While troubleshooting the tachometer there was a lot of info out there about the system and it's components but nothing on testing the tach amp with something basic like a digital multimeter or any other realistic way to get an idea of it's condition beyond dissecting it and looking at it or buying some high end test equipment. This is not for the guys with oscilloscopes and the like. That equipment is not on the list of home auto mechanic tools. I could always get such data but it still wouldn't be much use to most of us.

After the gauge itself tested ok and after replacing an absolutely shredded crank pickup sensor my last component was the tach amp. There's always a chance that there's a break in a wire somewhere but the odds were on the tach amp. I bought a used working one and decided to still get the info I was after for future reference. For what it's worth these are resistance readings from every pin combination. I compared it to the bad tach amp and not one single reading matched this good one.

The +++ symbol is to indicate the fact that the readings on some of the pin combinations would steadily climb. It would be ideal to isolate every component inside the tach amp but here it's not possible. I suspect that the variable readings have to do with the interaction of other components on the same circuit.

This is off the car with a room temp tach amp and an auto-ranging DMM. I don't know min/max working ranges or which readings might indicate impending doom. I just know that this tach amp works and these are it's readings.

Pins ______ Ohms
2-3 _______ 40 KΩ +++
2-6 _______ 8 MΩ and dropped to 6 MΩ
2-7 _______ .2 Ω
2-8 _______ .2 Ω
2-9 _______ 3.625 MΩ
3-6 _______ 27.10 kΩ
3-7 _______ .850 MΩ +++
3-8 _______ 1 MΩ +++
3-9 _______ 3 MΩ jumped to 20 MΩ
6-7 _______ 1.25 MΩ +++
6-8 _______ 1.3 MΩ +++
6-9 _______ 0L(beyond meter's range)
7-8 _______ .2 Ω
7-9 _______ 14 MΩ +++
8-9 _______ 25 MΩ +++

Because the amplifier has transistors, diodes, an IC and capacitors as well as resistors, a resistance check is pretty much useless. Add to that, the amps are quite often intermittent. The number one cause of failure is solder cracks around components. Even if you find a good one it should be disassembled and every connection re-soldered with fresh electronic grade solder.

It takes a while to get all the silicone rubber off the board first (trace side only) but it is not that bad with a small jewellers screwdriver and sharp tipped tweezers. Put a favourite radio station on and spend 1/2 hour picking away.

Also change the 100uF 16 volt electrolyic capacitor while you are in there. Use a 25 or 35 volt one instead of 16.

I just repaired 5 that I had accumulated and all were defective due to insuffient solder.

Again, this is for the guys who aren't going to do that, atleast not right off the bat, especially if it's a working amp. As long as the "other components on the same circuit" are within operating specs(the tach amp operates) then using this many resistance readings can establish a general baseline to rule in/rule out an amp.

This compares 15 different resistance measurements to 15 measurements of any other amp. For example this working amp had one 0L/Infinity reading. My non-working amp had eight 0L/Infinity readings. I think such a comparison can actually be quite useful. If I was coming here blind with doubts about my tach amp and all I had was a DMM and those were the readings I got I would easily call it dead, which it is. THAT in turn takes me to the decision to replace it, repair it myself, or send it to someone who repairs them. Anyone who makes it this far has either seen the tach amp repair threads already or they'll find them soon enough. It usually comes down to time vs money. Is the juice worth the squeeze?

I already dissected my dead one when the replacement was en route and it had a few burns on the upper disc. Maybe I'll go through it some day but for now it's the cruise amp. That one will be re-soldered and re-resistored.

The resistance measurements mean nothing in a transistorized circuit. The resistance will change with temperature, stored capacitance in the circuit, and the amperage the multimeter puts through the circuitry in question. If the tach amp isn't working or is working intermittently, reflow it. End of story.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Yes so do all 15 pin combinations have a transistor between them? If every single pairing of pins has a transistor in it's circuit and there's absolutely not one single telltale reading out of 15 possible readings then ok.

Correct, IF the tach amp isn't working then reflow it, trash it, etc. I'm trying to back up to the step where its condition got determined.

As a quick analogy of what I'm trying to do, which is find a way to get feedback without any of the right tools, if I wanted to see if a cylinder had any compression at all but I didn't have a compression tester I could put my hand over the hole and atleast tell if I have a 0 and make decisions from there. I suppose I'm looking for general data where it may not possible. Anyway I digress.

The resistors in the circuits rarely ever fail. What is the usual culprit is failed solder connections or electrolytic capacitors that have dried out and failed. A resistance reading won't tell you much since a poor solder connection can still test "good" but fail to pass any current causing a "fault". Any circuit with a transistor in there will give *a* reading, but the value is meaningless. You're time/money ahead to pull a faulty/intermittent tach amp and repair it. If nothing else, you'll retain more hair on your head.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The wires from the pins to the circuit board are hand soldered and leave a blob of rosin on the board that goes dark brown and can look like a burn.

If you indeed have burn marks on the board then it is likely the 33 ohm resistor frying from a shorted 100uF electrolytic capacitor.

The resistance readings are a useful troubleshooting tool. Specifying the type of meter is critical...as you did...DMM. An analog meter would be very different.


The one critical piece you omitted is the polarity. Which contacts were black DMM lead and which were red on your chart. If there are diodes in a circuit for example, they will read low resistance in one direction, but infinity with the test leads reversed.

__________________
Mark in MA
05 MB E320CDI 402k Granite Grey Metallic
05 MB E320CDI 267k Black
05 MB E320CDI 232k White
05 MB E320CDI 209k Tectite Grey
99 Dodge 2500 Cummins 5sp 148k
62 Jeep CJ-6 120k

I should have noted that I reversed the leads on each combination and the only difference was the meter showed negative "-" polarity on some combinations and no difference on others. It's a decent meter(Fluke 115). If you think about it someone at some point has rebuilt one of these tach amps that didn't need to be rebuilt for a few more years. I have to wonder if it really is impossible to cull out a useful reading(s) with resistors involved. Maybe using a range of temperatures.