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How To: Fix a 1990 DSM tach.

Discussion thread here :

This fix has worked for several vehicles now but I must say that there is no way to know what kind of side effects this may have. I've talked to one person who's had this for 2 years, but he is the only one I know of. If you use this fix and you have issues afterwards, then post up here please.

So today Sep 19th, 2011, I have found what I've needed to know for many many days. And this is VERY good news for the 90 guys.

This fix is so simple that it's mind boggling.


Items needed:

3-4 inches of 18-22 gauge wire.
Two MALE quick slide 22-18 gauge electrical connectors.

Now, to do this, you'll need a pair of wire strippers (or use your teeth, your choice) and wire crimpers.

1. Assemble everything you'll need. Ensure you know where the noise filter harness is in the car.
2. Take your wire and strip off about 1/2" on both sides.
3. Take the two connections and REMOVE THE OUTER CASE. I took a pair of pliers and yanked on the male side and locked the plastic part into a vice. A pair of pliers should also work. Take care to not crimp down on the wire crimp part yet.
4. Attach the two naked connections to the wire.
5. Bend the wire as shown in the picture below.
6. Slide wire into the noise filter harness plug.
7. Make sure it's tight, if not then secure it to the plug somehow.
8. Start the car and make sure the tach works.

It took me about 10 minutes to do this, but I have a good mechanical knowledge and background. Even a complete idiot should have this done in no more than an hour.

1. The stock noise filters resistance measure 2.3 kilo ohms.
2. The wire resistance measured .2 kilo ohms.
3. I did not need to ground the wire for this to work.
4. I did not go on an extensive test drive to test this out, but the tach DID function throughout the entire RPM range.

If any questions, give me a buzz on here. Good luck DSMers!


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Items needed. I used house wire as it's very sturdy and fit just right. You'll see the parts to the left and assembled to the right.

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My noise filter and area of the plug. Right behind the intake manifold. If your noise filter is still attached then it's only a 10mm bolt holding it on.

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Fix bent to fit into the connection nice and snug.

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Fix installed and working.

Now if you are wanting to be SUPER right about this, then install a 2.3 kilo ohm resistor on the line and see if that works. I didn't need to.


Theory of why apparently is dead wrong. So here is a quote from a more experienced electrical engineer.

luv2rallye said:
Ok here's the real truth about the filter from an experienced and degree'd electrical engineer:
The factory filter used is just a series resistor with a capacitor to ground after it. No values of these components are listed so the only way to know would be to open up a good one to look or measure the proper terminals with an ohmmeter and a capacitance meter. These components together form a low pass filter where AC components above a design cutoff frequency (derived from the resistor and capacitor values) get shunted to ground.

In answer to your question "So honestly, how can not having this noise filter affect the vehicle?", I assume you mean replacing it with a dead short wire as you propose. The only affect I can see would be the possibility of sending more pulses to the tach than should be sent. This is because the coil's tach output signal is the logical OR of the two coil primary voltage pulses (coil OR's them together before outputting the tach signal). But these primary voltage pulses have "ringing" (AC components) in them which may be detected depending on their voltage levels, as extra pulses in that coil's OR circuit which gets sent to the filter to be filtered out. [as an aside: The diagrams show an OR gate in the coil but it could just be an OR function, not an actual gate. If so, the output voltage peaks may also vary with rpm although most tachs should handle this ok if not too extreme. If it's a real OR gate the output voltage peaks will be constant with rpm (which is then nice for any instrument)].

The filter is only for the tach on the 90 as other years (except 2gnt) get it from the power transistor. As far as the ECU goes, the 90 ECU receives the signal before the filter so it is already designed to handle ringing. One of the things the ECU does with just the existance of this signal is keep the fuel pump running [why fuel pump only runs when engine spins (or more specifically when CAS and all other components produce ignition) on 2gT and all 1g]. So in a crash where the engine nearly always shuts off, the fuel pump gets disabled for safety.


A reason to why the stock noise filter no longer works. Just one theory.

I believe that on cars who's tachometer no longer works, the resistor finally gave out and separated from the wire, therefore blocking all signal and not letting the tach work. On vehicles that are intermittant, the heat/cold can change the metal just enough to let it make contact or separate it far enough to not let any signal through.


If there is any questions/concerns then feel free to PM me on here.

Good luck!



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