Thursday, May 2, 2019

Throwback: MerkMeter FAQ

Since I have a gauge cluster with a MerkMeter and needed to understand what it was, how it worked and how to reinstall it, I had to go find information about it. I finally found information and had to pull this out of the WayBack Machine, I figured I'd post it here, so there's a live website for this content again.


MerkMetertm Frequently Asked Questions

Q: Can I connect the MerkMeter directly to the EGO sensor wire in the engine compartment?
Absolutely no reason at all.  At the beginning of Step Four of the instructions this is mentioned as an alternate connection. It does involve taking a wire through the firewall and making a splice under the hood.  The wire provided with the MerkMeter installation kit may not be long enough to reach.  If you splice near the EGO sensor, make sure the splice is well insulated and away from the turbo heat.  The wire insulation on the OE Ford EGO is Teflon and can stand up to the high heat in that part of the engine compartment.  Make sure any wire you use will be as durable.  The connection at the computer is much easier, IMHO. The ECU can be pulled down pretty easily and does not normally need to be disconnected from the wiring harness. This also has the advantage of putting the splice in the cockpit where it is less susceptible to adverse environmental effects.  However, the call is yours.

Q: I don't want the MerkMeter on all the time.  Is there a way I can install an On/Off switch?
Yes.  The easiest way to do this is to splice into the black ground wire attached to MerkMeter and insert the switch of your choice.  The splice should be made in the wire between the MerkMeter and where it is attached to the back of the instrument cluster.  Some people have put a spare fog light switch into the empty spot where the cruise control switch fits.  If this is not desired, it is possible to just mount the switch under the dash where it is accessible.

Q: I don't like the standard LED arrangement.  Can I rearrange them?
An adhesive is used to secure the LEDs in place and it is not recommended that you try to remove them. For a nominal charge, we can customize your MerkMeter with any LED scheme you prefer. 

Q: Why is Dot Mode recommended over Bar Mode?
It really just comes down to personal preference.  Many people I have talked to about this find that one color is easier to distinguish in their peripheral vision.  Also, some find the "Christmas Tree" effect of Bar Mode to be distracting.  Others like the pizzazz of all those lights flashing in Bar Mode.  The MerkMeter will draw more current in Bar Mode to illuminate the LEDs.  However, it  will never draw more than ~95mA.  In the final analysis, it just comes down to what you like.

Q: Does the MerkMeter give average readings of the air/fuel ratio over time?
In Instant Mode, which is the default operating mode, it gives precise and immediate readings of the voltage generated by the EGO.  It can be set into Dampened Mode prior to installation.  Dampened Mode will slow the rate of change of the display so that it reflects a rough moving average. There is no computation taking place to calculate this "average", but the time constant chosen for the damping circuit works pretty well in practice.  When the ECU runs in Closed-Loop mode at about 2500 rpm, the typical EGO output varies from about 0.1 volt to 0.7 volt and back again about three times per second.  The MerkMeter in Dampened Mode will show a steady illumination of 0.4 to 0.5 volt.  This is right around the 0.45 volt indication of the stoichiometric air/fuel mix of 14.7:1 that the ECU is attempting to average.  In practice I think it is generally more useful to keep it configured in Instant Mode since the transient events are usually what you are going to want to see.  When you stomp on the accelerator you want to see if there are periods where the mixture leans out briefly.  Dampened Mode will mask these fast, short duration events that might be important.  I put the Dampened Mode into the circuitry because it was relatively easy to do and some people might prefer to see an average.

Q: Is the MerkMeter reliable enough to use for performance tuning?
The gauge shows the voltage generated by the oxygen sensor in response to the air/fuel ratio in the exhaust gas.  It is very accurate as far as this goes.
Perhaps the real question being asked here is "Can you correlate the meter display with a precisely defined air/fuel ratio?".  The answer here is only going to be as good as your EGO.  An old EGO is not as accurate as a new one.  The operating temperature of the EGO has a great deal to do with its accuracy.  Bosch has published some numbers that correlate voltage generated with air/fuel for given operating parameters.  The standard EGO is designed to have maximum accuracy around 14.7:1 and does this job very well.  It is much less accurate at the top and bottom of its operating range.  If you want to tweak the fuel delivery to get from a 12.6:1 to a 12.5:1 air/fuel ratio at WOT, then a standard EGO and the MerkMeter are not the first choice.  But neither is any other measuring instrument I have seen costing under $1000.  For this kind of accuracy you need a calibrated, wide-range sensor with precisely known operating characteristics.  These kinds of sensors are intended to be used in conjunction with dynos in test/tuning setups and are not normally used in everyday operation.  
When I talk about the MerkMeter I try to equate it to a coolant temperature gauge.  The temperature gauge is there partly to inform you as to current status of the engine temperature, but also to let you see if there are changes over time. If the engine seems to be running hotter now that it was a few months ago with the same operating conditions, then that could indicated a problem developing.  It is the same with a runtime air/fuel meter.  If a vacuum leak develops or injectors are not firing, you will see the mixture lean out immediately.  If the EGO is getting old and lazy you will see Closed-Loop crossovers slow down from where they were months or weeks before.

Q: Why does the reading on my meter change when I turn on my headlights?
(Many thanks to Allan Slocum and his Idea Factory for finding the answer to this particular problem and to Brad Anesi for pointing out a simpler fix than what I had in mind. -JRW)

This is a common problem on many cars, but particularly so it seems with Merkurs.  This is undoubtedly due to poor electrical grounds.  In tracking down this problem, I have seen as much as 160 millivolts between a given ground and the battery negative terminal.  For lights and such, this does not usually cause any problems.  In the case of an air/fuel meter however, this can be significant.  If you realize that the meter is comparing the voltage from the EGO sensor to what it thinks is a good ground (ideally 0 volts), you can see that a 100 millivolt error will cause the meter to read one LED lower than it should.  I have had reports from MerkMeter users that they have seen this same problem with other makes of A/F meters if the ground is bad.  Also, a bad EGO ground to the EEC-IV can cause the ECU to get bad data from the EGO sensor. 
The ground on the XR4Ti instrument cluster is particularly bad.  Although it made for an easier installation, my choice to take the ground off the stud on the back of the cluster was a really bad idea.  If this ground is bad on your car, you will see the MerkMeter reading drop at least one LED when you turn on the panel lights full "on" using the dimmer.  On all kits shipped after February 1, 2001, I have included new instructions and a short jumper wire for connecting the meter ground.  For installations made before this time, the problem can be easily corrected by grounding the instrument cluster to the chassis ground screw directly behind the cluster.  (See figure at right).  Adding a simple jumper (18 AWG or larger) from the ground stud on the cluster to the chassis ground is all that is needed.  If you need more detailed instructions and/or the jumper wire please contact me.  This fix has the added benefit of improving the overall ground to your instrument cluster.
Another notorious spot is the ground screw right under the battery tray on the XR4Ti.  This is baked by the turbo heat and is especially susceptible to corrosion.  
Also check your "EGO Ground" wire.  This is an orange wire that connects Pin 49 on the ECU to a screw on the turbo compressor housing.  If this wire is missing, disconnected, broken, or corroded, the EGO may not be properly grounded.  Our cars really could use a four-wire EGO that would provide a direct ground as well as a heater element.

Q: Is there a better way to secure the meter in an XR4Ti than using the double-stick tape?
This issue only applies to the XR4Ti installation since the tape is not used in the Scorpio.  Over time the heat under the dash might cause the double-sided tape to lose its adhesive properties.  The solution is to install the meter in a manner similar to that used for the Scorpio.  This method is a little trickier for an XR4Ti since there is less room to fit the wire tie.  However, with a little care it can be done and will hold the meter very securely.
The white wire tie with the eyelet on one end can be used to hold the meter in position.  Thread the end of the tie through the empty hole at the back-right of the meter.  You want to orient the tie so that the end can be pulled up through its locking slot with the eyelet above the meter.  Remember that the component side of the meter faces down.  Place the tie end slightly into its locking slot.  At this time only feed the end far enough into the locking slot to hold it in place.  Place a screw through the eyelet and thread it a couple turns into the hole in the cluster case just above where the meter is installed.  You want to leave as much slack as possible so the meter can be pivoted up and into the slot.  (One of the screws holding the front cluster lens in place can be used if no other is available).  Carefully slide the meter into its slot with the component side down.  Now carefully tighten the tie by pulling the end through the locking slot.  You may have to adjust the tie as you tighten and feed the excess through the hole in the meter.  Slowly tighten the screw and remove the slack in the wire tie in stages until the meter is secure in its slot.  Be careful not to over-tighten the screw or tie as the meter board could crack or break if too much force is used.

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