Mk. IV Voltage Regulator issue

(OddBrit) #1

I am just putting the finishing touches on a 1948 Mark IV 3.5 Saloon. Over the weekend I had an opportunity to run the car for a while. While doing so I noticed my amp gauge was pegged in the positive direction. I ran a volt meter across the battery terminal and I had 17+ volts showing. This leads me to believe I have an issue in the voltage regulator.

That being said when I purchased my car it came with a new wiring harness. When I was installing it I noticed it was for the late 1948 cars where the voltage regulator is mounted higher on the firewall than mine which is located just below the fuel pumps. The wire color was also different. I took the time to compare wiring diagrams for both and connected the wires based on the Terminal numbers/letters on the regulator.

Can anyone point me in a direction to correct this?

Thanks in advance,
Jim Bolinger (Canton, OH)

(Rob Reilly) #2

I’m no great wizard at electricals, but with a pegged needle my first thought is stuck contact points.
When you say the wire color is different, can you expand on that?
One thing I noticed when going over the regulator wiring on my '38 was that the wiring diagrams show the wire connections as viewed from the back, not the fuse side.
The '36-'48 and Mark V manuals say not to fool with the regulator (aka control box) because it is set at the factory and you can’t adjust it, but the Mark VII / XK120 manual tells you exactly how to do it over 5 pages. Here they are. Its a different model, but the principle is the same.



(Peter Scott) #3

I agree with Rob. It doesn’t sound like a wiring error but rather a faulty regulator.


(Roger McWilliams) #4

A good starting point is to identify the type and approximate age of the voltage regulator. An out-of-spec regulator sounds likely here. RF95 types often have a date code on the base. Originals tend to work well until they don’t. Some of the repros don’t work well at all. Attached are a couple of Lucas-related documents to help.

One possible step is to replace the possibly-faulty regulator with a regulator known to work. That helps guide whether the problem is in the regulator or the rest of the charging circuit.

Lucas_Generator_and_Control_Box_Tests.pdf (824.1 KB)

Section 5 Generator Output Control missing pages 15-18.pdf (1.8 MB)

LucasFault.pdf (2.4 MB)

(OddBrit) #5

Thanks all for your prompt and knowledgable replies. I think I’m going to start by cleaning the contact points in the regulator and see if that helps. I’ll let you all know the results.

Rob the wire colors on the late 48 harness that connect to the voltage regulator than on the early 48 and earlier harness. (i.e) The wire that connects to the “A” (ammeter) terminal on regulator is shown as White/Purple in the early 48 wiring harness diagram and Brown/White on the late 48 harness. Similar issues with other wires in the harness.

Thanks again, Jim