Tach needle dropping and jumping back up on '93 2.3L

[Tavis]

I have a 93 2.3L hatchback. Recently, as of the past month, I have noticed that the tach has started to "drop" and jump back up to normal. The tach needle will drop from where it actually should be, to about half way of where it should be on the dial, then suddenly/randomly jump back to where it's supposed to be. It does this either at idle or going 70 down the interstate. Sometimes it will work a long while, sometimes it will do it as soon as I start it up. If idle, it will drop to around 500RPM on the dial. The actual engine RPM is consistent - no drops.

Additionally, sometimes the check engine light comes on - I believe mainly when the needle shows RPM at 500.

The tach issue and Check Engine seemed to start after the power steering pump was replaced. I'm not saying it's related, but giving all info I can.

And lastly, the speedometer has been acting up for a long while. It is accurate until right over 45MPH, when you hit over that, it shoot past 55MPH and continue. I've noticed that over 45MPH it is about 10MPH off.

How do I need to fix the tach?

 

[91TwighlightGT]

Your ignition control module is probably failing which is causing your tachometer problem. This is a well documented failure point on dual plug 2.3l cars.

Check out this thread...

Tach problem

Tach is reading low. Idle is reading ~600, where is it's usually 800. When I'm on the throttle and it's reading ~1,500 it really should be closer to 2,500. When it would be normally around 3,000 it kicks up for a second to 3,000 and then drops right back down. I got a code scanner from my...

www.stangnet.com

The speedometer is likely an unrelated issue. It could be the speedo itself, or less likely the cable.

 

[junkyardwarrior]

did you dump the codes yet? What did it spit out? It will be 3 digit codes on a 93 4 cyl. Should have a code or two stored particularly since the CEL came on at at least one point.

 

[Tavis]

Update:
Ordered a new Motocraft brand ICM from RockAuto for my model. Got here today. Installed this afternoon (battery was disconnected prior to the install). Used white grease to replace the grease on the back of the ICM. I guess this is for better heat dissipation?
Anyway, after installing, I cranked up. It ran and idled beautifully. MUCH better than the original ICM. Let it sit and run for about 5 min. Continued to run and idle beautiful.
Had to go to the bank so I decided to take it for a test run. Ran GREAT...for about 5min. Suddenly, RPM on the tach dropped to half, started shaking bad under load and literally felt like it was running on only two or three cyl. CEL came back on. WORSE than the original ICM.

Limped back home. Made it home. Popped hood and nothing out of the ordinary. Checked ICM connectors - good and in solid. Checked ignition coil main connector and plug wires. Found that the rear coil main connector was easily bendable and a little broken plastic so assuming I possibly have a bad rear coil? I made a video of me trying to gently move the main connectors on both coils. https://photos.app.goo.gl/4moVnC2P5fjySx9f6

And then...I cranked it back up and it ran beautifully again. No CEL and accurate tach.

WTH? I assume I'll be ordering two new ignition coils. Unless you have other thoughts.

I don't have a code reader (yet, depending on price) to dump the codes. I only have an OBDII reader which of course does not cover it.

 

[91TwighlightGT]

I would suspect the ignition coils as well. I am not a fan of throwing parts at things - you can pull the plug wire off at the coil and see if it is arcing across, or with more work pull a plug and see if it is sparking when grounded.

Another test is swapping the coil packs front to back. With dual plug on these cars, only one coil is firing for ignition, the other is firing for waste spark. So, if your waste spark coil pack is acting up, then you will get some symptoms such as the tach acting up, but it will run okay. If the ignition side coil is acting up then you will get a running problem. These coils are poorly placed and susceptible to failing due to excessive heat from the exhaust manifold being so close. As the coils get hot they can start to act up, which is what you seem to be experiencing.

These coils have enough problems that some people always carry an extra just in case. Fashioning a better heat shield would also help. Without any codes and based on your description, I think you are on the right track.

 

[Tavis]

Since I have a few foxes, I ordered one of these from Amazon INNOVA 3145 Ford Digital OBD1 Code Reader.

And I'll be ordering two new coil packs. I'll report back once I get them installed/tested.

Thanks!

 

[Tavis]

Just received and installed the new coil packs with some fresh dielectric grease. Took it down the highway about 4mi and 4mi back. Performed flawlessly thus far! I believe the new coil packs and ICM did the trick. Thanks!

 

[91TwighlightGT]

Good job!

 

[glowstang93]

Been there done that. Best to just replace coil packs and Icm same time. Glad ur rolling again.

 

[7991LXnSHO]

@Tavis
The gunk on the back of ignition modules is supposed to be heat sink paste.
Something like this will be much better than grease at keeping the module from cooking itself.
Les

Heatsink Compound Paste Syringe High Performance Silver Thermal Grease CPU | eBay

Thermal Conductivity: 8.5 W/m-K. Thermal Impedance(/-in2/W): < 0.190. Color: Gray.

www.ebay.com

 

[cuda]

I would suspect the ignition coils as well. I am not a fan of throwing parts at things - you can pull the plug wire off at the coil and see if it is arcing across, or with more work pull a plug and see if it is sparking when grounded.

Another test is swapping the coil packs front to back. With dual plug on these cars, only one coil is firing for ignition, the other is firing for waste spark. So, if your waste spark coil pack is acting up, then you will get some symptoms such as the tach acting up, but it will run okay. If the ignition side coil is acting up then you will get a running problem. These coils are poorly placed and susceptible to failing due to excessive heat from the exhaust manifold being so close. As the coils get hot they can start to act up, which is what you seem to be experiencing.

These coils have enough problems that some people always carry an extra just in case. Fashioning a better heat shield would also help. Without any codes and based on your description, I think you are on the right track.

Maybe pickup coil?