Modifying an ancient (to modern standards) speedo to different tooth count - Page 1
Forum section: electronics / beginners

author: Vauxi
author_rank: Contributor
timestamp: September 30, 2015, 06:00:24 pm
content: A newbie saying hello.Old volvo 240 electric speedo reads car speed from back axle wich has 12 teeth gear. Plan is to turn it to read abs ring wich has 48 teeth. Some yankee guy allready done some investigation on this same gauge, but imperial units. Looks like kilometer units has slightly different pcb. Therefore I had to sign in to here and post a question.I've did some measurements. As sender at rear is vr type. I fed speedo with poor sinewave fromjimstimand scoped that 128hz is 60kmh spot on. And 213hz 100kmh. Where pcb differs a lot from mile version is input signal conditioner circuit. Kph meter has smd stuff and with miles, you have more root stuff, through hole. Now, when I "drove" at 60kmh a lm2903 does sinewave to square wave conversion. Then signal is fed to hef4024bt rippel counter. Looks like that rippel counter either drives the main cpu straight or via one transistor. Not sure about this. Either way, the output signal to main cpu input is terrible three square wave pulse signals with pause. Unfortunately I have not yet measured the output signal frequency or length. By uaf2115 datasheet. Cpu needs one pulse per one revolution. It makes me wonder why then put three pulses in a row.Picture from main cpu uaf2115 and this signal conditioner additional pcbQuestion is, how I can alter these three pulses to achieve 1/4 tooth readout when driving at 60kph at desk? I'm assuming it is done by changin ripple counter addition circuitry? I'm counting myself as newbie to electronics.I can post more pictures when needed.Speedo K=24364Volvo 242 -82, speedo from -86http://cleanflametrap.com/speedo.htmlMile meter backengineeringhttp://www.datasheetcatalog.com/datasheets_pdf/U/A/F/2/UAF2115.shtmlDatasheet to the cpu uaf2115As you can see, back engineered pin numbers are way off as writer states out at the end. This drives me nuts

author: mikerj
author_rank: Super Contributor
timestamp: September 30, 2015, 08:12:36 pm
content: You need to find out which output on the ripple counter is used. If the standard design uses output Q4 or lower, then you should be able to modify the PCB to use an output two orders higher which will give the divide by four function required (48 teeth/12 teeth).If not then you could adjust the RC time constant on the UAF2115 (it's not a CPU!) to get the speedometer reading correct, but the odometer will not be correct, or build a divide by four counter (e.g. using another 4024) on a small PCB and wire it between the LM2903 and the 4024 in the speedometer.

author: Vauxi
author_rank: Contributor
timestamp: October 01, 2015, 03:24:51 pm
content: Okay. I forgot to say that I have tried to modify rc time constant. And it does change speedo only. I'll look into the output of 4024.

author: Vauxi
author_rank: Contributor
timestamp: October 04, 2015, 12:55:15 pm
content: Looks like that pin 11 is used as output. It is q1. So then q3 should be correct one?

author: BradC
author_rank: Super Contributor
timestamp: October 04, 2015, 12:58:29 pm
content: I like what you are doing, but if you want to take the easy way out the early ABS 240's had a converter "relay" that did the division and fed the speedo to run a 12t speedo on an ABS 48t tone ring.

author: Vauxi
author_rank: Contributor
timestamp: October 04, 2015, 02:08:26 pm
content: Everyday you'll learn a new thing. Thanks. But I think when I eventually may find this divider I have been modified pcb a long ago. 240's with abs are so rare here.

author: mikerj
author_rank: Super Contributor
timestamp: October 04, 2015, 09:51:15 pm
content: Quote from: Vauxi on October 04, 2015, 12:55:15 pmIf q1 is definitely the output that drives the UAF2115, then modifying the PCB to use the q3 output should fix your main problem. It's also possible that the signal conditioning circuit (LM2903 and the passives around it) may not work properly at the four times higher input frequencies, but you'd need to test that.

author: tooki
author_rank: Super Contributor
timestamp: October 05, 2015, 10:54:14 am
content: Quote from: Vauxi on September 30, 2015, 06:00:24 pmI didn't know anyone made electric swimwear, never mind Volvo!

author: Vauxi
author_rank: Contributor
timestamp: October 05, 2015, 05:08:05 pm
content: Mikerj, Yeah. Signal conditioner has to work. I'll have to make an cheapo sinewave generator to drive "swimwear" at full speed. Even it does not match the real thing.

author: BradC
author_rank: Super Contributor
timestamp: October 06, 2015, 12:34:14 am
content: I calculated this out when I built an adapter to run a GM Delco cruise control in the Volvo.It's about 103hz for 60kph (12t). So 412hz for the 48t.

author: Vauxi
author_rank: Contributor
timestamp: October 10, 2015, 07:04:43 am
content: Allright. I cutted line from q1 between some spot and counter and soldered jump line from q3 to the spot. It did make an difference, a worse difference. Tooth is reduced but same time needle started wobbling a lot. :/ Have to scope it. I kinda understand reason for the wobble. But how to fix it. :/ I can not yet say does it need more hz to drive same speed. It didn't feel so.

author: Vauxi
author_rank: Contributor
timestamp: October 18, 2015, 09:01:31 am
content: Allright. Did some studying again. This time with screen captures. Last time I remembered wrong pulse count when using original HEF pinout. It was not three but two pulses and pause.And then compairing original and q3 we can see the difference.Should this cause the major wobble at meter ? Or did I had some soldering issue?At first it looked like there is no ground at my scope lead. Signal was way off the scale. After removing jump and re soldered original connection, signal at q3 and q1 were as showed above. And wobble is gone as it should be.PS: Dave must be laughin about my scope...

author: Vauxi
author_rank: Contributor
timestamp: October 18, 2015, 01:18:57 pm
content: Yes. There was something wrong with my measuring. But this is the output signal from q3 when connected to circuit. And of course. meter is jumping.

author: Vauxi
author_rank: Contributor
timestamp: October 21, 2015, 02:58:11 pm
content: There is no use of modifying rc time constant. Just tried varied caps and resistors. It just wobbles away. So I think that different kind signal output from hef should be the answer? Same amount of pulses, but different style.

author: Vauxi
author_rank: Contributor
timestamp: November 23, 2015, 05:10:40 pm
content: Oh yes!. I managed to modify tooth count so that now meter shows correct with 9% of accuracy. A littlebit of fine tuning is needed.For some reason I had sensor cables swapped wrong way around. That caused a lot of troubleBut this is beginners section..

author: RAPER
author_rank: Contributor
timestamp: July 16, 2022, 05:13:36 pm
content: Hello, is this counter working properly ?. Are you able to send me the parts list, with speedometer and odometer section ?? I want to make such a patent in w124. I already have the UAF2115 bone.greetings