OBD2b to OBD1 = No tach
09-11-2009, 10:02 AM
#1
OBD2b to OBD1 = No tach
Car is a 2000 Integra GSR. I have an OBD2b to OBD1 conversion harness from Xenocron. My stock tach does not work. The engine harness is uncut and perfect. If I plug in the stock ECU or AEM EMS the tach works fine.
From researching this problem I found that I needed to add a 1k resistor on Pins A21 and A25 as seen in this pic:
Well I added a 1/2 watt 1k ohm resistor as instructed. Now my tach moves but it will not go past 1500rpm. If I rev the engine higher the tach drops to zero. The ECU still reads rpms correctly (datalog). Do I need to use a different resistor?
From researching this problem I found that I needed to add a 1k resistor on Pins A21 and A25 as seen in this pic:
Well I added a 1/2 watt 1k ohm resistor as instructed. Now my tach moves but it will not go past 1500rpm. If I rev the engine higher the tach drops to zero. The ECU still reads rpms correctly (datalog). Do I need to use a different resistor?
09-11-2009, 12:08 PM
#2
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Re: OBD2b to OBD1 = No tach
the problem is that the tach line in the teg does not come from the distributor like earlier versions. The tach is fed from the ECU. (calculated from the other distributor sensors)
I've read multiple times that the resistor trick works, but I've never used it. When I did my swap, my solution was to run a wire from the tach line on the distributor to the tach line on the driver's shock tower. This sets it up exactly like the original OBD1 engine was.
I've had zero problems with doing it this way.
I've read multiple times that the resistor trick works, but I've never used it. When I did my swap, my solution was to run a wire from the tach line on the distributor to the tach line on the driver's shock tower. This sets it up exactly like the original OBD1 engine was.
I've had zero problems with doing it this way.
09-11-2009, 12:22 PM
#3
Re: OBD2b to OBD1 = No tach
Id rather not hack up my wire harness if I dont have to. Obviously the tach wire is present in the ecu harness so it just needs to be connected to the rpm signal from the distributor. That is essentially the same thing as running a wire directly from dist to tach. I would expect the adapter harness to do this but apparently it does not. And if running a wire directly works, then would I need a resistor?
09-12-2009, 07:50 AM
#4
Honda-Tech Member
Re: OBD2b to OBD1 = No tach
i converted my car from obd2b to obd1 myself and the tach didn't work at first. All i had to do was take the stock tach wire which ran to the ecu and "T" it into one of the tach signal wires
09-12-2009, 08:25 AM
#5
Honda-Tech Member
Re: OBD2b to OBD1 = No tach
If your adapter harness doesn't have a double wire coming out of A21 on the OBD1 side, one wire going to A19 and the other to B13 on the OBD2 side, then it wasn't built properly. That's the ICM feed from the ECU. One goes to the ICM (duh!) and the other goes to the tach.
09-12-2009, 09:27 AM
#6
Re: OBD2b to OBD1 = No tach
Jaker - It does, and it still does not work. OBD1 A21 (ICM output) splits to OBD2 B13 (ICM) and A19 (Tach).
Contacted Xenocron. Chris had no idea. Sent me a replacement harness. That one still did not work. I did more research. Found the above resistor trick. Installed the resistor. Tach moves now but not past 1500rpm.
Here is a pic of my adapter harness with the resistor installed acording to the above diagram:
Contacted Xenocron. Chris had no idea. Sent me a replacement harness. That one still did not work. I did more research. Found the above resistor trick. Installed the resistor. Tach moves now but not past 1500rpm.
Here is a pic of my adapter harness with the resistor installed acording to the above diagram:
09-12-2009, 07:11 PM
#7
Honda-Tech Member
Re: OBD2b to OBD1 = No tach
There must be some sort of issue elsewhere in your vehicle. We built literaly hundreds of these adapters using the exact configuration described above, and we never had any issues with tachs working or not working. I built my 1st OBD2B to OBD1 adapter back in 1999, and yes, the first one did not send the signal to the tach. Once we discovered the blunder, and included the extra connection, we never heard another word about it.
We never needed to introduce any resistors into the design. In fact, this is the 1st time I've ever heard about the need for one.
We never needed to introduce any resistors into the design. In fact, this is the 1st time I've ever heard about the need for one.
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09-14-2009, 02:08 PM
#8
Re: OBD2b to OBD1 = No tach
There cant be another issue. If there was, the tach would not work with the stock ECU AND my AEM EMS (30-1010). The engine harness is stock and unmodified.
A21 the ICM output is split to the ignition control module and tach. Wouldnt that make the electrical signal rather weak? I am thinking that is the logic behind adding the 12v from A25 but with a resistor inline to limit the voltage added to the tach/ICM output.
I could try another resistor but I really dont have any other proposed solutions.
A21 the ICM output is split to the ignition control module and tach. Wouldnt that make the electrical signal rather weak? I am thinking that is the logic behind adding the 12v from A25 but with a resistor inline to limit the voltage added to the tach/ICM output.
I could try another resistor but I really dont have any other proposed solutions.
09-15-2009, 03:35 AM
#9
Honda-Tech Member
Re: OBD2b to OBD1 = No tach
Jaques is correct. It is not that your car has a problem persay, but as I stated in the other thread, I have built over 2000 of these exact harnesses and there should not be this issue. I sent you a PM. Let me know so I can pull the specs on your car to look at a couple things.
09-20-2009, 08:40 PM
#11
Re: OBD2b to OBD1 = No tach
I swapped out my MSD Pro Billet distributor for the stock OEM distributor and the tach functions. Im happy to find the cause but this makes no sense. I've been using the MSD dist for over a year running the AEM EMS without any issues. The tach worked beautifully with the MSD distributor when running the stock OBD2 ECU and my old AEM EMS but it doesnt work when running an OBD1 ECU and adapter harness.
Does anyone know why the tach signal would be any different?
Does anyone know why the tach signal would be any different?
09-25-2009, 12:34 PM
#13
Re: OBD2b to OBD1 = No tach
Created a thread on MSD's forum for this:
http://www.msdignition.com/forum/showthread.php?t=14175
http://www.msdignition.com/forum/showthread.php?t=14175
10-11-2009, 08:29 PM
#14
Re: OBD2b to OBD1 = No tach
Here were the combinations with the original wiring config:
OBD2 ECU + MSD Dist = tach works
OBD2 ECU + Stock Dist = tach works
AEM EMS + MSD Dist = tach works
AEM EMS + Stock Dist = tach works
OBD1 ECU + Adapter harness + Stock Dist = tach works
OBD1 ECU + Adapter harness + MSD Dist = TACH DOES NOT WORK
Problem turned out to be MSD dist but unsure why. The solution was to seperate the ICM and tach wires on the adapter harness then connect the MSD aux tach output directly to the tach wire.
OBD2 ECU + MSD Dist = tach works
OBD2 ECU + Stock Dist = tach works
AEM EMS + MSD Dist = tach works
AEM EMS + Stock Dist = tach works
OBD1 ECU + Adapter harness + Stock Dist = tach works
OBD1 ECU + Adapter harness + MSD Dist = TACH DOES NOT WORK
Problem turned out to be MSD dist but unsure why. The solution was to seperate the ICM and tach wires on the adapter harness then connect the MSD aux tach output directly to the tach wire.
10-12-2009, 08:55 PM
#15
Re: OBD2b to OBD1 = No tach
The root cause analysis from MSD:
Originally Posted by Erik@MSD
Since OBD-0, OBD-1 and some OBD-2A get the tach signal directly from the igniter the ECU doesn’t have a designated tach out. OBD-2B on the other hand doesn’t use the tach output from the Igniter. Instead it uses a designated tach output from the ECU. Since your OBD-1 ECU doesn’t have a designated tach output the manufacture of the adaptor harness simply connected the tach wire from your OBD-2B harness to the ICM (Ignition Trigger) wire. This is where it gets complicated; The ICM trigger is a low voltage trigger (about 5 volts), the Tach output wire from the igniter is a 12 volt signal. By hooking the tach wire to the ICM wire you are already giving the tach a much lower voltage signal then Norman (but it works). However when you use the MSD igniter (PN 6305) it has a different inductance then the factory igniter. This can cause the voltage from the ICM trigger (normally about 5 volts) to slightly drop. Although this slight drop in voltage doesn’t affect the function of the igniter it will affect the function of the tach which is already trying to operate on low voltage signal. This explains why your tach will work with the factory distributor and not the MSD distributor when using an OBD-1 ECU.
07-03-2013, 10:20 AM
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Re: OBD2b to OBD1 = No tach
Jaques is correct. It is not that your car has a problem persay, but as I stated in the other thread, I have built over 2000 of these exact harnesses and there should not be this issue. I sent you a PM. Let me know so I can pull the specs on your car to look at a couple things.
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OneThirtyEight
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