How much is a voltage regulator worth to you? They only cost about 85 cents (USD). Well, I spent over $800 trying to sort out an overheating issue with my 1978 242, and the final culprit was a bad voltage regulator in all three guage clusters that I tried. On 1981 and later 240s, the voltage regulator is easy to replace – just unscrew the old one and screw in the new one. On the older cars, the voltage regulator is a non-solid-state box that, at this point, is very old and prone to failure. When it fails, your temperature and fuel gauges see battery voltage (12+ volts) instead of 10.0 volts. Consequently, both gauges will read higher than normal, making you think that the car may be overheating – as well as increasing your risk of running out of gas!
So, here is a quick guide for how to upgrade your 1973-1980 gauge cluster with the old voltage regulator to the more modern solid-state style. Note that this applies to 140, 164 and 240 series cars equally.
First, you need to test your old cluster. Remove it from the dash board and leave it plugged in. With the key in position II (“on” position), test the voltage at the three spade connectors on the brass-coloured box, on the left in this picture. The outside two should be battery voltage and ground, and the middle pin should be 10.0v. If the middle pin reads battery voltage, or anything different than 9.5 to 10.5 volts, it’s time to replace it.
The next step is to order your new voltage regulator. Since they’re only 85 cents, order 2 or 3 in case you get a defective one. You need a standard 3 pin 1.5A Fixed 10V Pos. Voltage Regulator. I bought mine from Mouser Electronics. The part you want is a Texas Instruments UA7810CKCSE3, or just 7810 for short. Here’s a link to the data sheet on TI’s website.
Once you’ve got that, check the data sheet for the wiring. One pin is input (12volts in), one pin is common (i.e. ground), and one pin is output (10.0 volts out). Now, the job is just to wire those three pins on the regulator to the three spade connectors on the back of the instrument cluster where the old voltage regulator was hooked up. They are labeled on the instrument cluster with a 12v or + for 12V, and an upside down ‘T’ for ground.
First, take off the old regulator – note that the left pin is marked by an upside down T (ground) and the right pin is marked with a + (12v battery voltage). The middle pin is 10v, since it only goes to two places – the nut at the top of the fuel gauge and the nut at the bottom of the temp gauge. Note that on this cluster light green paths are where electricity flows and dark green are the borders in between circuits.
Next, attach your new regulator to the cluster. The hole in the new regulator is just for attachment – it doesn’t have to ground to anything. I used a piece of old intake manifold gasket material to put between the regulator and the circuit board of the cluster to protect them, since it should be able to handle the heat if the regulator gets hot. A piece of rubber or hi-temp plastic should work too. Then just screw it down like so:
Next, you’ve got a couple options. Solder wires to the pins on the regulator or use some kind of connector. I had an old cooling fan from a desktop computer that had the same kind of 3-pin connector, so I just cut off the plug from the cooling fan and used that for my wiring. Any computer store worth its salt should have one of these 3-pin connectors for a few dollars. The next step is to strip the wires on the ends.
The wires were kind of thin (20 gauge I think) so I stripped longer pieces and doubled them up when I put on the spade connectors. This way there was more metal in the spade connector for it to crimp down onto. Make sure you get MALE spade connectors, as the connectors that are soldered onto the back of the instrument cluster are female. Now, just make sure the right wires go to the right spades, and you’re done!
When I got this cluster back in the car, I thought it didn’t work. Actually, it did, but the car was almost out of gas because the needle had been reading high. Before the new regulator, it has about 1/4 tank, and afterward, the more accurate measurement was in the red at the bottom of the gauge!