Tachometer adapter help because I have installed a BMW 735i Motronic engine.

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Granholt

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I have a Bmw 1969 E9 2800 cs that I have installed a Bmw 735 I engine from about 1982,it has motronic 1.1.
Have also a 5 speed overdrive gearbox.
I have had the car since 1980, and it have stand still in 29 years now, but now it is nice and restored.

The problem is that the tachometer does not work properly.
It can probably break if you connect it directly to the coil's minus pole I have heard.

Therefore I have made a small circuit that I found on the Bmw 2002 group, see the pictures.
But, the tachometer shows a bit too much at idle about 1500 and it jumps a bit sometimes.
Does anyone know of an adapter that I can use and that works?

Mybee I made the circuit wrong.
 

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i will start with - i presume you have checked the grounds at the instrument cluster ... that is the biggest cause of gauges not working correctly, that aren't mechanical in nature (like a speedometer / odometer).

on the coupe, the black wire that comes off of the back of the tach - goes straight to the tester plug and then to one side of the coil. what i don't know is if there is any difference in the way the car is wired for motronic 1.1. i don't think so, but not sure.
 
sfdon said:
Direct connection from coil to tach
No circuit needed
Click to expand...
Are you sure?
Because I have read that the signal from the coil is a little different than the original one that uses mechanic connection.
I read that on the minus side on the coil is between (lowest 3 to 5v) to 12 volts and a little different volts on the voltage peaks.
The old one i 0 to 12v + spikes at 200 300 v.
And some do not recommend connecting it directly, hmm... that must be wrong then!

Have you or anyone else you know, connected it directly to the coil? maybe that what I did 30 years ago I do not remember?
Because I don't remember that it not work when I put it down.

Possible grounding error then as rsporsche writes.
The tachometer is newly overhauled from Germany.
 

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rsporsche said:
i will start with - i presume you have checked the grounds at the instrument cluster ... that is the biggest cause of gauges not working correctly, that aren't mechanical in nature (like a speedometer / odometer).

on the coupe, the black wire that comes off of the back of the tach - goes straight to the tester plug and then to one side of the coil. what i don't know is if there is any difference in the way the car is wired for motronic 1.1. i don't think so, but not sure.
Click to expand...
Pin 1 from ECU(Motronic box) goes to 13 on the tester plug and to minus on the coil, black wire.

On my Bmw the black wire from the tachometer goes straight to the coil, via the headlight housing.

Where is the ground cable behind the dashboard connected to the body?
 
See video where ->Tacho attached direct to coil.
After previous engine start the needle was hanging on1600 o min.(Possible residual voltage that makes it).

And it is not working correct, mybee grounding problems as rsporche said.
What else could it be tacho was like new from overhaul from Germany

​

 
I have never had an issue connecting direct to coil negative. My M88 swapped car still runs original motronic 1.1 and I've no issues with tach being on coil negative. if possible run an extra wire and have tach removed from the vehicle with direct power/ground/signal wires to rule out wiring issues. If you have a good known spare tach/coil swap those and see.
 
I have done dozens of cars.
All the same.
Direct wire from - on coil to tach.
 
whether the wire goes directly to the coil or shortstops at the test connector does not matter if you have removed or do not use the test connector. in the factory configuration its just a stop + go at the test connector ...

ground on the gauge cluster ... there are several as i remember. its been a while since i disconnected mine and i will relearn when i go to put it back together in the near future. i think it is over near the fusebox. i know the ground wire(s) go through the 9 pin connector (where the gauge cluster wiring harness plugs in) and then runs to the ground - follow the brown wires - there are 4 that come thru the connector. 3 go to the engine compartment (1 goes to the water temp sender, 1 goes to the oil pressure switch, one goes to the brake fluid reservoir) and the free one goes to the ground.
 
rsporsche said:
whether the wire goes directly to the coil or shortstops at the test connector does not matter if you have removed or do not use the test connector. in the factory configuration its just a stop + go at the test connector ...

ground on the gauge cluster ... there are several as i remember. its been a while since i disconnected mine and i will relearn when i go to put it back together in the near future. i think it is over near the fusebox. i know the ground wire(s) go through the 9 pin connector (where the gauge cluster wiring harness plugs in) and then runs to the ground - follow the brown wires - there are 4 that come thru the connector. 3 go to the engine compartment (1 goes to the water temp sender, 1 goes to the oil pressure switch, one goes to the brake fluid reservoir) and the free one goes to the ground.
Click to expand...
Thanks for the reply.
Tested my other tachometer loose in the engine compartment and it worked fine.
Connected to the newly overhauled one in the car, it hangs up and is slow sometimes.
I took it and banged on the glass a little then it dropped down to the correct speed.
so I think the grounding is good, it's the needle that's a little slow.
It will probably get better as I use the car.

Now all I have to do is get the new windshield installed, it's probably worse.
 
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