Fuel Gauge Issues

[ChasR]

My fuel gauge is playing funny buggers on me. Sometimes it goes up to around halfway (with around £30-40 of fuel in it) and sort of bounces a little at that level and then it will tail off to almost hitting the bottom.

Today I went into the boot and took apart the awkward sound deadening around the fuel tank, and checked the resistance at the G and T terminals via Clark's guide at the sender. This showed around what I expected it to be. With bouncing the car around slightly, it moved slightly in accordance too.

However, when I put a 4 Ohm resistor to sender terminals on the wiring with the ignition on the sensor hit the bottom line, and as soon as I took it off, the sender went down to to the bottom.

My dad then took a look at reference voltages. At terminals G and T I think (I'll need to ask him) it only produced around 2.1 Volts when he thought it should be 5V. He then tried it via an earth on the car and it went to 8.6 volts.

I have taken the cluster out before and given the multiplug connections a clean with cotton buds, Super 10 switch cleaner and then some application of Vaseline (ooo err). I am suspecting that it is either a gauge fault, stabiliser fault or a ground fault.

Where is the ground for the sender and also where is the stabiliser and what voltage should be present at it.

It may be worth mentioning that the temp gage had a habit of going schitzo (maxing out) with the tapping of the clock sometimes bringing it down. Whilst it's not gone, it has much improved and it is much more stable as a result.

So where should I look next?

I should point out that the warning lights (parking brake, ABS etc.) Oil pressure and voltage all work fine as do the tachometer and speedometer.

 

[DavidL]

It's easy enough to replace the temp/fuel gauge- might be worth seeing if anyone around you has a spare you could try/borrow?

 

[scam75]

I had the exact same problem with my temp gauge. I fitted a different gauge and its now perfect. Would suspect gauge for temp issue if flicking it helps, fuel gauge is same instrument so can't hurt to replace it.

 

[ChasR]

Well I don't want to blow my own trumpet but I think I have fixed it now.

As I said I didn't need to tap the temp gauge anywhere near as much as before but the fuel gauge was still being dicky and not registering most of the time.

I took the cluster out and took out the oval pieces where the 8mm nuts hold them in. 3 of the 4 prongs were clean, but were given a clean anyway and were vaselined very carefully (only a smear on them), but the fuel gauge prongs were oxidised and black in places. I got the sandpaper out and sanded it until the black was off and vaselined them also.

Stuck it in the car, and flicked the ignition, and it shows that I have 1/4 of a tank . Only thing is I haven't had the red light come on in a while. How low do you have to run it until the red light comes on?

I'll put the pics up showing the dirty fuel item.

The Oval pieces I refer to are in DavidL's post here:
http://www.porscheclubgbforum.com/tm.asp?m=461380&mpage=1&key=temp&#461380

 

[Diver944]

The red light comes on extremely late in the range. In fact the needle is about to pass out from under the red line before the light comes on. Might be worth carrying a spare can and double checking if you haven't seen yours in a while (just in case its bust [] )