Tach Signals

[njautorepair]

Hey everyone. I am working on a project where the signal from the engine put into the car, does not match the signal the instrument cluster is receiving.

There is a chip in the cluster that takes in the signal from the ECU and converts it to a signal the tach understands and displays. The standard chip, looks like this one on the left, https://www.dropbox.com/s/s7v27dfnpmffa4u/IMG_5471 (Edited).JPG?dl=0 .

The one on the right is from another car that runs a very similar engine to the one in this car. It also has the same tachometer that reads 8,000 RPMS. I know that you can take the chip from the board on the right and solder it into the board on the left and you will be able to acquire a pretty accurate tachometer reading for the swapped engine.

So, I want to learn if it is possible to 'overwrite' the existing chip to distribute the signal properly? Or what would be the proper method in accomplishing this?

 

[Amar Dhore]

I could not locate the datasheet online for HLM. Do you have any link to it?

 

[njautorepair]

I am finding very limited information on that exact chip.
Here is what I have found:
http://www.ic2ic.com/search.jsp?sSearchWord=HML087&prefix=H#
http://www.alldatasheet.com/view.jsp?Searchword=HML087


These are some similar chips based on the description:

https://store.ti.com/Search.aspx?e=Frequency+to+Voltage+Converters

 

[Amar Dhore]

What did you change/ Why chip is not reading the TACH signal correctly? what BMW/car is it? How many cylinder model?

 

[njautorepair]

Original engine is a 4 cylinder engine, replacement engine is a 6 cylinder engine.

Car is a E30 M3 which had the 4 cylinder replaced with a E36M3 motor.

The tachometer is grossly inaccurate with the 4 cyl plug in the instrument cluster.

 

[Amar Dhore]

I do not have enough information online about this chip, but here is what I would do.

Measure TACH signal and compare it with the older signal. Find out how different the signal is compared to the older one. Is it half or double. Then you can buy or design a circuit which would convert the signal to cut/boost to the level.

I am not expert in the Engine ECU, I am sharing this best on my electronics knowledge.

 

[njautorepair]

The only problem I have here is that I don't have any way to measure the old signal vs the new signal.

Is there no way to compare the old chip with the new chip to see what differences they have?

I truly appreciate you trying to help me with this.

 

[Amar Dhore]

The only way I know to compare chip is, Datasheet. But I did not see any datasheet available online.