilloscope tell me when diagnosing an electrical issue in my 1973 911?

[Sunroof]

I remember as a kid going into a gas station garage and I could see those tall SUN Engine Analyzers that did some kind of magic providing vital info on the electrical and ignition system. Today its an oscilloscope and a laptop.

The shop is trying to figure out what's up with my 2.4 that has a bump or hesitation while running that they think is electrical. So what can the oscilloscope tell them and to what depth of analysis does it provide?

Bob

[FrankM_]

I use a (cheapo old analog) scope to check all kinds of things, but I have a 85 with the electronic injection (Motronic 1.x) :
- signal from AFM : continuous and (relatively) noise free (keeps having spikes on mine)
- signal from O2 sensor : lean/rich from 0.2 - 0.8 V and lively switching (difficult on voltmeters)
- fuel injector peak/hold signal form & duration correct on injectors (and possibly sticky injectors diag)
- speed sensor signal & reference sensor signal in limits and noise free
- signal from ECU to coil within required shape
- tach signal from ECU (low freq square wave)
- wheel speed sensor to speedo gauge (low freq square wave)
- ICV signalling from ECU
- with capacitive sensor around spark plug wires : spark plug trigger from coil/distributor - but haven't succeeded in getting a clean signal

Frank

[Sunroof]

Thanks Frank. I have the 2.4 with CIS, CDI, points, coil, etc, but I wonder if the device can analyze anything in the wire harness, coil and CDI output, voltage draw and resistance? I just hope they isolate the problem and fix the darn thing, but with a car that's almost 50 years old its anyone's bet!

[HarryD]

As a first reaction. Check your points.

[Trackrash]

Let us know how your scope diagnostics go. Is your hesitation consistent or intermittent?

I think a dwell meter and a timing light will tell you most of what you need to know. If you look at your motor running in the dark, any bad wires will be evident.

I had a hesitation in my car at one point. Weirdly, it was worse in damp weather. Turned out to be the rotor. It had a crack and would short out in damp weather. I don't know if a scope would have picked that up.

[tirwin]

Can the hesitation be consistently reproduced?

Can it be reproduced while stationary?

Can the shop interpret the data from the scope? The scope isn't going to do any analysis. You need grey matter to do that.

[Sunroof]

Top end of the engine was rebuilt, car starts up fine but hesitation well noted (no backfire). I thought it was fuel at first (mixture setting), but I took the car back to the shop and they tried changing out ignition wires and checked the new plugs. They think the issue is deeper so they decided to buy the oscilloscope to conduct more analysis. Points are fine as too the rotor and cap. Could be from the CDI to the coil or distributor, something in the fuser box (fuel pump gets power off fuse number 7), who knows? Thats why I wonder just what can the scope pick up.

[eastbay]

try a wiggle test and a water test on all your engine bay wiring, low tech, but tried and true

[jpnovak]

73. Is it a 73.5 with CIS or 73 with MFI?

Hesitation is either timing related or fuel related. different diagnosis procedures for each one.

If CIS check control pressures and fuel head piston for sticking. If MFI follow CMA to the letter. Neither one of these can use an oscilloscope.

For ignition you can use a scope to watch CDI input voltage, ignition input trigger (points signal), dwell (duration of points signal), ignition output (use a voltage stepdown from CDI or you will blow up the scope) and with an inductive pickup you can look at spark (coil) output.

There are more advanced diagnostics that can be done internal to the CDI if the ignition is still suspect but most shops do not know how to trace signals across the circuit board.

If this is the original CDI there can be old capacitors (dried out), over powered resistors and failing SCR. It is possible to check these with a scope.

[ClickClickBoom]

Might check the coil primary voltage and spark quality. If you aren't getting 12v at the primary side, the spark will be poor. If you have 12v at the primary side check the spark quality, if you arent getting a big fat blue spark the coil is bad. You can also check the ohms on the primary and secondary, but if the oil is gone it may ohm ok, but break down under heat and load.
Lots of people default to mixture, be it carbs or F/I, when the easiest, cheapest starting point is spark.

[jpnovak]

Careful here... The CDI coils do not have 12V on the primary side. These have approximately 250V input to the primary side and about 100:1 winding amplification.

Normal DVM and scopes will have problems if you put 250V directly into the inputs.

An inductive timing light will show a misfire during the hesitation and can be used to also check timing accuracy as well as advance curve.

@sunroof, Where is the hesitation? off idle, mid rpm, only on hard acceleration? Please provide more details.

[Sunroof]

Wow, lots of great input. My model 73.5T has the first CIS. The CDI unit was rebuilt, the coil is fairly new as too the wire harness between the CDI and distributor and coil. Points, rotor (6500 rpm limiter) and cap are new. Spark plug wires are okay.

Hesitation seems to be at the mid range rpm. The problem was getting progressively worse, which can mean a component is going. These guys at the shop will have to figure it out. Hell, electrical issues are a pain and with an old wire harness (rear engine harness replaced), it will be interesting to see how they trace it to the source. They do firmly believe its electrical as fuel pressures were spot on. The accumulator was suspect early on.

[Sunroof]

water and wiggle test? can you elaborate on that one, procedurally speaking.

Thanks

[jpnovak]

My guess is CIS mixture related. Check the WUR control pressures, cold and hot.

Does it always hesitate at light throttle, mid rpm?

I personally would pull the fuel head, clean it out, clean the plunger and check spray pattern of all 6 injectors. Often the fuel plunger on the metering plate can stick due to old varnish deposits. More throttle allows system fuel pressure to overcome the "stiction".

Is your shop familiar with CIS (K-jetronic)? Its a pretty simple system at that age.

[patkeefe]

I believe you can troubleshoot the CDI box with a scope. Look up oscilloscope and Early_S_Man in a search.

[kltarga72]

I had a 1972 911-S with 2.4 MFI. Purchased the car in 2008 and did the normal routine of replacing many parts. I replaced the fuel pump because it looked rusty and looked like the original. After several months of driving with no problems the engine developed a slight hesitation when the the rpm's were mid range. Replaced more parts, still intermittent hesitation. This problem occurred for the next year and I kept throwing parts at it. I was really concentrating on an electrical issue because it initially was a very slight hesitation, just enough to aggravate you. Finally one day I stood next to the car when the engine was idling and heard an odd noise coming from the fuel pump. Found out I had a Bosch fuel pump made in china, replaced the pump and problem disappeared.
Now I have two extra sets of spark pugs, wires, distributors, coils, solid state ignition, distributor caps, rotors, MSD 6Al, MSD Blaster coil, Bosch CDI, Permatune CDI, fuse blocks and books on troubleshooting electrical problems.
And yes I had good fuel pressure when ever I checked the gauge

Lash