Carpokes

The OP has an 83 so this idle adjustment does not apply. Good video and info for us later 944 folks though.

Yeah no diagnostics, so i'm not sure how to disable the idle stabilization. I know Clark's has this procedure http://www.clarks-garage.com/shop-manual/fuel-08.htm . However, Clark's only mentions the one idle screw adjustment and not the other that changes the stop for the cam.

The early cars do not have an idle stabilization valve. Idle bypass air is regulated instead by an Auxiliary Air Valve which reduces the amount of idle bypass air during the warmup cycle.

So back at this, i figured i could grab the signal off the back of the tach and just ground out to the battery for the O-Scope, i cant seem to get any readable waveform. Anyone have any experience diagnosing the tach signal on an early car? There’s 2 green wires which from wiring diagrams i figured either one is the incoming signal, one comes from the speedo and the other should come from the dme.

delgadostarside wrote: Fri Nov 01, 2024 2:33 pm So back at this, i figured i could grab the signal off the back of the tach and just ground out to the battery for the O-Scope, i cant seem to get any readable waveform. Anyone have any experience diagnosing the tach signal on an early car? There’s 2 green wires which from wiring diagrams i figured either one is the incoming signal, one comes from the speedo and the other should come from the dme.

Are you just looking for rpm's? You could always pick it up at the coil instead.

Yeah just looking for rpms right now since both my tachs won’t work right. Tryna get the idle dialed. Would i pull the signal off the green and the black of the coil or green and battery ground?

Green pulses. Go from green to chassis ground with your tach.

So, I think this segment of testing deserves a writeup of its own after. Doing some math in a spreadsheet. We can expect for a single cylinder in a 4 stroke, to fire the coil with 120ms between each pulse for 1000RPM (Given 2 rotations is 1 firing of the coil, converted from 1000RPM to 500 pulses/min to 8.33 pulses/sec to 0.12 sec/pulse to 120 Ms/pulse). Now this is for a single cylinder. Given its a 4 cylinder, I assume we should expect a pulse every 30 Ms for 1000 RPM then since it should fire every 180 degree crank rotation. Now when i throw my O-scope on the coil I'm seeing more like 58 ms between pulses! Here's a video of it running with the O-Scope (Timebase is 10Ms)



I'm not sure whats going on here. The car is audibly running much higher than what the pulses suggest. Anyone have better knowledge on this? I'm sure i'm misinterpreting something here. Regardless, while the time between pulses is so high, the tachometer also reads around 500 rpm which is what you would expect for 60Ms between pulses. The car is definitely not running at 500 RPM but i wonder if the car was trying to tell me this all along and the tach was fine its just the coil firing wrong? Is this a DME issue?

Note; that clunky sound you hear at the end, its either coming from the clutch or the trans, not sure what is though but that's another story for another time.

Well that's weird. Is the DME and ignition all stock? Where are you picking up the pulse? The motor sounds like it is running normally (i.e., one spark for every power stroke) so it sounds like the coil is getting a different pulse than the tach and scope? I'm wondering if the pulse to the tach is somehow messed up (in the harness, DME or inside the tach itself)? Maybe disconnect the tach, and see if the scope numbers change? Your math seems right to me.

Also, your balance shaft belt is too tight.

DME and Coil are stock, i changed the original coil to another OEM bosch on ebay since the casing on my old one cracked. This one from ebay is in spec too. I'm picking up the pulse straight off the green wire to the coil so that has to be what the coil is firing. Can't wrap my head around it because it sounds normal to me too no misfire. I'll try without the tach tomorrow but funny thing is it does this with both tachometers i have and they both read the same. I think the belt is tight too now that you mention it, my thumb rule is usually a bit over but you're right it probably is a tad tight. I use carpokes tool for the cam tensioning at least. Going back about the harness, if it really were shorted along the way somewhere, the scope would show a crazy mess or the car wouldnt even run at all no? I would say the same since it is 40 y/o wiring but i cant see that with getting any reading on the scope at all.