I work on a lot of 94-98 Ford Mustang Instrument clusters for people. I have noticed that Ford made an engineering change and it is not clear to my why they did it.

From 1994 through 1998 Ford used the same thermistor sensor that causes the gauge to read at the H mark if the sensor is at 9.7 ohms. it reads at the C mark when the sensor is at 74 ohms.

From 1994 through 1997, they used a simple air core gauge with a resistor in parallel with one of the coils. But in 1998, they changed the gauge coils and added a Zener diode and a rectifier. Since Ford wouldn't add a nickel to their cost without a very good reason, I am very curious as to why they added made these changes. I have not been able to figure it out. I am guessing that the 1n4001 is there to protect the Zener diode from an accidental polarity reversal, but it is not clear to me what the Zener does for the circuit.

Here is a comparison of the two circuits:



FYI, in 1999, Ford went to microprocessor-based instrument clusters with a completely different architecture.

Can anyone explain to me what the Zener does for this circuit?

 

hi gray,
The changes in meter resistance values indicate a change in the type of meter.

Changing the coil resistances and still using the same temp sensor would require changes to the current paths and levels through the new meter coils in order to give the same same temperature indication on the new meter type.

The 3v3 Zener would 'move' the zero point voltage to suit the new meter, and the diode the maximum point on the meter.
Basically, the diode and Zener are fitted to enable the new meter to use the old sensor.

Do you have a photo shot of the indicator panels of the two meter types that you could post.?
E

 

Does one Gauge-Cluster have a ~90-degree Needle-Sweep,
and the other have a ~270-degree Needle-Sweep ?

These are 2 very common Gauge configurations.
.
.
.

 

What I am really curious about is why Ford would make that change to the meter and add the two diodes. They are notorious for wringing costs out of their products. This had to add cost so it had to be done for a reason...

The gauges are all 90* sweep gauges and they all look the same from 94-98:

 

hi gray,
As I explained in post #2, changing the meter's internal coil design in order to use the same meter dial/facia display and temp sensor would mean modifying the external support circuit.

Perhaps the old meter style design became obsolete or more expensive.?

E

 

I've never been a Ford guy but if I had to guess - and this is purely a guess - it may be due to using the computer and a required voltage for the computer to register the temperature. A 3.3V Zener suggests it MIGHT be part of the CAN Bus. But this is purely a guess. Others may comment further on my "Guess", which may lead to a better understanding of why. I'd also imagine that a smaller gauge wire is required. Thus reducing weight AND copper costs in manufacturing. But once again, this is purely guess work.

 

hi gray,
As I explained in post #2, changing the meter's internal coil design in order to use the same meter dial/facia display and temp sensor would mean modifying the external support circuit.

Perhaps the old meter style design became obsolete or more expensive.?

E

I can see why they had to add the extra circuitry if the meter changed, although I would have thought that different resistance would be a cheaper solution than adding two diodes. It certainly wouldn't occur to me to add those two diodes if I were trying to compensate for a new meter.

Ford started using microprocessors in their clusters in 1999, a year later than this strange 1998 beast. They swapped two pins on the cluster connector for just this one year. They also used a different plastic casting for the cluster case to accommodate a different odometer stepper motor and they swapped two pins on one of the cluster connectors. They made a number of other changes to thinks like the fuel tank for the 98 model year only, making it challenging to find parts sometimes.

Maybe they were forced to change the meter due to a decision made by the meter supplier, but knowing Ford back then, I would be surprised if the supplier had much leverage.

Thanks all for putting some mental ergs into this. I have been curious for a while. I don't feel too bad now, not being able to figure out the technical/economic reasons for the change...

 

I've never been a Ford guy but if I had to guess - and this is purely a guess - it may be due to using the computer and a required voltage for the computer to register the temperature. A 3.3V Zener suggests it MIGHT be part of the CAN Bus. But this is purely a guess.

See next quote...

FYI, in 1999, Ford went to microprocessor-based instrument clusters with a completely different architecture.

A CAN gauge would not even be directly connected to the sensor. It would get it's information from the ECU. There was a time where there were two temperature sensors. One for the ECU and one for the gauge. Both of the TS circuits would be pre CAN circuits.

As for a reason in the change I don't have one. I do know from their oil pressure "gauges" they used in just about everything else how cheap they did make things. I actually added a real gauge to a few vehicles just to have a working gauge instead of their fancy warning light / "gauge".

I would agree that it was probably cost or obsolete parts. It may have been simply a stash or gauges somewhere in a warehouse that needed used up at the last minute (unlikely I know, but who knows). I don't know how much trouble it would be to manufacture a new circuit membrane (if it is even needed), but somewhere along the line it just made sense.

 

....this strange 1998 beast.

I had a 98 Explorer with the OHV engine that for some reason or another I had a lot of issues with getting the right parts. Sometimes it was people thinking I said I had the OHC, and other times is was kind of like something changed and not documented.

 

I had a 98 Explorer with the OHV engine that for some reason or another I had a lot of issues with getting the right parts. Sometimes it was people thinking I said I had the OHC, and other times is was kind of like something changed and not documented.

Sounds like the Mustang was not the only Ford vehicle with unique parts in 1998...

 

Sounds like the Mustang was not the only Ford vehicle with unique parts in 1998...

If I remember right they were still doing the California or not emissions and that caused a bit of trouble, there was some 99 parts on it, and the 4.0 OHV was on it's way out... most were OHC or DOHC. I would do it all over again in a heartbeat if I had the chance. Two doors, two wheel drive, and posi was a blast in the snow as long as you didn't get stupid with it. I called it the skid steer.

 

I drove a supercharged 96 DOHC Mustang with a limited slip differential for many years. I didn't need snow to skid steer.

 

72 Chevy Nova - standard rear end. California, rain, empty parking lot - tons of fun as long as you didn't hit any light poles.
302 high performance engine. Fastest thing I ever drove - - - 140 with a passenger and two surf boards on the roof. Yes, I was a stupid stupid kid back then. I'm no longer a kid.

Put my own oil pressure and engine temperature gauges under the dash along with volts. Put RPM on the steering column. Had a Woof Whistle with a bike brake handle for a trigger. Like I said, I'm no longer a kid. But boy I had a great young life. Lots of fun and luck. No tickets, no crashes, and I'm still here today.

[edit] forgot the CB radio and in dash 8 track.

 

72 Chevy Nova - standard rear end. California, rain, empty parking lot - tons of fun as long as you didn't hit any light poles.
302 high performance engine. Fastest thing I ever drove - - - 140 with a passenger and two surf boards on the roof. Yes, I was a stupid stupid kid back then. I'm no longer a kid.

Put my own oil pressure and engine temperature gauges under the dash along with volts. Put RPM on the steering column. Had a Woof Whistle with a bike brake handle for a trigger. Like I said, I'm no longer a kid. But boy I had a great young life. Lots of fun and luck. No tickets, no crashes, and I'm still here today.

[edit] forgot the CB radio and in dash 8 track.

You are bringing back a lot of memories! One of my buddies had a car very similar to that. I remember when we forgot to torque the rear driveshaft bolts and the driveshaft fell out on the street during our test drive. We did stupid kid things too. It's a miracle we lived through it all.