W108/109 RPM Gauge Problem

[racing3000]

Hi. Does anybody know that w108/109 rpm Gauge (7000rpm) can work properly on a w115 4 cycl.? We have installed today, but watched that the gauge shows the revolution remarkably less than the normal, even, could never pass 3000 rpm. Is it a fault to install 6 cycl. gauge on a 4 cycl. engine or is there another problem with the gauge? If the problem is about the gauge, is it possible to adjust it or should we change any element of it?

Thanks in advance..

[Frank Reiner]

The gauge is now receiving pulses from the 4-cyl. engine at a rate that is 2/3 of the rate of the 6-cyl. engine, and consequently reads at 66.7% of actual engine speed.

[racing3000]

Thank you. So, is there any option to convert the number of the pulses from 4 to 6 cyl. using any interface or something else?

[Benz Dr.]

I've never seen an electric tach in a 6 cylinder 108 or 109. This would lead me to believe that your part is for a V8 engine and is likely only showing half the actual speed of your engine.

[Tomguy]

I agree. In fact, I've only seen 109 tachs for the 3.5 and 6.3 (not even on the 4.5). If the redline is marked shy of 7k, it's for the 3.5. If it's shy of 6k, it's for the 6.3.

VDO currently offers brand-new gauges, you may be able to find a very close match to OEM-style, that will work with your 4-cyl.
Ex:
https://www.vdo-gauges.com/instruments-displays-and-clusters/by-series/series-1/programmable-tachometers/series-1-7-000-rpm-3-3-8-tachometer.html