1800es
 

Looking for ideas on an electrical fault on my 1973 1800ES. When I switch on the ignition the oil temperature gauge goes over to max. When the engine starts the needle drops back to zero. My initial thought was a faulty sender but I have disconnected that and the fault is still there. Just to make sure I am looking at the correct sender I am talking about the one low down on the engine block to the rear of the oil filter; it has a black wire going to it.

Roy

Attached is a snip from the wiring diagram for the 1973 1800 es. 66 is the oil temperature gauge and 67 is the oil temperature sender. The oil temperature gauge is supplied via the voltage stabilizer which also supplies the temperature gauge 29 and the fuel gauge (out of the picture). If the fuel gauge and coolant temperature gauge also behave erratically, I vote for a problem with your voltage stabilizer. On my 142 the voltage stabilizer would occasionally have fits causing the temperature gauge to pin in the red and the fuel level increase.

If the other gauges behave normally then the problem is likely
-in the gauge,
-in the gauge wiring, or
- in the sender

Refer to the service manual for the gauge test. The wiring diagram says that the sender wire is brown so you may have the incorrect sender. However, brown covered with oil and dirt can look pretty black. Check the service manual or get advice from somebody else on the correct location for the oil temperature sender.

I am familiar with the wiring diagram having studied at length while I was trying to work out how the cold start valve and thermo timer worked. And I have looked at this problem several times wondering what it could be. Now there has been some "rewiring" here as well. Looking at the wiring again I now notice that two wires come out of an outer sheath from the bulkhead; one is brown and one is black. They then go into a double connector and emerge as the same colour BUT then the brown has been chopped and a length of black wire attached to it; this goes to the oil pressure switch area under the manifold and I am now thinking should be the wire for the oil temperature gauge. The other black wire has also been chopped and a short length of black wire attached which goes to the what I think is the oil temperature sender.

So it looks like the wires for the oil pressure light and oil temperature gauge have been swopped. The oil pressure light does not come on when the ignition is switched on. I seem to remember it coming on when the engine was warm (can't be sure about this) so the light does work. This would make sense if the wires have been swapped and explain why the oil temperature gauge goes to max before the engine is started. Once the engine starts the oil light sensor would go open circuit and the gauge needle drop back to zero.

Roy

Your symptoms would be consistent with switched sensors. The oil pressure sensor is a switch, either 0 or infinity ohms which would be consistent with the temperature gauge being zero or max (when the engine is running). The oil temperature sensor is a variable resistance, typically high when cold and low when hot. This could cause the oil pressure light to come on as the sensor resistance drops as the oil temperature rises.

Roy;

142Guy has explained the symptoms with the Oil Temp Gauge well for you...

...and if you're still interested in how the Thermal Timer functions, see: https://www.sw-em.com/bosch_d-jetron...mo-Time_Switch

Cheers

Roy, the oil temperature switch on my 1973 ES is actually mounted (screw threaded) into the sump pan on the right hand side, it's the oil pressure switch that is in the engine block and from memory also houses the oil pressure gauge braided hose.

My ES isn't here at the moment but from memory the feed wire for the oil temperature switch is black and eminates from a terminal block on the bulkhead somewhere near the steering column area.

Hope this helps?

Paul

Update.

I've been wondering since I bought the car in March 2021 why the oil temperature gauge was behaving the way it has. Only today have I realised that it is the result of some PO's wiring.

I have separated the connectors on the two wires and connected them such that the brown wire goes to the oil temperature sender and the black wire to the oil pressure light and when I switched the ignition on the oil temperature gauge needle stayed on its stop and for the first time in my ownership a red light glowed in the oil pressure gauge; I didn't know it did this! I do recall seeing a faint orange glow once when the engine was warm which was the first indication that the car actually had an oil pressure light, so wondered why it didn't come on with the ignition. I had to read the owner's manual to see what the light was.

So I have cured two faults today! Why did some PO get this wrong? Tomorrow I'll be going for a drive and hope to get a reading on the oil temperature gauge. And of course see the oil pressure light go out when the engine starts.




Roy

Someone else suggested it would be in the side of the sump but on my car there is nothing on the sump so can only conclude that it is different on my car which is late 1973. The sensor the for oil pressure light and the for the oil pressure gauge are ahead of the oil filter under the exhaust manifold. A capillary tube goes from the sensor to the gauge. I'm going to take the car for a run tomorrow and hope to see some movement on the temperature gauge.

Roy

Roy;

Now that your indicators are connected correctly, great!...but don't be too upset if you don't observe too much action on the part of the Oil Temp Gauge! These are often suspected of not working, because in the first place, the oil temp lags (way!) behind coolant temp, and when it does finally move from the cold peg (after hours on the motorway...), it wont go up the scale very high at all! See: https://www.sw-em.com/voltage_stabil...800ES_Concerns

I do recommend you check and snug up the mount of the VStab so that it gets a good chassis connection...else the gauges supplied by it will indicate high as it is not cycling...see also: https://www.sw-em.com/voltage_stabil...Stab_Operation

Cheers

Attach a new earth strap from chassis to gearbox it’s earthing via the temp coil and raising your temp on gauge lol common fault and hurts when you touch coil lol only do it the once !

That will cure your problem