Red Ignition Light warning (MK IV)

The red ignition warning light on my 1.5 litre 1948 MK IV has started to come on. Initially I thought it would be the fan belt - but this seems to be at the correct tension and the dashboard ampmeter shows a nice positive charge and a meter put across the battery the correct expected voltage. This only occurs above 1,500 RPM. The red lamp is wired correctly to Jaguar drawing - so any suggestions please of why this fault should now happen and what to test or check?

Below 1,500 RPM the red ignition light is off.

Above 1,500 RPM the red light now comes on.

  1. Generator (dynamo) - clean the commutators with Scotchbrite and check the condition of the brushes.
  2. Voltage Regulator - clean all connections with muriatic acid and water rinse, and file the contact points

    I apparently failed to take a picture of the back of my regulator after cleaning in muriatic and water but all the brass looked like new.

I wonder if you have a bad earth connection to the control box? Try connecting an external wire between the E connection and any convenient good earth. Perhaps you can get someone to watch the lamp whilst you just touch the earth wire on. Be careful that you touch the correct connection. Unfortunately I don’t have access to my car at present so I can’t check it.



I think the connector I’ve circled on Rob’s photo might be earth. Rob will know.


Thanks. I will investigate the earth side first as I seem to have a good and constant charge from the dynamo.

Trying to figure out why the red light comes on at higher revs, and tracing through the wiring diagram. It goes off OK when the dynamo chargers when the engine speed increase from tick over, but then comes on at higher revs. There must be a sudden difference of voltage across the bulb to make it come on - it’s either off or on and no gradual brightening or dimming.

So I can check and test - What is the normal voltage to the Regulator box from the dynamo (D) and what is the Field voltage range to the dynamo (terminal F). Am I correct that as the dynamo spins faster the Field voltage should reduce to keep the output constant?

If you have an RF95 control box then the Earth terminal is well marked and easy to access.


The screw terminal E has a letter E under it at the arrow tip but is hard to see, and is indeed an earth. The other screw terminal has the letters AUX at the arrow tip and is negative hot. These two are not shown on the wiring diagrams.
You can power an inspection trouble light from these two. I have my starting carb solenoid connected here and to a switch under the dashboard, rather than the thermal switch on the water manifold.

Incidentally, when looking at wiring diagrams of this regulator, it is well to know that they are drawn as if you are looking at the back side, as you would be if you were connecting wires on the back, so the terminal labels AUX A, AUX B, H, S&T are in reverse order from the labeled fuse tabs you see on the front side.

My car has the RF95 regulator box and separate SF4 fuse box, but basically the same wiring and earth connections I need to investigate.

Not sure what date or models it changed from or to the single combined RF91 unit.

One step to consider is replacing the RF95 unit with a known working RF95 from the 1950s. I have found what appear as new RF95 units can be problematic. Since the wiring in the photo looks pretty new, maybe the RF95 unit was replaced with a new replica that is not functioning correctly. Note the cap is unimportant, sometimes people put an old cap on a recent replica.

The wiring is all new last year as the old cotton covered looms had dropped apart and not safe, but the relays, fuse boxes, etc are all original - just stripped down and cleaned up before being refitted.

Problem solved this afternoon - my technical error!!! When my thermostat developed a fault and sprayed boiling water around I took off the dynamo to make sure it was OK. For safety I disconnected the battery and the dynamo wiring to the RF95 box. Spot the deliberate mistake in the photo below - I’d only put the ignition warning light cable back incorrectly to the field F terminal and not the D dynamo output!!! Consequently as the engine speed increased the regulator was dropping the field voltage (to keep the dynamo output constant) and consequently the red warning light came on. School boy error, but I’ve learned my leason!!!

The ignition light wire (yellow/red) was to F

It should have been in terminal D

Job done, and working as it should.

Well spotted! Glad you got it sorted.