Early Tacho - Half RPM

pieboyfresh

I'm a bit stuck, i've managed to get an early tachometer which came from a 140 series, thanks to a very nice and knowledgable bloke named Wayne, who wrecks early Volvos here in Perth.

I've fitted it, and it works, but the RPM reading appears to be exactly half of what it used to be.
When I rev to what sounds like the peak of the rev range, it reads as 3.5k RPM, when i'm assuming it should be showing 7k RPM, which is the peak of the gauge.

I had a good search on here before posting, I could only find lasercowboy's findings here, but that was concerning a later model tach. On the rest of the web I found this post on TurboBricks which seems to be the same problem exactly, however there was never a fix for it.

The back of the tacho is stamped with 4Zyl./4Takt at the top, which confirms it is for a 4 cylinder engine.
It is orange between 6-6.5k and red between 6.5-7k. A few of the things it says on the back are 230/65/1 (VDO part number?), 12.73 (Dec 1973?), 14000 and JMP.

The cables connecting the back of the tacho to the two connectors either side of it on the cluster are black to positive (driver's side) and the brown to negative (passenger's side)

I have installed it with the "1" terminal on the back of the tach connected to the negative terminal on the coil, which I have read in a few places is the correct method. I used the wire which was already run through the firewall for it with the rest of the loom.

There are four screws on the back, and two on the face, none of which are labelled. As far as I can tell they are just for keeping the rear plastic and metal face on.

Does anybody have any ideas what might be causing this, or how to adjust one of these? Or if I have the wrong part?
My car is a 1980 244 GL sedan, with a B21E and M46.

pieboyfresh

Loads of photos below, as long as I've gotten nothing wrong i'll write up a step by step tutorial.

Major Ledfoot

pieboyfresh;8684 wroteDoes anybody have any ideas what might be causing this, or how to adjust one of these? Or if I have the wrong part?
My car is a 1980 244 GL sedan, with a B21E and M46.

I can tell you there's 2 different part numbers for the 4 cylinder tacho - 282772-3 for the 140 series, and 283546-0 for the 240 four potters; I can't tell you what the differences are between them. Both of them fit into late 140 / early 240 clusters. I speculate that the later versions use the SAK215 IC chip, and the early versions used something else.

Your description of how you've connected the tacho appears correct. See below for factory guide for the 282773.

At 100 km/h in 4th gear, you should be doing about 3000 RPM.

Any chance of swapping the tacho you have over with another one?

familyman

I swapped one from an early 70s 244 to a slightly later car. Just had to run a wire to the coil, screw the gauge in place, and it was done - and its count was correct. (Engine idle is about 700-900 RPM, so a bit below the "1".)

Even though the 140 was probably miles and the 240 km, the signal is in RPM on both, so the signal TIMING should be about the same. Although the 140 may have had mechanical distributor points (not sure) whereas the 240 probably electronic. If so, that might be something to do with it - a difference in the tacho internal components so it can read the two different types of spark.

You could find and play with swapping values of the tacho's internal components, but you'd be guessing at RPM without another method of checking RPM at the same time. In the end it's easier to get one from a 240 of the same vintage.

It could be a fault with the the green circuit board, but again, the easiest way to test is obviously to try a later tacho from a 240 of the same vintage. If it produces the same half-speed behaviour, then it's time to inspect and fix, or swap the circuit board over - and then fit whichever tacho you plan to keep.

BTW... The tachos are common enough. I used to see one or two fitted out of every five 244s I saw.

Angus242164

Faulty tachometer or low voltage to it due to a cluster PCB fault I suspect. Points vs. electronic ignition makes no difference, the signal comes from the coil negative which is downstream of the coil switching. Many 240's with carby engines had points ignition and used the same tachometer.

Any difference between 140 and 240 tacho's is likely to just be where the redline is marked.

pieboyfresh

I'm guessing that's the case, thanks for confirming the difference in part numbers. Some people have reported the 240 tachos working in 140s, but nobody has said the opposite, probably because it doesn't work. I'd say you're probably right about the ICs too.

That's exactly how I've done it. Thanks for sharing those instructions.

I've got an M46 box, 4spd with OD, at 100kmh in 4th it's about 3500 RPM and 4th with OD it's about 2750 RPM.

I don't have another, but the guy I got it from does. Since he mainly deals in early parts I'd say it's the same as the one I have but I'll check regardless. If not I'll post in the wanted section and see if anyone has one they're willing to sell.

pieboyfresh

Sorry guys I just realised I replied to a cached version of the page on my phone so I only saw and replied to your reply @bgpzfm142

@familyman My car idles at about the same according to the gauge, about 1200-1300 when cold and 700-800 when warm. That's the plan at the moment. Yeah I thought they were fairly common, quite surprised that when I posted on the WA Volvo Enthusiasts Facebook group I only got one reply out of a few hundred Volvo owners. I might have to ask some of the local Volvo shops instead.

@Angus242164 I think you may be right, if I can't find another tacho locally i'll probably order a multimeter that can read RPM and see what that gives me in different places. I think they're actually marked in the same place, judging by some forum photos from elsewhere. The only difference I can notice is part numbers, without pulling two apart and comparing the internals.

Dauntless

A generic $20 eBay tacho would work for testing. I have a Tiny-Tach, which is a battery powered digital tacho which works simply by wrapping a wire around one of the ignition leads. It's perfect for testing things like this.

familyman

The reason the tachos don't seem as common now, is there's not as many of the cars themselves around. ;-) I've scrapped two myself due to excessive rust.

Good to read what Ash said... The method of producing spark was the only alternative I could come up with apart from a fault in the cluster itself, why it would halve RPM. (I was thinking going electronic might have done something like producing a shorter spark so the gauge might have been 'averaging out' two 'blips' as one, etc.) But I also vaguely remember something similar happening to me years ago - and using a later tacho solved it. So I guess in that case it must have been the gauge itself (not a fault in the pod).

They're very simple circuits with big and clear circuit tracks - so unplug and completely remove it from the dash (to remove any circuits completed elsewhere in the wiring harness), put it on a table... Use a magnifying glass, etc. to look for faults visible to the eye. If none can be seen in the tracks themselves, then melt any solder connections, suck up at least some of the old solder and add fresh solder in its place. (Reason: I always hit lots of air bubbles on my 70s cars' solder and could tell by the 'crinkling' surface that took longer than expected to cool, it was past its expiry date and would probably produce more faults soon enough. So fresh is always best.)

Oh - and it could be lead solder too... Short exposure shouldn't do anything, but that, and the fact it 'spits' at you are both good reasons to sit back a-ways.

Then refit and do a test.

If no-go, take it out again and check each circuit track for continuity with the board still in place. If no joy, then loosen those 'roofing iron' screws on the back holding the circuit board in place. Check each circuit track for continuity with a multimeter while pressing/flexing the board SLIGHTLY. Tightening the screws down at the factory and years of vibration might have flexed and put enough stress on the board to crack open a circuit track - which connects again by removing the screws, making it disappear until screwed back in place. So flexing the board slightly can reproduce the fault. (It's ok, press down and it's broken, it's ok, press and it's open again...)

Of course trying a second gauge first might negate all this. As I said, I vaguely recall having a faulty gauge years ago which gave me the wrong impression a difference in the internals from 140s to 240s could have been the reason.

Oh - and that large brass-coloured regulator standing up from the board on the rear can cause weird faults. I've swapped one of those before too. Again, try resoldering the connections first - leaving the regulator in place if you can without sucking all the heat out of the iron - just in case the connections want to splay off in some weird direction.

pieboyfresh

Hey again guys, I still haven't changed the tacho yet, but since i've changed over the fuel system stuff for the parts @jamesinc gave me, i've tried driving it and revving it higher, and it doesn't mind it.

I'm thinking maybe there was a bigger problem with the fuel distribution block I had before, because when I ran it at high RPM before I would hear a bit of detonation (so I never did it), but now nothing? I can rev it out to what the gauge says is 6500rpm and it doesn't seem to hate it. I don't know, however, if the gauge is correct still so I won't be doing too much of that until I plug another gauge in. I hope that the "6500rpm" isn't really 9000rpm...

familyman

What RPM does it show at idle? Depending on if it's a high idle (where the engine sounds like it's running a bit fast and could be turned down a bit) or a low idle (where it's spluttering and shakes every few seconds like it would stall if RPM went any lower), then it should be showing SOMETHING from say 0.5 to 1 on the tacho.

Or to put it another way... If the tacho shows pretty much 0 RPM most of the time when the engine is idling ok (i.e. without being near stalling) with only the occasional blip into the 0.5 or greater range, then there's a fault.

pieboyfresh

When I have the idle adjusted to where it sounds and behaves correctly, it reads as 1000rpm. Judging by that I think it's probably correct.

In terms of sound, the shift point of an automatic box would equate to what my gauge reads as about 4000rpm.

familyman

I don't know what the RPM should be going by my gearbox upshift. But with the 4x 240s I've owned, I've found an idle of 1000 RPM (going by the Volvo tacho, which may not be accurate at idle anyway) was too high. At 1000 RPM, sitting on level ground required 'a bit too much' foot on the brake for my liking. It was as if it was dragging itself through red lights. I like it set so the car is at a very slow creep on level ground, or, just holds itself in place without rolling backwards on a slight uphill slope without any brake pedal. I think that's usually about 750-900 RPM. But it's been quite a while since I've looked at the tacho.

What you really need to do if it bothers you, is find someone with a timing light with a tacho gauge, auto multimeter with RPM, etc. And test revs at a few different ranges above idle, because the resolution (increment markings) on the Volvo tach leave a bit to be desired.

familyman

There seems to be some phone apps that measure RPM. Not sure how accurate they are. Maybe that helps?

pieboyfresh

I'll look into those options, might even be worth buying a multimeter that will read RPM since i'm sure I will need to test it again at some time or another. Thanks for the advice :+1:

familyman

About $18 AUD...

https://www.aliexpress.com/item/Professional-Car-Exclusive-Non-Contact-RPM-Meter-Motor-Speed-Gauge-Gun-Style-Surface-Speed-Tach-Meter/32852996783.html

About $47.50 AUD...

https://www.aliexpress.com/item/UNI-T-UT107-LCD-Automotive-Handheld-Multimeter-AC-DC-voltmeter-Tester-Meters/32470104238.html

This is not a recommendation because I have no idea how good those are. I have chosen links to sellers that have sold a few of each, but are still among the lowest price offered. If you think it's worth the risk but haven't dealt with Aliexpress before and want some tips, just ask. (I won't type it all unless you haven't dealt with them before and definitely plan to buy.) There are some thieves on there, and in China, whatever you can get away with is seen as perfectly normal business practice. But I think I've learned most of their dirty tricks now, so you won't get cheated.

I'd try a phone app first though. But I'm a cheapskate.

Dauntless

Chinese copies of the TinyTach are $8 shipped on eBay, and would be much simpler to use.