Easier than expected. The tachometer in my car showed too high RPM. I measured it and in idle 850 RPM it showed 2000 RPM. In various forums you can read about a hidden adjustment screw which requires drilling, or you can send your tacho to an instrument repairer. But if you have the old type of Tachometer chance is that it wont fix anything. I got a call from Christer, a gentleman in Kolmården also a radio technician who had had exactly the same problem. He has a white -61 in very nice condition. He said he could send me some appropriate resistors. If you have the same problem, your fix can be simple. You dont even have to remove the meter, just what I was hoping for. This is his car.
From 1961 to sometime 1964 the Tachometer had a sender unit placed in the grille. Reason is that it got hot. Apparently they contain condensator(s) that age which is likely to be the main reason for the tacho showing too high revs. For those that have read this blog, early on I found out that the tank meter is in effect a volt meter. The old type of Tachometer is in effect a Ampere-meter, measuring current. Any meter has a internal resistance and using the logic of Ohms Law you can lower the current (Ampere) by introducing a resistor before the Amp-meter.
So how to identify if you have the old Tachometer (also check the bottom of this post)? According to Volvo, chassi# 1 – 9999 had the external sender unit with very few exceptions (Karl-Eric Målberg found these exceptions: 9441, 9883-9887 and 9905-9911). Still, the (probably) most secure way is to check the backside of the meter. It’s a very cramped space but I used a light and my iPhone switching to the front camera. In this way it works like a mirror and you can video while looking. If you look on the meter from behind you want to see two blade connectors way up at the top, the left marked with “+”. Dont touch that, that is the 12V feed. The right one is the signal from the sender unit in the grille. Then you need something like this:
NOTE: BEFORE DOING ANYTHING WITH THE ELECTRICS, DISCONNECT THE BATTERY.
Christer had guessed I needed 300 Ohm and he had even mounted it. Then it is a 5 minutes fix. So with the video in mind you can feel your way to the right blade connector and loosen it. Then mount the 300 Ohm resistor and fit the female to the meter and the loosened sender unit’s female to the male in the image. To avoid any possible short circuiting make sure you have shrink tubes at the resistor and use electric tape to isolate the coupling between the sender unit and the male above.
So, these are not ordinary resistors, these can take some power(/watts). The power is not very high, probably less than a Watt. According to some old Smiths/Volvo documentation the current should be 10mA and voltage 5V at 2800 RPM. I dont understand all parts fully but if current (Ampere) is correct, the power law gives P=U*I=12*0.1=1.2Watts as voltage wont go higher than 12V with a car battery as the base. And in a series coupling, the sender unit takes some volts, the meter takes some volts and then we have the newly introduced resistor taking further meaning that the Watts is probably lower than this. Here’s the effect on my meter with a 300 Ohm resistance. Just below 1000 RPM instead of the 2000 RPM it showed without the resistor.
Also, I wrongly assumed that the previous error was constant. I.e. that when driving with RPM 5000 the actual value was again some 1100-1200 below. This is not likely to be the case. I will check this further with some revving and driving but right now it is very rainy here. And for the time being I am happy to for the first time see the actual value at idle on my Tachometer.
Disclaimer. No guarantee it work for others. But if it does, make sure you use a enough-power resistor that can handle some power consumption. A simple small 300-500 ohm resistor found in the nearest electronic shop is likely to get hot and introduce fire hazard. We wont do that (!!).
How to determine which Tachometer you have?
Here’s quite a blurry still from the iPhone video I did to find out myself. You can see the two blade connectors at the top, the left one marked with a faint “+” above the screw, that is the 12V feed. To the right is the one to fit the resistor on. Visible is also the red cables grounding the two lights, one on each side and below the blade connectors. This is how the old type tacho looks.
The 2nd generation tacho (but still old) look different on the backside. They dont have the two blade connectors at the top. Instead the 12V feed is to the left in the middle and to the right (still in the middle) there’s some kind of pickup loop. If you have this type of tacho, there are reports of some succeding drilling a hole according to the image below and finding some kind of adjustment screw below.