Fuel gauge question for anyone with an aftermarket ecu or engine swap

[Escy]

Has anyone got an aftermarket ecu or an engine swap? Have you been able to retain fuel gauge functionality?

[specboxCO]

The ECU doesn't talk to the fuel gauge at all, at least on a non canbus car. Its a direct feed from the sensor to the gauge.

[Escy]

I don't think that's correct. ECU plug D, pin 33 is fuel level and pin 34 is for the reserve light. I was reading on another forum about someone fitting a Boxster cluster into a 914, from full to 1/4 of a tank it works from the sender, the opinion on the forum was from 1/4 tank and lower it used calculations based on the ECU consumption data.

I want to see what the score is from someone that's using the standard gauges but not the standard ECU.

[Reebuck1]

I installed a 3.4 996 motor in my 1998 Boxster and also added a 996 gauge cluster.
I re-flashed my original DME with a 996 ROW flash. My gas gauge worked fine but I needed to reprogram the cluster for oil pressure and MPH.
I know there are multiple sending units for certain years gas tank and believe your gauge and sending unit has to match or you will get a distorted reading. Hope this helps

[Escy]

My sender and gauge match but whatever function that pin 33 and 34 are used for will be lost which wouldn't have been the case in your swap.

[itsnotanova]

I didn't have any issues with my fuel gauge. My swap was pretty simple though. I used a 2001 3.4 engine, DME, Immobilizer, key fob, cluster, 4 stalk all from the same donor and installed them on my 2000 S. The only wiring I had to do was for the OBC, oil pressure gauge and the water temperature gauge.

[specboxCO]

Originally Posted by Escy

I don't think that's correct. ECU plug D, pin 33 is fuel level and pin 34 is for the reserve light. I was reading on another forum about someone fitting a Boxster cluster into a 914, from full to 1/4 of a tank it works from the sender, the opinion on the forum was from 1/4 tank and lower it used calculations based on the ECU consumption data.

I want to see what the score is from someone that's using the standard gauges but not the standard ECU.

I've done some more digging. On a 986 for the first 3/4 tank the cluster takes a reading directly from the fuel sensor. Once the reading from the sensor hits a certain level, a signal is triggered from the cluster through the fuel status pin #22 on the blue connector to DME pin #39. This then starts the the dme sending a signal from the fuel consump pin #83 linked to the injector duty cycle to the cluster pin #13 of the white connector. The cluster then calculates the fuel usage and displays the last quarter tank.

In short, as long as you dont go below 1/4 tank the cluster will work. Once you go below 1/4 it wont work unless you rig up a signal from the aftermarket ECU to mimic the DME.

[Escy]

Thanks, seems like a lot of hassle to get the last 1/4 working.

[Qmulus]

Originally Posted by specboxCO

I've done some more digging. On a 986 for the first 3/4 tank the cluster takes a reading directly from the fuel sensor. Once the reading from the sensor hits a certain level, a signal is triggered from the cluster through the fuel status pin #22 on the blue connector to DME pin #39. This then starts the the dme sending a signal from the fuel consump pin #83 linked to the injector duty cycle to the cluster pin #13 of the white connector. The cluster then calculates the fuel usage and displays the last quarter tank.

In short, as long as you dont go below 1/4 tank the cluster will work. Once you go below 1/4 it wont work unless you rig up a signal from the aftermarket ECU to mimic the DME.

Are you 100% on that? I know that you need to do this on cars with saddle tank, like turbos and C4Ss where the sender can't get to the bottom of the tank, but I thought that on the two wheel drive cars the cluster reads this directly.

I guess this is easy to test, I have 986, 996 C2 and 996 Turbo clusters on the bench and can just simulate the tank sender. That isn't a circuit that normally has any problems, so I have never needed to dig deep on those.

[specboxCO]

Originally Posted by Qmulus

Are you 100% on that? I know that you need to do this on cars with saddle tank, like turbos and C4Ss where the sender can't get to the bottom of the tank, but I thought that on the two wheel drive cars the cluster reads this directly.

I guess this is easy to test, I have 986, 996 C2 and 996 Turbo clusters on the bench and can just simulate the tank sender. That isn't a circuit that normally has any problems, so I have never needed to dig deep on those.

Nowhere near 100%. Trying to put together information from various sources and understand how the system works. I read straight from the sender into my race data logger, but the accuracy is so bad at the bottom end, if the cluster doesn't use the injector volume, i have no idea how it would get any kind of reasonable read out.

The DME 5.2.2 also has all of the necessary pinouts and data, and was only used on early boxsters and C2s, so I have no idea why they would implement it, and not use it.