Some months ago while in the local U-Pullit i happened across a late model P38 with a updated cluster.

The guage faces have a green background and the legends etc are all a bit more modern, so i bought it.

Finally went to fit it today, and after sorting some niggles (airbag warning light/message due to a missing bulb) and syncing the odometer, it all seemed to be working, except the two small guages.

The fuel guage seemed to display 1/4 tank. The petrol tank is nearly empty and with the old cluster the light was on and the guage was just off the red. Though at various points it did correctly display "fumes" as it should, it also at various points returned to showing 1/4. I took it out for a drive for a mile or so and it slooooowly dropped to the line before the red, however the light didnt come on. I dont know if it would have eventually sorted itself out. When i got back, i cycled the ignition a few times and eventually it did show the low fuel light, but after checking some things with nanocom, it went back to showing 1/4 tank again....

The test drive also highlighted a second issue. Once it warmed up the temp gauge pinned its self round against the right hand side and the red warning light came on. Checked with nanocom, and the ECM was reporting 93c as normal. Does the temp sensor perhaps differ between Gems and Thor? Any way to alter the calibration?

Info is all in RAVE

**Engine Coolant Temperature Gauge**
The engine coolant temperature gauge sensor has
the capability to sense from –40 
 C to +130 
 C. Petrol
and Diesel resistance valves are different.
Both Petrol and Diesel sensors are interfaced to the
Instrument Cluster (Z142) via the BeCM (Z238). The
resistance value then being represented by an A/D
data transfer generating the required angular
deflection on this gauge. 

**Fuel Gauge**
When the fuel tank level is low (“E”), the resistance
value of the gauge sensor is 270W for petrol and
diesel engines. As the fuel level increases, the
resistance of the sensor decreases. When the fuel
tank is full, the resistance value of the sensor for
petrol engines is 19W and for diesel engines 25.8W .
When the fuel gauge sensors resistance value
increases to 175W (9 liters/2.25 US gallons, the low
fuel warning light will illuminate to warn of the fuel
status. Both Petrol and Diesel sensors are interfaced
to the Instrument Cluster (Z142) via the BeCM
(Z238). The resistance values then being
represented by an A/D data transfer generating the
required angular deflection on the gauge.

Okay additional data points

Fuel guage behaves the same way when I plug the original cluster back in. I guess the becm will dial itself in once I'm driving.

Temp guage I suspect there is a calibration difference between the gems and Thor engines.

Measured the sensor on the engine at 150ohms at around 50c and according to some online resources measures about 600ohms at room temp.

Measured my Thor sensor and it's at 150ohms around 80c and ~1200ohms at room temp.

I would have gone about it a different way. Retain your existing main PCB and satellite board with the message centre on it but swap the actual gauges with their faces. That way, any differences between sensor voltages, will still drive the gauges to the same place. GEMS uses two temperature sensors, one for the ECU and one for the gauge but Thor has a single one containing two separate senders so it is quite possible the temperature/resistance outputs are different.

Yeah I could have gone that route, but if it went wrong I'd be staring at two broken clusters 🤣

I figured swapping them would be easier.

Just need to figure out either a matching sensor that will fit the 1/8" NPT hole, or a way of attaching the Thor sensor to the gems manifold.

From googling online the Thor dual sensor and the GEMS engine sensor both use the same thread. So it should screw right in there.

Just need to see if the Engine part of the Thor sensor uses the same resistance range as the GEMS one. I only have one datapoint for the GEMS engine sensor currently: ~4.5k at 5c

However the Thor sensor was around 2.8k at 20c, and some googlefu suggests that sensors reading ~2.8k at 20c will often read around 4.5k at 5c

I've put it in the fridge and will measure it tonight to get some more points to confirm. But we may be in luck and i can just fit the Thor sensor in place of the ECM one, rewire and job done.

Surely the easier method is to make sure the correct engine type is selected in the BECM. The BECM converts the analog reading from temp sender & fuel sensor to a value from 0-255, which then drives the gauges. It's more likely the A-D conversion changes than the gauge settings ?

But then there are many different instrument pack part numbers

As he is fitting a later cluster to an early car, the engine type will be correct in the BeCM. Changing it from GEMS to Thor may correct the temperature gauge but there's no telling what other things would be changed. I think the main differences are going to be in the cluster pcb, in much the same way that the tacho will read differently if a diesel cluster is fitted to a petrol car or vice versa. The petrol ECU sends 4 pulses per rev while the diesel sends 3 and the correct reading is calculated by the instrument cluster itself.

You'll need to add an extra ground too. The early GEMS had a single wire to the gauge sender with the ground through the body (later GEMS have a different sender with two pins so has a ground wire to it), the Thor one has a 4 pin connection, two signal and two ground wires.

Pete12345 wrote:

Surely the easier method is to make sure the correct engine type is selected in the BECM. The BECM converts the analog reading from temp sender & fuel sensor to a value from 0-255, which then drives the gauges. It's more likely the A-D conversion changes than the gauge settings ?

But then there are many different instrument pack part numbers

My suspicion is the becm simply measures the voltage and sends the signal across to the cluster as a 0-255 digital value. It doesn't do anything to the reading other than measure and pass it on.

The cluster takes that digital value and applies a lookup table for the guage deflection.

The reason I suspect this is the becm diagnostic doesnt actually show the temperature, it only displays the digital value.

I tried resetting the gems engine type and writing settings again just incase doing that would somehow pass it on to the cluster, but it didn't make any difference.

Gilbertd wrote:

You'll need to add an extra ground too. The early GEMS had a single wire to the gauge sender with the ground through the body (later GEMS have a different sender with two pins so has a ground wire to it), the Thor one has a 4 pin connection, two signal and two ground wires.

Yeah I noticed the extra wire, I guess I'll fashion up a wire to the manifold to provide that. Still haven't managed to fully test the Thor sensor yet. Maybe tonight!

GEMS: 260ohm at 90c (engine fully warmed up after a drive)
Thor: 280ohm sitting in a cup of water out the kettle (will be close to 90c but perhaps a little less)

Seems close enough for me! Same reading at 5c and same reading at 90c.

Now i just need to find time to fit it!

sensor all swapped today

Cluster is installed and working as expected