1986 EFI to Carb ~ Speedometer interface?
Thread Starter
Burning Brakes
Joined: Sep 2008
Posts: 810
Likes: 58
From: Alva Florida
1986 EFI to Carb ~ Speedometer interface?
Ok, I went to the forbidden territory and swapped my EFI to a 600 holley carb. Everything works perfectly...except my speedometer.
I looked into the Dakota Digital interface, called them and the guy said I should be able to connect the VSS directly to the Dash Cluster, and the cluster proccesses the signal to report the speed.
I did that and all I get is "0", all the time.
The FSM seems to say the ECM is part of the equation.
I've looked everywhere and no one seems to know the answer to this question.
How can I make the speedo work???
I looked into the Dakota Digital interface, called them and the guy said I should be able to connect the VSS directly to the Dash Cluster, and the cluster proccesses the signal to report the speed.
I did that and all I get is "0", all the time.
The FSM seems to say the ECM is part of the equation.
I've looked everywhere and no one seems to know the answer to this question.
How can I make the speedo work???
Thread Starter
Burning Brakes
Joined: Sep 2008
Posts: 810
Likes: 58
From: Alva Florida
According to what I could find (FSM), the signal from the VSS goes directly into the cluster, and then is directed to the ECM to monitor the cruise control, and torque converter lockup functions.
I think there is a processor in the cluster, because the manual say's if there is current at the pins attached to the cluster when the VSS is spinning, to replace the cluster.
I think I'm going to send the cluster in to be checked.
I just wish someone knew something about this subject.
I think there is a processor in the cluster, because the manual say's if there is current at the pins attached to the cluster when the VSS is spinning, to replace the cluster.
I think I'm going to send the cluster in to be checked.
I just wish someone knew something about this subject.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
VSS signal goes direct to speedo , cluster then outputs speed signal to ECM.
'90+ does opposite; VSS to ECM , then signal to speedo
Stock speedo should work without any adapters
Last edited by rodj; Dec 31, 2011 at 06:51 PM.
Melting Slicks
Joined: Jan 2005
Posts: 2,658
Likes: 4
From: Clifton Park NY
Let me try my question again.
If the ecm requires a certain type of signal.. I.E a certain wavelength,voltage etc. and the Dakota cluster is looking for a different type of signal then it won't work.
Ask me how I know this...ran into the same problem with the communication between ECM and analog gauge on mine. I have to use a convertor on mine for the signal to match the ecm
Look in the paperwork for the gauge it should tell you what type of signal its looking for. Then see if those match what the ecm is looking for.
If the ecm requires a certain type of signal.. I.E a certain wavelength,voltage etc. and the Dakota cluster is looking for a different type of signal then it won't work.
Ask me how I know this...ran into the same problem with the communication between ECM and analog gauge on mine. I have to use a convertor on mine for the signal to match the ecm
Look in the paperwork for the gauge it should tell you what type of signal its looking for. Then see if those match what the ecm is looking for.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
Just had a similar problem on a SD HSR swap into my brother '56 Chev.
As noted above, in that case the '730 TPI ECM has 2 speed outputs (4000 and 2000 pulse ) but neither would drive his Dolphin Gauges speedo.
Luckily after talking with Dolphin we found the raw unprocessed signal direct from the VSS would run the speedo which can be calibrated for a range of pulse inputs
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Thread Starter
Burning Brakes
Joined: Sep 2008
Posts: 810
Likes: 58
From: Alva Florida
Was speedo working correctly before carb swap?
Well, yes, but Also changed the engine, trans, diff, interior, & repainted it. The car is running and driving fantastic, and I had the cluster rebuilt. Don't ask me how much time and money I have in it...
All the other gauges work perfectly, by the way. Everything is hooked up factory, except the EFI system.
So many changes, all good..except now I'm not sure what might be not grounded or what by not having the ECM running the engine. That's a good question, but I really don't know the answer.
So far as the sine wave or interface, I think the cluster gauge itself converts the input to the the reported speed on the cluster.
I just wish I knew what took place inside that cluster.
Thanks much!
Well, yes, but Also changed the engine, trans, diff, interior, & repainted it. The car is running and driving fantastic, and I had the cluster rebuilt. Don't ask me how much time and money I have in it...
All the other gauges work perfectly, by the way. Everything is hooked up factory, except the EFI system.So many changes, all good..except now I'm not sure what might be not grounded or what by not having the ECM running the engine. That's a good question, but I really don't know the answer.
So far as the sine wave or interface, I think the cluster gauge itself converts the input to the the reported speed on the cluster.
I just wish I knew what took place inside that cluster.
Thanks much!
Last edited by cadmaniac; Dec 31, 2011 at 07:07 PM.
Melting Slicks
Joined: Jan 2005
Posts: 2,658
Likes: 4
From: Clifton Park NY
I agree.. but I'm understanding the OP's post as he's using aftermarket
Dakota speedo?
If he's using the stock gauge cluster then his carb mod would have no effect on the operation of his speedometer unless the problem was there prior to the mod. or he might have accidently removed a wire without his knowledge.
To the OP-- are you using a aftermarket gauge or stock?
Dakota speedo?
If he's using the stock gauge cluster then his carb mod would have no effect on the operation of his speedometer unless the problem was there prior to the mod. or he might have accidently removed a wire without his knowledge.
To the OP-- are you using a aftermarket gauge or stock?
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
My thoughts
Have you confirmed voltage at cluster or just reading from the FSM so far?
Have you confirmed voltage at cluster or just reading from the FSM so far?
Last edited by rodj; Dec 31, 2011 at 07:25 PM.
Race Director
Joined: Apr 2003
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From: moraga ca
When I swapped my 84 injection to a carb years ago, I had no problem with the speedo. Ok before the swap and ok after. Just plug in the same connector to the speedo sensor. You said you changed trannies. What are you running now?
Thread Starter
Burning Brakes
Joined: Sep 2008
Posts: 810
Likes: 58
From: Alva Florida
Factory Guage cluster is being used. I originally thought since I wasn't using the ECM, I would have to find something to replace it. That's why I called Dakota to see if their universal converter would work. He said to connect the VSS signal directly to the cluster.
I did that (connected the VSS wires directly to the harness at the cluster)but it still didn't work...
I've been chasing this problem on and off for a couple of months. I went from a 4+3 to a 700R4 trans.
I fixed all the other things and now this is all that is left. Can you believe it, only one thing wrong with an '86 vette!!!
I pulled the cluster and put a voltmeter on the 2 wires coming out of the harness to the cluster. Since I bypassed everything else, I got a very consistent a/c voltage from the VSS at the harness connector.
The FSM says if you are getting the signal at the harness to the cluster, to replace the cluster.
That's why I think I should send it in and have it checked.
I did that (connected the VSS wires directly to the harness at the cluster)but it still didn't work...
I've been chasing this problem on and off for a couple of months. I went from a 4+3 to a 700R4 trans.
I fixed all the other things and now this is all that is left. Can you believe it, only one thing wrong with an '86 vette!!!
I pulled the cluster and put a voltmeter on the 2 wires coming out of the harness to the cluster. Since I bypassed everything else, I got a very consistent a/c voltage from the VSS at the harness connector.
The FSM says if you are getting the signal at the harness to the cluster, to replace the cluster.
That's why I think I should send it in and have it checked.
Melting Slicks
Joined: Jan 2005
Posts: 2,658
Likes: 4
From: Clifton Park NY
Well Dakota is full of it I think.
Wire your speedometer according to the FSM..not the way Dakota told you to first before you do anything else.
It it doesn't work your gonna need to use the FSM wiring schmatic and a DVM and start checking voltages,wire condition all the way thru. Even the connections that go into the ECM,Custer,and Speedo
Make sure your using a wiring diagram for the 700r4 and you need to use your existing ecm
Wire your speedometer according to the FSM..not the way Dakota told you to first before you do anything else.
It it doesn't work your gonna need to use the FSM wiring schmatic and a DVM and start checking voltages,wire condition all the way thru. Even the connections that go into the ECM,Custer,and Speedo
Make sure your using a wiring diagram for the 700r4 and you need to use your existing ecm
Race Director
Joined: Apr 2002
Posts: 10,036
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From: Anaheim CA
The ECM has nothing to do with the speedometer function. The cruise control is also done inside the instrument cluster, not the ECM. You can make the TCC work (normally controlled by the ECM) by connecting the 700r4 4th gear switch to the TCC. You'll only have TCC in 4th gear, but at least you'll have it.
The speed signal goes from the VSS sender to the cluster and gets converted to MPH there. There are different VSS senders that put out different frequencies (2000 pulses/mile in early C4s -- I think it's 4000 pulses/mile in early '90 cars). You need to make sure your VSS sender is compatible with your cluster. Also the VSS sender gear in the tailshaft needs to match your differential gear ratio or your speedometer will not be correct. You can get around this by using a converter.
The speed signal goes from the VSS sender to the cluster and gets converted to MPH there. There are different VSS senders that put out different frequencies (2000 pulses/mile in early C4s -- I think it's 4000 pulses/mile in early '90 cars). You need to make sure your VSS sender is compatible with your cluster. Also the VSS sender gear in the tailshaft needs to match your differential gear ratio or your speedometer will not be correct. You can get around this by using a converter.
Last edited by Cliff Harris; Jan 1, 2012 at 02:13 AM. Reason: Added VSS frequencies.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
Melting Slicks
Joined: Jan 2005
Posts: 2,658
Likes: 4
From: Clifton Park NY
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
Melting Slicks
Joined: Jan 2005
Posts: 2,658
Likes: 4
From: Clifton Park NY
Using a 88 corvette schamatic (which I'm assuming is the same for the 86)- A10 (ecm) is wired using a brown wire that goes direct to the dash cluster.
A7 (ecm) is connected to the TCC switch
Assuming the OP spliced into that Brown wire there by eliminating A10 from the ECM then I would assume his Speedo wouldn't function because he's connected directly to the sender. And if he is then where is the speedo getting the .60 to 12V from?
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
You will note there is no terminals labelled VSS on the 86 - '89 '165 ECM. ( so how would speed signal get to speedo from the ECM ?)
Check the IP diagram in your FSM ;
you will see the two wires (purple / yellow ) from the VSS go directly there