View Full Version : Speedhut Speedometer...wierdness
Chopper
07-15-2022, 09:47 AM
Hey,
I have the FFR (Speedhut) gauges, non GPS. Recently, my speedometer has developed a quirk after working fine for years. It works fine when the car is moving, and as you come to a stop. However, maybe a second after the car is stopped, the speedometer will move up to showing 20mph, sometimes 40mph. As soon as you start to accelerate from the stop it drops to zero and behaves correctly again. It will also raise to show one of these random speeds when the car is started and sitting idling. However, once moving, same thing...works properly.
As a matter of troubleshooting, I had a spare speed sensor (T5 transmission), so I swapped them out. Behavior was exactly the same with the other sensor, so I think that rules it out.
Anyone have any ideas of what to do to troubleshoot? I was thinking about calling Speedhut to see what they thought.
Thanks in advance.
-Brandon
The speedo is responding to a signal to display the speed. Could you have a chafed wire between the sending unit and the gauge? It could also be caused if you have a noisy wire, say from a coil that is running beside the speedo wire. Have you done any recent changes to the ignition system? New plug wires, coil, etc.?
Chopper
07-15-2022, 01:07 PM
Good call! When I upgraded from Pro-flo2 to Pro-flo4, I also had to change my coil from a TFI style to a canister style. The tach feed runs off of the negative terminal on the coil and feeds into the RF harness to go up to the gauges, same as the transmission pigtail, they call go through the wiring harness hole in the driver's footbox upper corner. I am wondering if the tach sensor wire could be transmitting the interference in this case?
I can check and report back in a few days. I think I'll have to pull my dash and get a really good look. If this is the case, I'll either need to shield one or both wires (tach and speed sensor), or separate the speed sensor wire and run it differently.
-Brandon
Rdone585
07-15-2022, 01:15 PM
If this turns out to be the problem, I highly recommend shielding and perhaps rerouting the offending wire. If it's interfering with the speedo wire, it could also be interfering with other wires or signals and you just don't know about it yet.
narly1
07-15-2022, 01:31 PM
Perhaps the addition of a bypass capacitor to ground on the speedometer?
I suggest a call to Speedhut before you do though...
Earl
Good call! When I upgraded from Pro-flo2 to Pro-flo4, I also had to change my coil from a TFI style to a canister style. The tach feed runs off of the negative terminal on the coil and feeds into the RF harness to go up to the gauges, same as the transmission pigtail, they call go through the wiring harness hole in the driver's footbox upper corner. I am wondering if the tach sensor wire could be transmitting the interference in this case?
I can check and report back in a few days. I think I'll have to pull my dash and get a really good look. If this is the case, I'll either need to shield one or both wires (tach and speed sensor), or separate the speed sensor wire and run it differently.
-Brandon
Easy check would be to take the tach wire off the coil and see if that makes the problem go away.
Hacksaw84
07-15-2022, 05:00 PM
If you cant figure it out or isolate it to the gauge, Speedhut's customer service is really good. They will take care of you.
Chopper
07-18-2022, 08:45 AM
Papa was right, I disconnected the tach feed and the speedometer works perfectly. I had reused the tach feed when installing the PF4 as it seemed like the easiest thing to do at the time. The PF4 ECU does have a tach output, I'm going to see if I can convert to using that and eliminate the wire from the coil to the tach entirely. The connection then happens entirely behind the dash with no chance for interference. For the ECU connection Speedhut says I need a 4.7k ohm 0.25w resistor, I don't have any in stock so I ordered some.
I'll report back when I get it tested out, in case anyone is curious if the PF4 ECU can drive a speedhut tach.
-Brandon
Interestingly, while my car was being tuned yesterday, the tuner called and said he was getting odd data from my brand new O2 sensor when he hit about 3000rpm. As it turns out, my coil wire was running right along with the O2 wires. He rerouted the coil wire and the issue went away.
Chopper
08-01-2022, 12:38 PM
To close the loop on this, I pulled my dash and changed the tach output to run off of the Pro Flo 4 ECU. Works perfectly. I also used the opportunity to rewire my fan control to use the ECU instead of a thermo switch. Seems cleaner to me. Cleaned and removed the old tach and fan control wires under hood, I'll pull the thermo switch and replace with a plug next time I change coolant. Thanks all for helping me solve this!