I took the dash apart and cleaned the grounding for the instruments in the dash, I also cleaned the positive connections and all connections on the tach itself. No difference. I even added alligator clips between known good grounds and the ground connections in the dash but this didn’t change anything either.
The coils both measure 0.8 ohm, it must have been a measurement error when checking the first time.
When driving as normal the engine is very smooth and has plenty of power, regardless of whether the tach is bouncing or not, it is only at idle when the engine is lumpy and seems to correlate somewhat with the tach bounce.
I am afraid it is not really feasible for me to install a permanent second ground strap since I’ve had the car inspected with this setup with a power shutoff switch mounted between battery - and the ground point; this is mandated by my insurance company, which allows me to get insurance for 150€/year instead of over 200€/month (!!).
To try and diagnose the issue I have added temporary ground straps between several points, among others the battery - and the intake manifold, and between the ground point on the fender and the air injection bracket. This makes no difference.
I have tried using three different amps, the original GM amp has little on idle, but reverses at 2500 rpm, as described in earlier posts. The new ACDelco amp has quite a bit of bouncing at idle, and seems to reverse at 5000rpm, and a HELLA amp I’ve tried suffers very heavily from bouncing at idle, I haven’t tried to rev with the HELLA.
I have also noticed that the tach sometimes bounces on startup, between 0 and like 100kmh when cranking, this only happens sometimes.
I let the car run for a few minutes while just sitting in the car and trying to analyze the rev gauge jumps, it seems like it does so whenever the engine ‘stutters’ almost as if the spark is faster than otherwise. It doesn’t feel like a misfire, but rather like the spark is too fast, or too soon. The ‘fast-fire’ is always accompanied by a rev bounce, but not the other way around - ie. the rev bounces more than the engine ‘fast fires’.
I also replaced the fuel pump and fuel regulator, which resulted in what feels like a somewhat smoother idle. But I’d like to clarify that the idle isn’t necessarily uneven, but rather plagued with the occasional misfire which shakes the car a bit - unsure whether related to low compression, bad spark, or bad fueling.
I also managed to inflict this when I dropped the FPR on the fender…
I am sure you understand how satisfied I feel with my car purchase right now…
Nevertheless the search still continues. I’ll try to list some of the facts so far:
The tachometer is showing the wrong RPM:s depending on which ignition amp is used when accelerating. Using the original GM amp the tach is showing 2500 rpm, after which it ‘turns’ down to 2000 after which it continues to climb normally.
Using a new AcDelco amp the tach initially climbs to 5000 rpm after which it turns, unsure whether it ever turns back up.
The tachometer is also ‘bouncing’ up and down about 100-200 rpm at idle, however remains steady when engine is held at higher RPM:s. This issue is consistent with three different amps, however to different extents.
There also seems to be a slight misfire at idle, and some kind of issue which results in what feels to be the engine firing faster, this seems to be linked to the tachometer bouncing.
The issue first appeared after the distributor had been taken out of the vehicle and serviced, initially the problem was discovered when the engine was set to rev to 3000 rpm for timing, however the tach was observed not following engine RPM. I suspect the issue was already present before the distributor was taken out for servicing, however this is not verified. After replacing the amp with an aftermarket model the tach seemed to follow engine rpm, and the timing was set, during the test drive the car suffered from ignition failure, probably related to amp overheating, a new aftermarket amp was used but this also overheated. When a new AcDelco amp was installed the amp no longer seemed to suffer from overheating, however the issue of tachometer bouncing now occured. This issue now appears regardless of amp used.
So far the following parts have been checked:
- Replacement coil DAC6093, which didn’t solve the issue, old coils measured within spec.
- New spark plugs, gapped correctly.
- Distributor mechanical advance greased and working.
- Vacuum advance hose replaced, now working.
- All connections from coil reshoed on coil end.
- Ignition amp replaced, non GM/AcDelco amps suffer from overheating.
- Grounding points inside dash- and in engine compartment cleaned and remounted.
- Connector between distributor pickup and ignition amp redone with new spade connectors.
- New fuel pump and B-bank FPR installed.
- Spark plug wire resistance measured, all within spec.
- Distributor pickup gap measured, within spec.
- Engine timing set to 18* BTDC @ 3000 rpm, possibly wrong since tach is suspected showing incorrect RPM:s.
My thoughts on how to continue:
- Take out the distributor from the car and re-service it in order to stabilize the magnetic pickup and possibly notice something ‘out of place’.
- Remeasure spark plug wires.
- Figure out a way to test the magnetic pickup in order to decide whether it should be replaced.
- Possibly purchase a full AB14 ignition module.
So, does anyone know of a way to test the magnetic pickup? I suspect you’d have to use an oscilloscope, however I don’t have access to one as of writing this post.
Did the straight 6 engine have the same AB14 unit as the v12? I haven’t been able to find a scrapyard with a v12 here in Sweden, but several straight 6 cars, and I’m hoping to be able to get one cheaply from there.
The ‘fast firing’ issue being related to the tach bounce seems pretty reasonable, as far as I understand the tach is just using the same impulses as the coil, a fast impulse would lead to the tach jumping thinking the engine was at ex. 5000 rpm instead of 800 rpm, but it wouldn’t have time to get up enough to display the interpreted rpm, I’d need an oscilloscope to verify this however.
As always, any ideas of ways forward?