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MG MGA - Cruise control installation issues

I have two issues with Dakota CRS-3000-4165 with HND-2 dash switch that I purchased. The manufacturer of the is Rostra Precision Controls, North Carolina. The servo is Rostra electric universal cruise, with the dash switch being Rostra p/n 250-3593 (open circuit control switch), and the mMagnet kit Rostra p/n 250-461.

My issues on my 1500 MGA:

(1) Dash switch coming on by itself. It has engaged twice by itself
I can create it by starting the car, or when the turn signal switch comes to rest, or when headlight switch is turned off.

(2) Surging is my second problem. At 50 mph, it goes to 52 the 48, and continues this rolle-coaster.

Dakota Digital had me do the following without success:
• 1 mF capacitor on white (microfarad ceramic 50v) & 2.2 m F capacitor on gray
• White & gray to ignition side of fuse box; then to ignition switch, own post
• White & gray, with a toggle switch, to battery side of starter solenoid
• Disconnected gray from everything (cut in half)
• Black & blue to two different grounds and to negative battery terminal
• New dash control switch
• Servo wiring: Red direct to MGAs brown battery wire for constant voltage (hot side of brake switch is key activated)
• Tried settings #3, 4 and 7 on. Had surging. Said to try changing pulse/mile settings from 8,000. Try up one. And, try down one.
• Changed turn-signal switch: L=Green blue, R=Green yellow, F=Green
• Changed flasher unit: X=Green, P=Light green, L= Green brown to terminal 1 on relay box

Rostra has had me do the following without success:
• Tested switch with independent battery outside car. Switch worked fine. Problem is the car.
• Ck’d voltage on 4-pin connector - all getting 12v.
• Ck’d ohm on violet wire to brake switch - good.
• Ck’d ohm on servo ground wire – good, 0.1 ohm.
• Ck’d ohm on dash switch ground – good, 0.5 ohm.
• Tech suggested self-engagement issue may be due to defective flasher unit which I replaced (TBD).
• Suggested for surge:
o No more than 3 beads (I have 2 between two connectors).
o ¼” slack in the cable (don’t want it too loose or taunt).
o Remove two magnets, leaving two 180 degrees apart.
o Program for 4,000 ppm. Only #3 and #7 should be on.
o Ran TACH wire temporarily to ground
This did NOT work. Set at 50, it goes up & down between 51 & 49

A friend built the following filter: 1 mF capacitor to ground, a series choke coil to block any remaining noise, and a 1000 mF capacitor and a diode to act as a power reservoir.

• With servo power wire (brown) disconnected, switch turned it self on, so I connected the above filter between ignition switch and dash switch without success.

I've been spinning my wheels for over three weeks now and getting no where. Any suggestions would be most welcome.
D Quinn

How does one edit their own post? This thread is my first on this site. There was a couple things I wanted to clean up a little.

By the way, I installed an electric cruise (not vacuum) many, many years ago on a 1600 MGA and the only issue I had was the trial/error to get it to run smoothly. I recall trying many different settings before it worked properly. Sadly the car was totaled and the paperwork lost. But, a bigger issue to me is having it turn itself on from 'noise' in the circuitry, and worst yet when it engages itself!
D Quinn

15 to 20 years ago I installed a few vacuum powered cruise control units, after market parts. I ordered one cruise control to be installed on a new car at the dealer, but they had some independent contractor type guy install an aftermarket unit. All of the aftermarket units gave various levels of problems during and after installation, and they all failed within 5 years (some sooner). I ultimately gave up and run my MGA without one, even on long road trips.

On the flip side, I have had a few cars with factory cruise control the worked flawlessly and never fail. One on my '87 RX7 is still working well (which may be one of the few electrical things on that car that does).

My take on CC units is, if it doesn't work well within one hour of installation, send it back. If you can't get it to work via the included installation instruction sheet, a call to tech assist is probably a waste of tine.
Barney Gaylord

What Barney says, especially since tech support usually just opens up the instructions and verifies that you did what they asked. They don't know anything. Best bet within 15 minutes with tech support is to say, "I am still having problems, can I talk to a supervisor?"
Mike Parker

engineknow what most of this means (and don't want to!), but:
Is the dash switch a mechanical switch or some solid state thing? Is there a control box someplace?
Your signs are that there is some sort of electromagnetic pulse doing this. Voltage spikes do all kind of weird stuff to solid state "switches", and such spikes can be induced by loose connections. ( I had a nearby lightning strike turn on my stereo, turn volume to max, put it on PLAY TAPE and RECORD at the same time; scared the bejeezus out of me and erased the middle of my favourite tape, which happened to be in the player at the time. It was OFF at the time and had been left on RADIO at normal volume.)

These are suspect, esp the 0.5 ohm one:
FR Millmore

These links show the manuals for the two main components:

1. Servo is Rostra electric universal cruise p/n 250-1223
Link to manual:

2. Dash switch is Rostra p/n 250-3593 (open circuit control switch)
Link to manual:

Barney, 15/20 years ago I started out installing a vacuum CC which did not work to well. I switched to an electric one that I was told at the time was designed for larger vehicles such as motor homes. It lasted the life of my car. The brand was AutoVox (identical in design to the Rostra - both servo and switch. CC is especially important to me because of a bad ankle that I've had since high school.

Mike, I intentionally purchased the unit from Dakota Digital based on the recommendations that they provided excellent technical product support. After working with them for days it was clear they were unable to clear things up. I then contacted Rostra, whom I believe to be the largest manufacturer of after-market CC and who sell them under various names. At Rostra I've been talking to their senior oldest technician, who specifically said he is the only one who understands what MG's are, etc. Heck, he even knew they were originally positive ground. :)

FRM, the switch is definitely solid state. Two toggle switches- on/off, set/resume with small circuit board behind them. Has two power wires (switch power, backlight power) and two grounds, plus a four pin connector for the toggles to the servo.

To check the grounds I had the ohm meter on it's lowest setting of 200 to obtain the results above. Both I and the tech considered 0.5 (one-half of one ohm) to be a good ground. It's constant with the car running. I don't recall what the reading was on the brake switch but the tech said, that's fine. The brake shut-off works as well.

The switch worked fine to an independent 12-volt battery outside the car. The wires to it all carry 12 volts. The two grounds had the 0.5 reading. If that is not good enough (frankly I don't know), then I can I can try them in another location. However, I done this a couple times already. The issue may simply be that MGA as a whole have a serious problem when it comes to grounding - some worse than others it seems based on the noise coming back. I'm just a shade-tree mechanic and don't have access to a fancy scope, etc., so I can't measure things as well as a shop might.

Right now I'm at a loss.
D Quinn

I also installed a couple of vac operated cruise units in the 1980's. Never had a relibility problem with them.

In general, the MGA with its relay-regulated generator is a very noisy system. The 1500, with the brake/turn signal realy is another electrical noise source. It sounds like you may have 2 separate problems, one ust general noise and the other related to the relay box. If any wires are routed around the relay box and/or the gen regulator, the best first try would be to move wiring from these devices. Troubleshooting electrical noise issues can be a difficult task.

I'd try some antiparallel diodes directly across the relay box coils.

Do you have headlight relays? If so another antiparallel diode directly across that coil too might help. An R/C circuit of ~ 1uf and say 100 ohms across the coils might also be needed but save that till later if the diodes don't work.

As for the switch panel energizing by itself, I think you will find that the swtiches are ust PCB mounted NO push button switches. These lines may have to be desensitized to prevent noise from energizing the system. The wiring harness between the panel switch and the control box should definitely be moved away from all other MGA wiring as a first start.

according to the installation manual:

On a vehicle with a manual transmission, the TACH wire connection is not required only when clutch DISENGAGEMENT SWITCH (Kit# 250-4206) has been installed; this will take into acount the TACH over-rev safety feature. The TACH wire should be grounded when using a clutch disengagement switch to ensure that the wire does not introduce
Chuck Schaefer

Chuck, thanks for the fresh ideas.

The 1500 relay box is the one big difference over my prior 1600. I've suspected it could be the source but did not know how to approach it. That does seem like a good place to start.

Yes, I have headlight relays. I've installed an aftermarket harness that handles greater load with the protection of a relay for each headlight.

I initially connected the TACH wire to the negative side of the coil. Then, to conduct tests, I disconnected it and ran it to ground. Doing so, did not change anything. If it had, I would have left it to ground and ordered the clutch disengagement kit. At the moment it is still to ground, so hopefully I can rule it out as part of the problem.

D Quinn

I installed silicon diodes (RadioShack IN4003-Micromini) to terminal 4 and terminal 8 on the relay box at cathode end with the other to ground. It was fun of trying to figure out how to attach and ground them with the heater box in the way but I was able to do so. Another failure. They merely took the turn signals out of commission.
D Quinn

Here's an organized recap of my issues and attempts as of August 2, 2011

(1) Control coming on by itself
Comes on when car is started.
Comes on when turn signal switch comes to rest.
Comes on when headlight switch is turned off.
(2) Control engaging ‘set’ by itself
This has happened twice during road testing
(3) Surging

Tech contacts used
Dakota Digital, 800-593-4160 - Derrick
Rostra, 800-732-4744 - Thomas

Control solutions suggested, tried, and failed

• Control power wire (white) with1 mF (microfarad ceramic 50v) capacitor installed
• Control backlight power wire (gray) with 2.2 mF capacitor installed
• Controls power wires to ignition side of fuse box
• Controls power wires to ignition switch using it’s own post
• Controls power wires, with toggle switch, to battery side of starter solenoid
• Control backlight power disconnected
• Control ground wires (black & blue) to two different grounds
• Control ground wires to negative battery terminal
• New control installed
• New control tested with independent battery outside car - worked fine.
• Control’s 4-pin connectors voltage check - all get 12v.
• Control’s ground resistance tested 0.5 ohm.
• Servo’s ground resistance tested 0.1 ohm.
• Disconnecting servo power and control still came on
• Super filter (little box with 1 mF capacitor to ground to shunt any noise to ground, a series choke coil to block any remaining noise, and a 1000 mF capacitor and a diode to act as a power reservoir to keep the output voltage constant to the cruise even if the system voltage drops a little after switch-off). Installed between control and ignition switch.
• Silicon diodes (RadioShack IN4003-Micromini) to relay box terminal 4 and terminal 8 (cathode end to terminal with other to ground).

Servo change
• Brake switch hot wire (red) rerouted from brake switch (per manual) to car’s battery wire (brown) for constant voltage (per Dakota tech) as brake switch is key activated.
• Checked ohm resistance on cold side brake switch wire (violet) – good per tech.

Surging solutions suggested and tried

• Tried settings #3, 4 and 7 on with pulse/mile settings at 8,000.
• No more than 3 beads (I have 2 between two connectors).
• ¼” slack in the cable (don’t want it too loose or taunt).
• Removed two magnets, leaving two 180 degrees apart, per Rostra tech.
• Tried settings #3 and 4 on with pulse/mile at 4,000 ppm.
• Ran TACH wire (blue) temporarily to ground (can’t leave like that; need clutch disengagement kit)
D Quinn

Sorry the diodes didn't work out. I have no explaination why they affected the operation of your signals though. Unless you are running pos ground.

All of your symptoms, except the surging, can be realted to noise being injected into the control panel or its wiring. If I understand it correctly, when you feed the control panel with a remote 12v batter, everything works well???? Try relocating the control panel. Lay it loose and move it around to see if there is a place where it does work properly. Move the wiring around between the control and the servo too.
Chuck Schaefer

Chuck, it's definitely a negative ground system.

As for the diodes, the only reachable ground was the lower screw holding the relay box on the bulkhead. I used that for both grounds and came around each bottom side to attach them to 4 and 8.

The control is a plastic case mounted firmly onto a L brace I made to attach it to the bottom ridge of the dash. This is an identical location that I've used in the past.

As for the wiring placement, naturally there are huge limitations given the limited space under the dash and in the bulkhead area. Both control power wires simply run across the underside of the dash to the key switch.

At the current time I don't even have the servo connected to power, as I've focused on trying to solve the control issue.
D Quinn

Hi Dave,

I just listened to your phone message so you do have the correct Dave. I'll need to find my notes/look at the car to refresh my memory before I call you back. I frankly don't remember what I got connected to where or the dip switch settings I used.

I'll call tomorrow if that works for you.

Dave Ahrendt

Dave, that would be great; any help would be appreciated big time!
D Quinn

Dave, At the moment I've got the white/gray/blue/black wires (power/ground) all disconnected as I was going to try something different. Pending your info, I will hold up doing anything till I hear.
D Quinn

It took a while but this story has a happy ending. Thanks in large part to this site and it's archives. I was able to track down another owner with a similar kit and with his assistance we made some discoveries.


The first break came when a MGA owner with a similar cruse system reported he tired to duplicate my issues, i.e. turning the turn signal on and off, while using an oscilloscope to analyze things. To his surprise and everyone, he reported getting voltage spikes of 150 volts for a few microseconds from the turn signal switch. He commented, “I'm not surprised now that a modern electronic device would have fits living in that environment. These modern circuit boards are not set up to handle that.” His recommendation was to use a 1uF capacitor on Directional Indicator Relay Box terminals #4 and #8. I used Radio Shack p/n 272-0996 non-polarized caps. They worked. Turn signal issue resolved. Since the Relay Box is unique to 1500’s later models would not have this to deal with, although they may pickup stray voltage from other sources. Additional tests by my new friend show the heater blower motor switch can arm it; multiple use of the horn, wipers, headlight switch, etc. can arm it. So placing a cap on any of these sources might be beneficial.


The British wiring expert sent me a very small black box he constructed containing a 1uF capacitor to shunt any noise to ground, a series choke coil to block any remaining noise, and a 1000uF capacitor with a diode to act as a power reservoir to keep any output voltage constant if the system voltage drops after switch-off. I connected the servo brown power wire and the controller white power wire on one side (yellow wire) and the box’s white wire to the green side of the fuse and the black wire to ground. At first try it did nothing. I then ran the servo’s blue Tach wire to ground instead of the coil with the super filter in place. It worked! I had tried the Tach wire to ground before and it made no difference but with the filter it did. I never really pinpointed the cause or source of the dirty signal when the car was started but these steps eliminated it. Possibly it was the car’s starter solenoid. I should have tried a cap on that. If you ground the Tach wire, you need a clutch release kit, which I ordered.


With the turn-on issues resolved there was still drivability to deal with. It needed tuning. The car’s speed was running like the stock market – up and down quickly and not at all predictable. There are literally hundreds of combinations possible as nine of the twelve switches have either three or four group combos. Thankfully I hit the perfect one after a handful of tires. When all the switches were placed in the off position except for numbers 4, 7, and 9 it provided a perfectly smooth operating cruise. It is the equal of any original equipment manufacturer. I could not be happier. The cruise now runs as smooth as any of my modern cars, at least on the flats and foothills of Michigan. I suspect the settings should be equally as good in really steep grades.
D Quinn

I have followed this thread with interest since I was planning to install a Rostra CC on my car. Problem is, the dealer that Rostra referred me to won't sell me one and says they haven't sold a magnet kit is years. Any suggestions?
PS: I work in an aerospace electronics firm. Capacitors to suppress (electrical) noise (spikes) are used everywhere!
G T Foster

Glad yout got to the root causes and have a functioning system. Now, with your new fangled device, your MGA will no longer be nuclear EMP safe.
Chuck Schaefer

GT, This is where I ordered mine from and this is the unit.

If you buy it and would like more specifics on my installation; let me know. I have taken extensive notes.
D Quinn

Gerry, here's p/n's on mine:

Dakota Digital CRS-3000-4165 (that p/n includes the magnet kit) with HND-2 dash switch for $275.

Local installers have no idea how to install the unit on our 50 year old cars. They go by a "book" and our MGA's are certainly not in there. So, if you want it, you have to do it yourself. Their manual can be pretty confusing but, as I said, I can clear up most of the mystery if you are game to try.
D Quinn

I have a question for D Quinn who started this thread.

From the time you took possession of this new CC unit (sometime before Aug 1) until you got it installed and working on Aug 14, how many man hours in total did you spend installing, inquiring, corresponding, tinkering, modifying, etc. Also what did you spend on additional electronic components and capacitors?

The object of this question is trying to determine how much the installation process actually cost if your time was worth minimum wages. When we have the answer, this is why I initially said, "If it doesn't work well within one hour of installation, send it back".

Now that you did all of the necessary research and development to make it work, do you intend to give this information to the manufacturer? And if so, what are the chances they might change the design so it could actually work immediately after first installation?

This is a major problem with lots of aftermarket gadgets being installed on MGs. Either they weren't intended to be installed on MGs, or there wasn't enough R&D work done to make it work on the MG (or all cars in general). If it does not work on all cars, then the manufacturer should include in the sales ad a list of cars it has been tested and verified to work with (which is likely to be a shorter list than the cars it won't work on). Of course this will never happen, because if they did it would dramatically cut into sales (when they can't sell it to the unsuspecting souls who don't know that it won't work).
Barney Gaylord


Total time, over 200 hours since I had no resources to draw from. Tech support, both the manufacturer and seller, could only give suggestions rather than solid answers. The manufacturer's tech, after three conversations, refused to talk to me anymore, so I have no desire to try to call him again.

With what I know today, I would think installation in an MGA would take two-thee days for a beginner; a day for good mechanic.

Frankly, I don't think there is anything the manufacturer could do to make it work better. The issues are with the our car and not with their unit. The manufacturer is well aware that certain cars produce dirty signals. The manufacturer does not list MG's in their list of applications at their web site BUT they do sell a generic global unit that is intended for hot rods, old cars, etc., provided the buyer has the aptitude to make it work.

I'm preparing a technical write-up. I can send you a copy and you can decide if you want to include it on your site. I have one more thing I wish to test before printing it.


D Quinn

Mr. Quinn,
Thanks for the information.
G T Foster

Gee Barney, whatever happened to the fun of backyard engineering?
How many thousands of dollars and hours are put into getting another .1 second off an ET or lap time? Is that wasted time? Do we just give up after an hour of work?
The world would be pretty boring if everything was PlugNPlay.
If a few weekends are spent making the car more enjoyable to the OP then why not?


Dave Ahrendt

Update: I have now confirmed that the super filter, ref above, was not needed. I installed a 2.2 uF capacitor on the car's starter switch (starter side) and reattached the unit's TACH wire to the distributor side of the coil. All works with no dirty signal to the controller. So the two major issues (starter and turn signal) were corrected with three capacitors. A pretty darn simple fix once I was able to pin point the exact causes.
D Quinn

This thread was discussed between 01/08/2011 and 17/08/2011

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