I think i may need a wiring diagram

[Ado1379 0]

Been chasing fault with my instrument cluster for a while now. My water temp and oil temp on the MFA are not that reliable.

-The water temp would sometimes sit at 110 degrees or it would correctly sometimes. 

-The oil temp reads perfectly when Idling staying at 102 degrees for as long as the engine is running. As soon as i go for a drive it climbs up to 160 degrees, but if i turn the car off and immediately start it again and flick the MFA over to oil temp it will be reading 110 degrees or so.

The whole coolant system is brand new, The oil is brand brand new with new filter and new sensors. 

Now here is the good bit... I took the clocks out so i could replace some bulbs and have general look around behind there and noticed 2 burnt out pins on the instrument plug. Traced the wire back and all i can say is (as i have only had the car for less than a month) someone is extremely lucky they didn't burn the car out! ( see pictures)

 

Now what i need is a wiring diagram for 1994 2.0 16v. I have found a few online but they don't seem to correlate with what i am seeing albeit i only had a quick look. The burnt out pins are 3 and 5, and i have a feeling these are ground. This could explain my dodgy readings as i would expect the resistance to be off. But either way this needs sorting. 

 

[Cressa 44]

I've just looked in the Bentley manual and have only found the instrument cluster wiring for  VR6.

May be useful..... have a look

 

[Dox 23]

Oil temp for sure earths on the block (single wire sensor). I'd put money on it the coolant temp does too (eyelet to the block?).

 

Make sure the main engine earth is in good condition - broken / corroded / loose will mean heavy starter current turning over the engine will try and earth via any wiring connected to the block instead

[Ado1379 0]

You are correct in both sensors earthing to the block. The gauges themself need earthing also. And i believe this wiring is definately the issue. Ive reterminated them to the bracket that holds the instrument cluster in and took it for a drive. A lot less irratic and oil temp topped out at 118 degrees. Although when i turn ut off and immediately back on it reads 96 degrees. I think the earth point isnt great so i will find a better one tomorrow but all good so far. Now need to find out what caused this problem in the first place!

 

Thsnks for your help

[Dox 23]

1 hour ago, Ado1379 said:

The gauges themself need earthing also

 

 

Nope, 12v (actually 9 or 10V via a the clusters built in voltage regulator) to one side of the gauge, negative via the variable sensor on the block.

 

 

[fendervg 33]

Correct - they are single wire sensors, earthed to the engine block via the mounting threads - this then has to go back from the block to the battery via the engine earth - even a small amount of corrosion or a nicked wire can cause issues.

[Ado1379 0]

Okay so still on this issue. 

Changed the sensors and found out where those wires terminate to. So i now have a new issue. 

On start up, the battery light stays on. Battery reading 11.7v with alternator running. Give it a rev and it jumps to 13.6 - 13.7v. I think i may have a earthing issue but here is what i have found:

-All earths read 0 Ohms with engine off (including casing to the alternator)

-With engine running, all engine earths read 0.8 Ohms but the casing to the alternator is 2.8 Ohms.

I have replaced all earth straps and cleaned all earth studs. Still no difference. 

I have had this car for just over a month now, and drove it all the way from Manchester to Belfast with no issues. The only work I have carried out is replace the front engine mount. Surely this would not cause these issues? 

 

Can anyone shed some light on why all resistance between chassis and engine to the battery increases with the engine running? 

[Dox 23]

Low idle speed, too larger pulley on the alternator (lower rotational speed) will both cause the alternator to not charge until revved. AS will a failing alternator - remove the brush pack and check the ends of the bushes for length - check they move easily against there springs - soot on the end of the brush is a sure sign of arcing.

 

Resistance causes voltage drop, testing for voltage drop is more meaningful than resistance (unless you do this day in, day out).

 

Meter to DC Volts

Start the car and rev it.

The source of power is the alternator, positive lead to the alternator cable tag, negative to the alternator casing - note the result.

Move the negative to the earth point of the battery, note the result and compare, any loss is the voltage drop due to resistance in the circuit (alternator body - alternator bracket - engine block - gearbox bell housing - bolt / nut to earth tag on the earth lead - the lead itself - battery connection).

 

If you just want to search for the loss, place the positive lead on the alternator body, the negative to the earth connection on the battery - this is potential (voltage) difference in the circuit

 

 

 

 

[Ado1379 0]

I should have stated that im a 14 year avionics engineer so elwctronic principles are kind of my thing. 

 

Iver ordered a new voltage regulator as they arent expensive. So ill replace that anyway. 

 

My question still stands though, has anyone come accross anything that would cause high resistance even once the engine is running? As i said there is a good bonding between battery, block, and chassis.