308 Tachometer "Jumping" | FerrariChat

308 Tachometer "Jumping"

Discussion in 'Technical Q&A' started by Sophia, Jun 21, 2004.

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  1. Sophia

    Sophia Formula Junior

    Aug 17, 2003
    298
    Dash Point/Federal W
    Full Name:
    Peter Barbin
    '78 308's tach is jumping intermittently. The first indications of a problem showed up at idle, bouncing from 900 rpm up/spiking toward 2-3K rpm briefly.

    Now the problem is during cruise too. The needle jumps up a few thousand rpm then settles back to its proper rpm...DURING the needle rise I hear relay clicking type noises from under the dash.

    Now its time to chase the gremlin. The wire from the coil is brown/black str. On the wiring schematic I noticed that there is an associated relay that gets a signal from the tach. Obviously I'll check for a good connection at the coil, and then head for the back of the instrument to check for good connections.

    Questions:

    1. Has anyone experienced this symptom?
    2. What does the relay do that is associated with the tach circuit?

    Thanks, PeterB.
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,799
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    The relay you hear "clicking" may be item 128 as shown on the US '78 OM schematic. It's a special relay that also uses the tach signal coming from the coil to actuate the electrovalve that causes the diverter valve to dump the air injection air out under certain conditions (i.e., the clicking relay isn't causing your problem -- rather it clicks for the same reason your tach reads wacky since it sees the same "bad" input signal.)
    Does the engine also actually mis-behave when you have the problem, or is it just an instumentation problem?
     
  3. Sophia

    Sophia Formula Junior

    Aug 17, 2003
    298
    Dash Point/Federal W
    Full Name:
    Peter Barbin
    Thanks Steve, the engine behaves PERFECTLY normal during BOTH these events: the idle "spike" and the "cruise needle rise." The first time I saw the tach jump at idle I was springloaded to listen/feel for any power pulses/ popping caused by ignition shorting somewhere. And during cruise, again...smooth running, no perceivable power loss. If I wasn't looking at the needle, nor hearing the clicking of the relay, I wouldn't have known there was a problem.

    What type of pulse is generated from the coil to the tach via that brn/blk wire? A grounding pulse or voltage pulse? I'll be back on line tomorrow...off to work! Thanks.PeterB.
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,799
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    Peter -- (Sorry for the rather poor image quality, but) The jpeg below shows the typical primary coil voltage waveform (although I'm unsure if "0" means 0 volts or 12 volts and if up or down is positive). There's a fairly large voltage spike on the primary side (like several hundred volts) at the start of firing (and my guess would be that the {inverse} time between these spikes is what the tachometer gauge is detecting/representing, but I don't know for sure what feature in that primary waveform that gets "detected"). I think you're on the right track to first make sure that you've got good, clean connections at the coils -- as an experiment you might try intentionally disconnecting the brown/black wire and see what the tach does in that condition when running -- just a thought.
    You could also unplug the 128 relay temporarily as a test just to confirm that it's not causing the problem.
    Alternatively, you could do an internet search on something like "primary voltage waveform points coil dwell" and you should get a lot of references to check/read -- good hunting...
     
  5. Sophia

    Sophia Formula Junior

    Aug 17, 2003
    298
    Dash Point/Federal W
    Full Name:
    Peter Barbin
    Great input Steve, I'm on the road and I'll be home to work on the car Thursday morning...Yup, I'll clean and dielectric grease the suspect connections, and pull the brn/blk wire and see if the tach drops to min., or pins to full rpm. Pulling the relay to check tach performance is a great idea. I just put in a new oil pressure sender and replaced a missing adell clamp holding the coolent pipe near the oil filter housing...and my hands were "messing around" with stuff in the diverter valve area.

    Ultimately, all my smog stuff (but not the evaporative collection equipment) is coming off, and if it's sole function is to direct diverter valve dumping, it won't be necessary either. When you say it directs the diverter valve to dump air, where is the excess air from the air pump dumped?

    Next question, where does this "USA" relay live? It's not on the main board. I hear it "clicking" but I haven't yet seen where it lives. Your expertise and experience is always appreciated. PeterB.
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,799
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    "where is the excess air from the air pump dumped?" -- The air from the air pump is released to atmosphere from a screened (IIRC) vent directly built into the diverter valve itself.

    "where does this "USA" relay live?" -- Behind the passenger footrest panel with the slowdown warning light ECUs and the timer for the fuel evap blower motor. Item 25 in this jpeg:
     
  7. Frari

    Frari Formula 3

    Nov 5, 2003
    1,194
    brisbane australia
    Full Name:
    tony
    I have had similar problems with both a 308 and a porsche and found that the problem was the car battery was on its last legs and with a mix of poor alternator and poor battery this caused a fluctuautin in the tacho, don';t ask me why but when both were replaced all was ok. Tony
     
  8. Sophia

    Sophia Formula Junior

    Aug 17, 2003
    298
    Dash Point/Federal W
    Full Name:
    Peter Barbin
    Well, I've been to electrical hell and back...well not really, but a great learning/familiarization exercise in the ignition system of this car.

    SteveM, I thank you for the jpegs; duh, they were in the owners manual. I studied the wiring diagram more and discovered that the wire that ultimately goes to the tachometer...gets there via the relay/electric control unit: "B"-Electrovalve switch controlled by engine speed...for those tuning in this relay is located, as mentioned earlier on this thread, in the forward compartment of the passenger foot-well. (p110 of the 1978 US 308 owners manual)

    I thought I'd go to the source first though to check for corrosion. At the coils I discovered a bit of rust on the (-) terminal, not on the coil itself, but on the post of what appears to be a ballast resistor (with "80K" markings) secured below each coil. The points' wires from each distributor connect to each resistor block on one end, the other end of the block has a jumper wire that leads to the (-) post of the coil. On ONE of these blocks you will find the brown/blk wire that gives engine speed readings to the electrovalve switch and ultimately, the tach.

    I first found that my car's wiring was backwards at the coils, not that it matters for function though. But, the 1-4 bank is, per wiring diagram, the bank/coil that hosts the brown/blk lead.

    A little voice kept echoing, "check the points" while you're there...I pulled each distributor, sure enough mega peaks and pits on the points, particularly on the R2 points Did a quick 'fiat' roadside file job, checked gaps, re-installed the distributors, timed and dwell within spec for R1's and R1/R2's in parallel. New pnts and cond. soon! Easy method to get to all this stuff: LR wheel and fairing off, voila!

    ROAD TEST: So far, the tachometer is STABLE, exhibiting none of the earlier mentioned dastardly traits. I won't say I 'fixed it,' but again I found RUST on the jumper leads of the (-) coil "blocks" and PITTED POINTS.

    Could this be a symptom of the "sticking points" I heard mentioned?
     

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