Tachometer Quits at 4,000 RPM, need help diagnosing problem Options

[jfort]

tach has been to Palo Alto Speedometer twice. Checks out fine there according to Hartmut Mees. Seems to be OK until 4,000 RPM, then it falls back and flutters.
CD Box: Bosch 0 227 200 001. The "coil" is new, Bosch 0221121001. Car runs fine. How can I isolate and fix the problem?

[jimkelly]

if engine runs past 4000 - i say - try another tach first.

jim

[So.Cal.914]

I know, but I have to ask. Do you have a rev limiter? They can go bad.

My .02

[tod914]

Points or Pertronix ignition? The pertronix can do whacky things
when they are on their way out. But you'd probally have running
issues coupled with the tach issue.

[marks914]

tach has been to Palo Alto Speedometer twice. Checks out fine there according to Hartmut Mees. Seems to be OK until 4,000 RPM, then it falls back and flutters.
CD Box: Bosch 0 227 200 001. The "coil" is new, Bosch 0221121001. Car runs fine. How can I isolate and fix the problem?

Did Palo alto redo your tach or just check it?

Go buy the cheapest tach you can find at the auto parts store and try it


Mark

[r_towle]

tach has been to Palo Alto Speedometer twice. Checks out fine there according to Hartmut Mees. Seems to be OK until 4,000 RPM, then it falls back and flutters.
CD Box: Bosch 0 227 200 001. The "coil" is new, Bosch 0221121001. Car runs fine. How can I isolate and fix the problem?

Did Palo alto redo your tach or just check it?

Go buy the cheapest tach you can find at the auto parts store and try it


Mark

I suspect the CD box wiring for the tach is the issue.
To much spark at the high reve probably makes the tach not perform.
see if Bosch has a wiring diagram for the correct way to hook up the tach...I think that you are using an early 911 CD box...so the diagram may be on one of the early 911 forums/boards.

Rich

[jfort]

thanks for the responses, guys. it is points. palo alto repaired then when i sent it back, checked it. i think wiring is OK. i don't think there is a rev limiter. the thing is, it worked fine until a few months ago. the CD box was looses and hence not grounded. i saw arcing at the coil and i think that's what burned out tach. palo alto repaired. got CD box well grounded and replaced coil, now i have this problem. i like the cheap tach idea to begin isolating the problem. other new variable is the coil, but I assume it is correct or the car wouldn't run correctly, as it does. i am also going to check all connections. thinking about hooking tach up out at the engine to eliminate wiring variable. then maybe try new coil and CD box, respectively.

[r_towle]

I happen to have a car here running a bosch CD box.
Tell me your part number and I will see if it is the same.
I can give you a map of the wiring for his CD box...his tach works fine.

Rich

[jfort]

Rich,

CD Box: Bosch 0 227 200 001. The "coil" is new, Bosch 0221121001.

Just to be clear, engine is strong all the way.

Hartmut Mees at PAS emailed me last night. He is going to send me 3 test tachs. My mechanic thinks I have a wiring issue. Tonight I will check all the connections and the continuity between the (-) at the coil and the instrument cluster. Someone said in another thread that fuse #4 can affect tach. I'll check that, too, while waiting for Hartmut's tach. I'll let you know what happens.

Again, thanks for the suggestions and help.

[r_towle]

Same one,so here is how its wired.

Looking at the numbers on the box, there are two terminals vertical, then three horizontal at the bottom.
If you are using the part numbers, (our is upside down) but lets use the numbers for reference..I will explain as if the box was mounted on the firewall, with numbers right side up.

So, the connector closest to the face with the numbers.
This wire goes to the ground screw on the distributor that also attaches to the advance plates inside the distributor...so its a direct ground.

Next wire is also a ground.
This goes to the fan housing, and the negative side of the coil also goes to the fan housing...another ground post.

Then we have the plug with three spades.
Bottom connector goes to the postive side of the coil.
Middle connector is the heavy black wire from the ignition switch. (12vdc)
Top connector is the green wire to the condensor AND the little black tach wire (its a double spade connector deal)

That works.
I think the two grounds are important..one to the dizzy and one to the motor...
Remember, its throwing alot of multiple sparks.

Rich

[jfort]

Rich, I'm looking at the coil, since it is new. The CD box and its wiring haven't changed since before the tach worked fine. There are two wires to a terminal lug on the + side of the coil. I assume one of those goes to the tach since the tach gets its RPM signal from the + side of coil, correct? On the - side of the coil is one brown ground wire that goes a few inches to the fan housing.

I think I'll clean up all the connections and check the ground. Don't know what else to do right now.

[r_towle]

Rich, I'm looking at the coil, since it is new. The CD box and its wiring haven't changed since before the tach worked fine. There are two wires to a terminal lug on the + side of the coil. I assume one of those goes to the tach since the tach gets its RPM signal from the + side of coil, correct? On the - side of the coil is one brown ground wire that goes a few inches to the fan housing.

I think I'll clean up all the connections and check the ground. Don't know what else to do right now.

My response earlier.

Same one,so here is how its wired.

Looking at the numbers on the box, there are two terminals vertical, then three horizontal at the bottom.
If you are using the part numbers, (our is upside down) but lets use the numbers for reference..I will explain as if the box was mounted on the firewall, with numbers right side up.

So, the connector closest to the face with the numbers.
This wire goes to the ground screw on the distributor that also attaches to the advance plates inside the distributor...so its a direct ground.

Next wire is also a ground.
This goes to the fan housing, and the negative side of the coil also goes to the fan housing...another ground post.

Then we have the plug with three spades.
Bottom connector goes to the postive side of the coil.
Middle connector is the heavy black wire from the ignition switch. (12vdc)
Top connector is the green wire to the condensor AND the little black tach wire (its a double spade connector deal)

That works.
I think the two grounds are important..one to the dizzy and one to the motor...
Remember, its throwing alot of multiple sparks.

Rich

Your initial response that got me going on this thought.

the thing is, it worked fine until a few months ago. the CD box was looses and hence not grounded. i saw arcing at the coil and i think that's what burned out tach.

I wanted to put all of this together on one page.
You mentioned that the tach dies after you had a grounding issue with the CD box.
Look at how mine is set up.
Lug one , closest to the part number plate is grounded to the motor.
This same lug goes to the negative side of the coil.
Lug two is grounded to the distributor.

Ground is very important here.

the tach gets its signal from negative side of the CD box on my setup, same lug as the condensor.
Tach cannot get a signal from the coil, it will get confused.
Anytime a CD box is used, you can no loger get a valid tach signal from the coil.
IT MUST get its signal from a terminal off the CD box, or an adapter.

The CD box is shooting multiple sparks. This signal is what activates the coil.
So, instead of getting your tach signal from the coil, you gotta get it from the CD box or adapter. This bosch box seems to provide a port for it. Our is hooked up this way, so give it a shot.

Rich

[jfort]

Thanks, Rich, for taking so much time to write a detailed response. I am not confident or smart enough to start taking apart the wiring to see if mine is the same as yours. The top of the CD box has one big connector with multiple wires. I haven't taken it off. I assume it is correct because the tach and everything else worked fine until I had the "lack of CD box ground leading, I think, to the burned out tach" issue. Palo Alto Speedo is sending some test tachs. I'll try that and, if that doesn't reveal a tach problem, then take apart the wiring. I did check the continuity of all the grounds and cleaned up all the connections, but that did nothing.

[sixnotfour]

A small point gap will make the tach needle jump around.

[jfort]

thanks, six. Another possible clue: I don't think the tach is correct even below 4,000. I think I am actually about, say, 5,000, and it says something below 4,000. As the revs increase, it just drops and flutters.

If the point gap were that small, would the engine still be running so well?

[jfort]

here's the latest. Harmut at Palo Alto Speedo sent 3 tachs to test in my car. None of them worked right. One of them acted like mine -- slow up to 3,000 - 4,000 RPM's but nothing above that.

i am going to try swapping a CD box to see if that could be the problem.

again, car runs fine. tach worked fine, too, until it just didn't work. so i don't think i need to change wiring. history: car ran rough. discovered arcing at coil while car idling. then car quit. replaced coil and tightened CD box down. i'm sure it was properly grounded. car then ran fine, but no tach. repaired tach but it doesn't work above 4,000 RPM's

this is frustrating. any suggestions?

[jfort]

Need some help, guys. Just put in new Bosch CD box and that did not fix the problem. We put in a new coil when the problem started. It is Bosch 0221121001. Could someone with a six check and see if this is correct? Otherwise, I am stumped. Not the tach, not the CD box. This wiring has not changed and it worked before. HELP!

[jfort]

where does the tachometer get the RPM signal from? At coil? At CD box? I am going to try to connect tach out at the engine but don't know where. Would someone please look at a schematic or know first hand? Thanks

[ericread]

When you tried the three tachometers, did you place them in your circuitry the same as your original tach? If it is a wiring problem, you are just re-creating the problem. How about attaching the tachometer with one wire directly to the coil and the other directly to the negative terminal on the battery. Just place the tachometer on a pad on your rear trunk. Bump up the throttle and see what happens. If the problem remains, at least you've isolated the wiring as the problem/not the problem.

Eric Read

[jfort]

that's my plan, too, eric. I am just trying to confirm that the connection is to the coil.